Effect of intrinsic foot muscles training on foot function and dynamic postural balance: A systematic review and meta-analysis.

This systematic review aimed to analyse the effects of intrinsic foot muscle (IFM) training on foot function and dynamic postural balance. Keywords related to IFM training were used to search four databases (PubMed, CINAHL, SPORTDiscus and Web of Science databases.) for relevant studies published between January 2011 and February 2021. The methodological quality of the intervention studies was assessed independently by two reviewers by using the modified Downs and Black quality index. Publication bias was also assessed on the basis of funnel plots. This study was registered in PROSPERO (CRD42021232984). Sixteen studies met the inclusion criteria (10 with high quality and 6 with moderate quality). Numerous biomechanical variables were evaluated after IFM training intervention. These variables included IFM characteristics, medial longitudinal arch morphology and dynamic postural balance. This systematic review demonstrated that IFM training can exert positive biomechanical effects on the medial longitudinal arch, improve dynamic postural balance and act as an important training method for sports enthusiasts. Future studies should optimise standardised IFM training methods in accordance with the demands of different sports.

1 Introduction

Whilst running, the feet act as the starting body part of the lower limb kinetic chain. Aside from functioning as shock absorbers, weight support structures and locomotive effectors [1, 2], the feet can resist deformation, provide a stable base of support and lever the arms to propel the body efficiently [3]. Given that the feet are the most distal aspect of the lower limb and the first part touching the ground [4], many studies have explored their potential mechanism in transmitting ground reaction force and established that impact forces can be distributed through the active modulation of the activity of muscles, such as the plantar flexor, tibialis anterior and calf muscles [3, 5–7].

The main Intrinsic foot muscles (IFMs) are abductor hallucis (ABH), flexor digitorum brevis (FDB) and quadratus plantae (QP). Their principal function is to provide foot stability and flexibility for shock absorption [8]; improve dynamic alignment; stiffen the foot arches and stimulate proprioceptors on the sole of the feet [9-12]. IFMs are also categorised as active subsystems in the foot core system and play an important role in static posture and dynamic activities [1, 13]. During the early stance phase of rearfoot strike running, IFMs are passively stretched as the rearfoot initially touches the ground, and the arch of the foot is slowly compressed to absorb impact energy, which is stored in the relevant plantar elastic structure [6, 12, 14]. In the terminal stance phase, the compressed arch begins to rebound, releasing previously stored elastic energy and providing improved propulsion to runners in the push-off phase [12]. This spring-like mechanism of foot muscles provides 8%–17% of the mechanical energy to the body during every step [2, 15, 16].

IFMs can be trained by using several methods, such as short foot exercise (SFE), toe-posture exercises, towel curl exercises and metatarsophalangeal joint (MPJ) muscle training [17-21]. Amongst these methods, SFE is the most studied because it utilises the IFMs to draw the metatarsal heads back towards the heel whilst minimising distal interphalangeal flexion [18, 22, 23]. Through IFM training, weakened or inhibited IFMs are activated and foot–ankle neuromuscular control is improved [24], which may help prevent running-related injuries, such as plantar fasciitis [25], foot pronation [26], hallux valgus [27] and chronic ankle instability [28].

While a number of isolated studies have shown benefits of IFM, the applicability of these findings is still limited, to date, no previous study has systematically studied these effects nor has a meta-analysis been applied to get an overall estimate of the effect of IFM training. Therefore, the current study aims to identify and determine the effect of IFM training on foot function and dynamic postural balance.

2 Methods

2.1 Search strategy

This systematic review was conducted in accordance with the PRISMA guidelines [29] and registered in PROSPERO (CRD42021232984). PubMed, CINAHL, SPORTDiscus and Web of Science bibliographic databases were searched by 2 independent authors to identify potentially relevant articles from January 2011 to February 2021. The following search terms were applied in the database search: (‘foot muscle’ OR ‘intrinsic foot muscle’ OR ‘plantar muscle’ OR ‘intrinsic flexor foot’ OR ‘toe muscle’ OR ‘hallux muscle’) AND (‘training’ OR ‘exercise’ OR ‘strength’ OR ‘strengthening’) AND (‘foot function’ OR ‘foot morphology’ OR ‘foot structure’ OR ‘foot posture’) AND (‘dynamic postural balance’ OR ‘dynamic balance’ OR ‘posture stability’ OR ‘posture control’ OR ‘postural’ OR ‘balance’). The Scottish Intercollegiate Guidelines Network criteria were used to describe the include studies [30]. An example of the search strategy for the PubMed database is attached in the supporting information. The search strategy was limited to publications in English.

2.2 Study selection

After duplicate articles were removed, the search results were screened independently by 2 authors based on titles, abstracts and full texts on the basis of the following criteria:1) research specific to IFM training as an intervention (treatments, such as SFE, that emphasise the neuromuscular recruitment of the plantar intrinsic foot muscles), 2) having at least 1 desired foot biomechanical parameters (such as navicular drop, foot posture index) and 3) randomised controlled trials (RCTs) or pre-/postintervention studies assessing the effectiveness of an intervention.

2.3 Data extraction and analyses

The following data were extracted: (i) author (year), (ii) study design, (iii) population characteristics (e.g. sample size), (iv) interventions (e.g. exercise prescription [sets/repetitions]), (v) outcome characteristics (e.g. foot posture index to describe the parameters of foot function) and (vi) main findings. When the information was unclear, the corresponding author of the study was contacted via email for clarification.

2.4 Quality assessment

The methodological quality of the included intervention studies were evaluated by two researchers independently using the Physiotherapy Evidence Database (PEDro) scale [31], which is found to be a reliable and valid measure to evaluate the quality of intervention trials [32], with higher scores indicating lower risk of bias. Each item scoring “yes” contributes 1 point to the total score, except for the first item, which relates to external validity. The total PEDro score thus ranges from 0 to 10 points. Studies with a total score of at least 6 points are considered to be of adequate quality [32, 33]. Notably, if the trials/studies were listed in the PEDro database (https://www.pedro.org.au/), those scores were used in this review.

2.5 Quantitative data synthesis and analysis

The training effects were calculated and illustrated based on difference between the pre-intervention and post- intervention parameters using forest plots with Review Manager version 5.3. Random effects models were used to calculate standardised mean differences and 95% confidence intervals (CIs) for the control and experimental groups. The I2 statistic was used to verify heterogeneity (χ2) between the included studies. The risk of publication bias was also assessed by using funnel plots.

3 Results

The electronic database search yielded 249 articles. After duplicates were removed (29 excluded), a total of 220 eligible articles were included. Then, 203 articles were excluded after reviewing the titles and abstracts, reducing the number of articles to 17. After full text screening, 1 article was excluded [34]. Finally, the remaining 16 articles met all the inclusion criteria and were included in this systematic review (Fig 1).

Fig 1

Flow diagram of literature search.

3.1 Methodological quality

The results of the risk of bias assessment using the PEDro scale can be found in Table 1. The total scores for the methodological quality ranged from 1 to 8 points. Eight studies [11, 16, 18, 22–23, 26, 35, 36] were moderate quality (PEDro score ≥5) and the others [10, 17, 19, 37–41] were poor quality (PEDro score < 5). The following items were most commonly reported in the articles: random allocation (69%), concealed allocation groups (19%), similar at baseline (63%), blinding of the therapist/subject reported in none of the articles, and blinding of the assessor in ten articles (63%), follow-up > 85% (38%), intention-to-treat analysis (25%), between-group comparison (88%), Point measures and measures of variability (100%).

Table 1

Results of the risk of bias assessment using the PEDro scale.

Study	Eligibility criteria	Random allocation	Concealed allocation	Groups similar at baseline	Subject blinding	Therapist blinding	Assessor blinding	Follow-up > 85%	Intention-to-treat analysis	Between-group comparison	Point measures and measures of variability	Total score
Day and Hahn, 2019 [37]	Yes	Yes	No	Yes	No	No	No	No	No	Yes	Yes	4
Fraser and Hertel, 2019 [16]	Yes	Yes	No	Yes	No	No	Yes	No	No	Yes	Yes	5
Goldmann et al., 2013 [19]	Yes	Yes	No	No	No	No	No	No	No	Yes	Yes	3
Hashimoto and Sakuraba, 2014 [38]	Yes	No	No	No	No	No	No	No	No	No	Yes	1
Kim and Kim, 2016 [39]	Yes	Yes	No	Yes	No	No	No	No	No	Yes	Yes	4
Mulligan and Cook, 2013 [10]	Yes	No	No	No	No	No	Yes	No	No	No	Yes	2
Okamura et al., 2020 [11]	Yes	Yes	No	Yes	No	No	Yes	Yes	Yes	Yes	Yes	7
Pabon-Carrasco et al., 2020 [26]	No	Yes	No	Yes	No	No	Yes	Yes	Yes	Yes	Yes	7
Sánchez-Rodríguez et al., 2020 [40]	No	Yes	No	No	No	No	No	No	No	Yes	Yes	3
Sulowska et al., 2016 [17]	Yes	No	No	No	No	No	Yes	Yes	No	Yes	Yes	4
Sulowska-Daszyk et al., 2020 [18]	No	Yes	No	Yes	No	No	Yes	No	No	Yes	Yes	5
Sulowska et al., 2019 [35]	No	No	No	Yes	No	No	No	Yes	Yes	Yes	Yes	5
Taddei et al., 2020 [23]	Yes	Yes	Yes	Yes	No	No	Yes	Yes	Yes	Yes	Yes	8
Taddei et al., 2020 [22]	Yes	Yes	Yes	Yes	No	No	Yes	No	No	Yes	Yes	6
Taddei et al., 2018[36]	Yes	Yes	Yes	No	No	No	Yes	Yes	No	Yes	Yes	6
Unver et al., 2019 [41]	Yes	No	No	Yes	No	No	Yes	No	No	Yes	Yes	4

Note: Scoring of eligibility criteria specified does not contribute to total score

3.2 Study characteristics

The studies included 14 RCTs with sample sizes ranging from 14 to 118 [11, 16–19, 22, 23, 26, 37, 39–41] and 2 pre-/post-test designs with sample sizes of n = 12 and n = 21 [10, 36]. The intervention time of the 16 studies varied: 4 weeks [10, 16, 26], 5 weeks [39], 6 weeks [17, 18, 35, 41], 7 weeks [19], 8 weeks [11, 22, 36, 38], 9 weeks [40], 10 weeks [37] and 16 weeks [23]. The details of the study and participant characteristics are presented in Table 2.

Table 2

Characteristics of the included studies.

Author (year)Study design	Samplesize	Intervention group (IG)	Control group (CG)	Outcome measures	Results
Day and Hahn (2019) [37] RCT	n = 20competitive distance runners	n = 11IFM strengthening exercises4 weeks, 3 times/day, 3 days/weekisometric, concentric and eccentric exercises	n = 9Not prescribed any additional strengthening protocol	• Toe-flexor strength,• MPJ and ankle mechanics• Running economy	• Toe-flexor strength increased• MPJ and ankle mechanics did not change• Running economy did not change
Fraser and Hertel (2019) [16] RCT	n = 23healthy recreationally active young adults	n = 11SFE, toe-spread-out, hallux-extension and lesser-toe-extension4 weeks, daily, 3 times/day104 repetitions/day in 12 sets Progression: from sitting to double-limb stance to single-limb stance	n = 12Received no intervention	• Clinician-assessed motor performance• Participant-perceived difficulty• Ultrasound imaging motor activation measures of the ABH, FDB, quadratus plantae and FHB were assessed during toe-spread-out, hallux-extension and lesser-toe-extension exercises	• Improved motor performance and decreased perceived difficulty when performing the exercises• No changes in the ultrasound imaging measures of IFM activation in the IG compared with those in the CG
Goldmann et al. (2013) [19] RCT	n = 27healthy males	n = 15Heavy resistance toe flexor muscle training with 90% of the maximal voluntary isometric contraction (MVIC)7 weeks, 4 times/week4 sets of 5 repetitions (3 s contraction, 3 s relaxation)	n = 12No training programme and continued their daily activities	• Maximal MPJ and ankle plantar flexion moments during MVICs were measured• Motion analyses were performed during barefoot walking, running and vertical and horizontal jumping	• MPJ plantar flexion moments in the dynamometer, external MPJ dorsiflexion moments and jump distance in horizontal jumping increased significantly
Hashimoto and Sakuraba (2014) [38] Pre/post	n = 12healthy males	Toe flexion interphalangeal/MPJ 3 kg load8 weeks, 3 days/week200 repetitions/day	-	• Digital grip dynamometer• Foot arch measurements (longitudinal and horizontal planes) during static standing using the Berkemann footprint• Dynamic test items: single-leg long jump, vertical jump and 50 m dash	• Significant changes observed for intrinsic foot flexor strength scores, foot arches, vertical jumping, single-leg long jumping and 50 m dash time
Kim and Kim (2016) [39] RCT	n = 14university students with flexible flatfoot	n = 7SFE5 weeks, 3 times/week, 30 min each time	n = 7Arch support insoles	• ND tests• Y-balance tests	• SFE group showed significant decreases in ND tests• SFE group and arch support insole group showed significant increases in Y-balance tests
Mulligan and Cook (2013) [10] Pre/post	n = 21asymptomatic subjects	SFE: 5 s hold up to 3 min per day for approximately4 weeks, daily, 30 repetitions/dayProgression: from sitting to double-limb stance to single-limb stance until reaching 3 min	-	• ND difference between the seated and standing navicular positions• AHI calculated by dividing the dorsum foot height by the truncated length of the foot in seated and standing positions to obtain a ratio• Intrinsic foot musculature test• SEBT	• Subject ND decreased by a mean of 1.8 mm at 4 weeks and by 2.2 mm at 8 weeks• AHI increased from 28% to 29%• Grade of IFM performance during a static unilateral balancing activity improved from fair to good• Significant improvement during a functional balance and reach task in all directions with the exception of an anterior reach
Okamura et al. (2020) [11] RCT	n = 20 patients withpes planus	n = 108 weeks SFE3 times/week3 sets/time10 repetitions/setseach repetition was held for 5 s with a 45-s rest period between setsProgression: from sitting to double-limb stance to single-limb stance	n = 10Received no intervention	• Foot kinematics during gait, including dynamic ND—the difference between navicular height at heel strike and the minimum value• Time at which navicular height reached its minimum value• Three-dimensional motion analysis to assess static foot alignment via the FPI and ND test• Thickness of the intrinsic and extrinsic foot muscles was measured by using ultrasound	• FPI scores with regard to calcaneal inversion/eversion improved significantly• Time required for navicular height to reach the minimum value decreased significantly
Pabon-Carrasco et al. (2020) [26] RCT	n = 85asymptomatic participants	n = 42SFE reinforcement: maintain the position of maximum shortening for 30 s from sitting to standing position to standing unipodal4 weeks, daily, 50 repetitions/day	n = 43Nonbiomechanical function exercise	• Foot posture was evaluated twice via the ND test• FPI	• Comparison of foot posture before and after training found no statistically significant differences between the experimental group and CG• FPI was modified in both groups with respect to its initial state and the ND value decreased
Sánchez-Rodríguez et al. (2020) [40] RCT	n = 36healthy adults	n = 189-week intrinsic and extrinsic foot and core muscle strength program2 sessions/week40 min/session	n = 18Received no intervention	• FPI scores	• IG showed significantly reduced FPI by 1.66 points, whereas the score of the CG was the same as that preintervention
Sulowska et al. (2016) [17] RCT	n = 25long-distance runners	n = 12SFE and balanced loading of the 3 support points of the foot6 weeks, daily, 2 times/day, 15 min/timeProgression: from sitting to standing to half-squat	n = 13Vele’s forward lean and reverse tandem gait exercise	• FPI scores• FMS tests	• Significant improvement in the FPI-6 (inversion/eversion of the calcaneus after SFE intervention)
Sulowska-Daszyk et al. (2020) [18] RCT	n = 80long-distance runners	n = 48SFE, balanced loading of the 3 support points of the foot6 weeks, daily, 30 min, repeated 30 timesProgression: increasing the load and level of difficulty every 2 weeks in seated, standing and half-squat positions	n = 32Received no intervention	• Quality of movement patterns with the FMS was evaluated before and after intervention• Muscle flexibility was evaluated before and after intervention	• Significantly increased FMS values in individual tasks and in the total score after 6 weeks• Significant improvement in muscle flexibility at baseline and after 6 weeks (e.g. external rotation muscles)
Sulowska et al. (2019) [35] RCT	n = 47long-distance runners	n = 27 with neutral foot6 weeks, daily basis for 30 minVele’s forward lean and reverse tandem gait exercise, SFE and stability exercise	n = 20 with slight and increased pronation	• Knee flexor and extensor torque, work• Power on isokinetic dynamometer• Running-based anaerobic sprint test	• Increased values of the peak torque of knee flexors• Increased values of maximum power
Taddei et al. (2020) [23] RCT	n = 118healthy runners	n = 5716 weeks of foot core training (8-week training course, followed by 8 weeks of remotely supervised training)4 times/week (1 time by a physical therapist and 3 times given through online videos)Both groups were instructed to perform their respective exercises 3 times/week up to the end of the 12-month follow-up	n = 615 min placebo static stretching protocol3 times/week on the basis of online descriptions	• Assessments consisted of 3 separate biomechanical evaluations of foot strength• FPI• Weekly report on each participant’s running distance, pace and injury incidence over 12 months	• CG participants were 2.42 times more likely to experience an RRI within the 12-month study period• Time to injury was significantly correlated with FPI and foot strength gain scores• Foot exercise program showed evidence of effective RRI risk reduction in recreational runners at 4–8 months of training
Taddei et al. (2020) [22] RCT	n = 28healthy recreational long-distance runners	n = 148-week foot–ankle exercise during weight-bearing activities (with a physiotherapist once a week and at least 3 times at home over the entire course of the study)	n = 148 weeks of 5 min warm-up and full body muscle stretching protocol	• Hallux and toe strength• Foot function• Cross-sectional area and volume of the ABH, ADM, FDB and FHB• MLA range of motion and stiffness• Vertical and anteroposterior propulsive impulses during running	• Volume of all investigated muscles and muscles for vertical propulsive impulse during running increased in the IG relative to those in the CG Correlations were found between vertical propulsive impulse and volume of ABH, ADM and FDB
Taddei et al. (2018) [36] RCT	n = 30healthy recreational long-distance runners	n = 158-week foot–ankle muscle strength (trained in weekly sessions by a physiotherapist and instructed to perform the same exercises at home at least twice a week)	n = 155 min placebo warm-up and muscle stretching protocol	• Hallux and toe muscle strength using a pressure platform• Foot muscle cross-sectional area using magnetic resonance imaging• Foot kinematics during running using 3D gait analysis	• Cross-sectional area of the ABH and FDB increased significantly at 8 weeks in the IG
Unver et al. (2019) [41] RCT	n = 41 patients withpes planus	n = 216-week SFE training daily	n = 20Received no intervention	• ND• FPI• Foot pain• Disability• Plantar pressures	• ND, FPI, pain and disability scores significantly decreased• Maximum plantar force of the midfoot significantly increased

Note: SFE, short foot exercise; ABH, abductor hallucis; FDB, flexor digitorum brevis; ADM, abductor digiti minimi; FHB, flexor hallucis brevis; MPJ, Metatarsophalangeal joint; MLA, Medial longitudinal arch; FPI, foot posture index; ND, navicular drop; SEBT, star excursion balance test; FMS, functional movement screen; AHI, arch height index

RCT, randomised controlled trial; IG, intervention group; CG, control group.

3.2.1 Sample characteristics

The 16 studies included a total of 627 participants. Seven studies included elite long-distance runners (n = 348) [17, 18, 22, 23, 37–39]. Three studies explored the IFMs in patients with pes planus (n = 75) [11, 39, 41], and 6 other studies included only healthy or asymptomatic subjects (n = 207) [10, 16, 19, 26, 36, 40]. The sample sizes of the included studies ranged from 12 to 118 (mean = 39). The validity and statistical conclusions of the study by Hashimoto and Sakuraba (n = 12) were the lowest [38]. Overall, the proportion of males was slightly higher than that of females (54.10%). The age of the runners ranged on average from 20 years old to 45 years old (mean = 27.78), and the body weight of the runners ranged from 50 kg to 76 kg (mean = 67.17). For the foot morphology characteristics, the foot posture indexes in the existing studies ranged from 1 to 10 (mean = 6). Table 3 shows the details of the sample population characteristics.

Table 3

Sample sizes and participant characteristics for each included study.

Included studies	N		Sex, M/F		Age, y		Height, cm		Body mass, kg		BMI, kg/m2		FPI
	INT	CON	INT	CON	INT	CON	INT	CON	INT	CON	INT	CON	INT	CON
Day and Hahn [37]	11	9	NR	NR	24(6)	30(12)	173(1)	172(1.1)	60(8)	62(8)	NR	NR	NR	NR
Fraser and Hertel [16]	11	12	6/5	6/6	23.6(6.6)	19.6(1.2)	170.9(11.5)	166.5(13.8)	70.5(12.0)	64.9 (9.5)	24.0(2.0)	23.5 (3.1)	6.7 (4.2)	6.0 (3.9)
Goldmann et al. [19]	15	12	15/0	12/0	24.0(4.0)	26.0(2.0)	185.0(7.0)	181.0(6.0)	77.0(9.0)	77.0(5.0)	NR	NR	NR	NR
Hashimoto and Sakuraba [38]	12	-	12/0	-	29(5)	-	172.5(7.3)	-	64.9(12.8)	-	NR	-	NR	-
Kim and Kim [39]	7	7	6/1	4/3	24.0(1.9)	24.1(1.5)	172.2(6.9)	167.0(6.7)	68.2(12.9)	63.3(17.6)	NR	NR	NR	NR
Mulligan and Cook [10]	21	-	3/18	-	26.1(3.7)	-	168.4(7.1)	-	69.3(13.6)	-	NR	-	NR	-
Okamura et al. [11]	10	10	1/9	2/8	19.7(0.9)	20.2(1.5)	158.6(6.1)	159.5(8.8)	49.7(4.5)	53.7(7.7)	19.8(1.4)	21.1(2.1)	9.7(1.9)	9.0(2.1)
Pabon-Carrasco et al. [26]	42	43	24/18	18/25	19.5(0.4)	20.9(1.1)	NR	NR	NR	NR	24.1(4.2)	21.65(3.4)	6.8(0.6)	6.35(0.3)
Sánchez-Rodríguez et al. [40]	18	18	7/11	8/10	23.6(5.9)	21.6(1.9)	NR	NR	NR	NR	23.2(3.2)	23.9(2.6)	8.1(1.7)	8.0(1.2)
Sulowska et al. [17]	12	13	NR	NR	NR	NR	NR	NR	NR	NR	NR	NR	NR	NR
Sulowska-Daszyk et al. [18]	48	32	31/17	26/6	32.5(6.8)	33.4(7.8)	175.0(8.7)	177.7(7.9)	69.8(9.7)	71.0(10.6)	NR	NR	NR	NR
Sulowska et al. [35]	27	20	NR	NR	NR	NR	NR	NR	NR	NR	NR	NR	NR	NR
Taddei et al. [23]	57	61	28/29	33/28	40.5(7.9)	41.3(6.8)	167.4(8.2)	171.0(9.1)	68.2(12.3)	72.1(13.2)	24.2(2.9)	24.5(3.2)	1/0	2/2
Taddei et al. [22]	14	14	5/9	9/5	41.9(7.4)	41.6(6.0)	166.4(7.8)	169.4(9.2)	68.3(12.7)	75.1(13.9)	NR	NR	2.5/1.5	2.5/2
Taddei et al. [36]	16	15	11/5	7/8	39.4(8.5)	44.8(8.7)	169.6 (9.4)	168.7 (8.8)	70.7(12.4)	67.8(12.7)	NR	NR	1/0	2/2
Unver et al. [41]	21	20	5/16	11/9	21 (1)	21.4(1.7)	NR	NR	NR	NR	22.9(3.3)	23.1(1.9)	9.0(1.5)	8.4(2.0)

Note: NR, not reported; BMI, body mass index; FPI, foot posture index; INT, intervention group; CON, control group

3.2.2 IFM foot interventions

All included studies have various differences in IFM foot interventions. The interventions varied in terms of training methods, exercise prescriptions and timeframes. In broad terms of the training approaches, the included interventions can be categorised as 1. SFE [10, 11, 26, 39, 41], 2. series of foot–ankle muscle training exercises [22, 23, 36, 40], 3. SFE and stability training of the foot [17, 18, 35], 4. SFE and toe/hallux-extension exercises [16, 37] and 5. interphalangeal joint and MPJ loading exercises [19, 38].

3.3 Outcome measures

For the characteristics of IFMs before and after training, the included studies measured the muscle activation ratio (contracted measurement/resting measurement) [16] through ultrasonographic imaging and the thickness [11], cross-sectional areas [22, 36] and volume [22] of the foot muscles by using magnetic resonance imaging (Table 2). Additionally, IFM training was measured directly through the hallux or toe muscle strength test [19, 22, 23, 36, 37, 41] and intrinsic foot musculature test [10] by using a custom-made dynamometer.

Medial longitudinal arch morphology was evaluated on the basis of the navicular drop [10, 11, 26, 39, 41] and arch height index [10]. These parameters were verified to provide accurate changes in the medial longitudinal arch. Six studies (n = 326) used the multidimensional and comprehensive evaluation of the foot posture index for the pronation/supination of the feet [11, 17, 23, 26, 40, 41]. This index has been verified to have clinical applications in assessing the risk of injury in athletes (Table 2).

Dynamic postural balance was evaluated by using some function tests, such as the functional movement screen test [17, 18], the star excursion balance test [10, 39] and clinician-assessed motor performance [16] (Table 2). Additionally, Fraser and Hertel [16] explored the participants’ perceived difficulty during the toe-spread-out, hallux-extension and lesser-toe-extension tests.

3.4 Data analysis

The results of cross-sectional area indicated no significant effect on the muscle characteristics of the Abductor hallucis (ABH) (P = 0.07), Abductor digiti minimi (ADM) (P = 0.08), Flexor digitorum brevis (FDB) (P = 0.22) and Flexor hallucis brevis (FHB) (P = 0.20).

IFM training was observed to have a significant effect on the medial longitudinal arch. The navicular drop (P = 0.02) and foot posture index (P = 0.0003) after IFM intervention had significantly decreased relative to those after the control treatment. The mean difference was −1.97 (95% CI: -3.57–-0.36) for the navicular drop (Fig 2) and -0.69 (95% CI: -1.06–-0.32) for the foot posture index (Fig 3). No significant heterogeneity was observed amongst studies (navicular drop: I2 = 40%, P = 0.17; FPI: I2 = 35%, P = 0.18). The bias funnel plots of the navicular drop (Fig 4) and foot posture index (Fig 5) did not suggest evidence of publication bias in the studies included in this meta-analysis.

Fig 2

Forest plot illustrating navicular drop of meta-analysis comparing IFMs training with control group.

Fig 3

Forest plot illustrating foot posture index of meta-analysis comparing IFMs training with control group.

Fig 4

Funnel plots showing publication bias among studies used to compare IFMs training and control groups.

Fig 5

Funnel plots showing publication bias among studies used to compare IFMs training and control groups.

A significant difference was found for dynamic postural balance after intervention. Although various function tests were included and were difficult to synthesise, the included studies all demonstrated that IFM training can exert positive effects on dynamic postural balance.

4 Discussion

This systematic review performed a meta-analysis to summarise the current studies that explored the effect of IFM training on foot biomechanical outcomes. Although potential differences in IFM intervention type, time or frequency may contribute to the potential heterogeneity of the included studies, the current studies verified that IFM training would bring positive biomechanical effects and ameliorate dynamic postural balance.

Four included studies (n = 102) explored the effects of IFM training on muscle morphology [11, 16, 22, 36]. However, no significant difference was found in terms of the parameters of IFM thickness, cross-sectional area and volume. Possible explanations for these discrepancies maybe explained by previous studies indicating that small volumes of IFMs are covered by plantar fascia, which would bring barrier to detect the slight changes in foot muscles [11]. Additionally, Taddei et al. [22, 36] also established proposed that ABH, FDB and FHB have various origins and insertions, different lever arm lengths and may be trained from different degrees during the intervention. Thus, the strength change of single IFMs may be different and hard to detect. In light of the difficulty in measuring the small cross-sectional area of IFMs, future studies should utilise advanced technology, such as magnetic resonance imaging, to measure the IFM fat infiltration and cross-sectional area after training.

Another direct parameter used to describe muscle characteristics is IFM strength. Considering that no gold standard for measuring IFM strength exists [42], the studies included in this review measured IFM strength by applying various approaches, such as pressure platforms [22, 23, 36, 41] or the intrinsic foot musculature test [10]. Although Day and Hahn [37] verified the positive effect of IFM training on muscle strength, no significant difference was found after pooling the data of the included studies. One possible reasons for this conflicting result might be related to the compensation of extrinsic muscles, such as the tibial posterior muscle [43, 44]. Although studies have attempted to avoid possible interference factors by placing the lower limb in a special position, external muscles are still involved during the test. Unlike previous results reported that enhanced IFM strength can provide additional propulsive impulses, making the foot similar to a stiffened spring during late stance [44, 45], the current study also did not observe any differences. Hence, the actual effect of strength training on IFMs needs to be studied further.

The navicular drop and arch height index are 2 common parameters that describe medial longitudinal arch morphology and dynamic function. IFM exercise is believed to activate weakened IFMs and increase IFM recruitment by intensifying and optimising the tension of the medial longitudinal arch, thereby preventing the excessive lowering of the medial longitudinal arch [10, 46] and related running injuries. This systematic review included 4 studies (n = 80) that utilised the navicular drop [10, 11, 26, 41] and 1 study (n = 21) that utilised the arch height index [10] to explore the changes in the medial longitudinal arch. The included studies all demonstrated that the morphology and function of the medial longitudinal arch significantly improved after several weeks of intervention. Even though IFM morphology and single muscle strength showed no significant difference, the overall effect of the medial longitudinal arch was improved. This finding indicated that IFM training can be recommended as an effective measure to improve medial longitudinal arch function and might provide further benefits to people with pes planus. Moreover, the foot posture index is a validated measure for quantifying foot posture. Five studies (n = 103) demonstrated that foot posture index can rectify abnormal lower extremity alignment and stress on the foot and related structures [11, 17, 26, 40, 41].

Amongst the included studies, several measured the dynamic postural balance after IFM training by utilising the functional movement screen [17, 18], star excursion balance test [10], clinician-assessed motor performance and 1-legged long jumping [18]. Although various methods can be applied to assess dynamic postural balance, the results of the included studies established that IFM training has significant positive effects compared with other interventions. Additionally, of the 2 included studies that subjectively assessed IFM training difficulty and foot pain in different situations, the difficulty in motor function perceived by the participants seemed uncomplicated, and the pain in the pes planus was alleviated.

The main limitation of this systematic review is that the included studies varied in terms of their interventions’ approaches, time and frequency and their participants’ characteristics, this variation might compromise this study. In addition, the included studies utilised different methods for assessing IFM strength and dynamic postural balance. Potential heterogeneity and slight publication bias in the analysis may exist. Therefore, caution is warranted when interpreting the findings of this study.

5 Conclusion

Although the interventions of the included studies seemed inconsistent, this systematic review demonstrated that IFM training can exert positive biomechanical effects on the medial longitudinal arch, improve the postural balance of the lower limbs and act as an important training method. Future studies should optimise standardised training methods in accordance with the demands of different sports.

Searching terms.

(DOCX)

Click here for additional data file.

PRISMA 2020 checklist.

(DOCX)

Click here for additional data file.

International prospective register of systematic reviews (CRD42021232984).

(PDF)

Click here for additional data file.

21 Sep 2021

A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.

A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.

An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

If applicable, we recommend that you deposit your laboratory protocols in protocols.io to enhance the reproducibility of your results. Protocols.io assigns your protocol its own identifier (DOI) so that it can be cited independently in the future. For instructions see: https://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols. Additionally, PLOS ONE offers an option for publishing peer-reviewed Lab Protocol articles, which describe protocols hosted on protocols.io. Read more information on sharing protocols at https://plos.org/protocols?utm_medium=editorial-email&utm_source=authorletters&utm_campaign=protocols.

We look forward to receiving your revised manuscript.

Kind regards,

Peter Andreas Federolf

Academic Editor

PLOS ONE

Journal requirements:

When submitting your revision, we need you to address these additional requirements.

1. Please ensure that your manuscript meets PLOS ONE's style requirements, including those for file naming. The PLOS ONE style templates can be found at

https://journals.plos.org/plosone/s/file?id=wjVg/PLOSOne_formatting_sample_main_body.pdf and

https://journals.plos.org/plosone/s/file?id=ba62/PLOSOne_formatting_sample_title_authors_affiliations.pdf

2. We note that the grant information you provided in the ‘Funding Information’ and ‘Financial Disclosure’ sections do not match.

When you resubmit, please ensure that you provide the correct grant numbers for the awards you received for your study in the ‘Funding Information’ section.

3. Thank you for stating the following financial disclosure:

“This work was supported by the National Natural Science Foundation of China  [11572202]”

Please state what role the funders took in the study.  If the funders had no role, please state: "The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript."

If this statement is not correct you must amend it as needed.

Please include this amended Role of Funder statement in your cover letter; we will change the online submission form on your behalf.

4. Thank you for stating the following in the Funding Section of your manuscript:

“This work was supported by the National Natural Science Foundation of China [11572202”

We note that you have provided information within the Funding Section. Please note that funding information should not appear in other areas of your manuscript. We will only publish funding information present in the Funding Statement section of the online submission form.

Please remove any funding-related text from the manuscript and let us know how you would like to update your Funding Statement. Currently, your Funding Statement reads as follows:

“This work was supported by the National Natural Science Foundation of China [11572202]””

Please include your amended statements within your cover letter; we will change the online submission form on your behalf.

5. In your Data Availability statement, you have not specified where the minimal data set underlying the results described in your manuscript can be found. PLOS defines a study's minimal data set as the underlying data used to reach the conclusions drawn in the manuscript and any additional data required to replicate the reported study findings in their entirety. All PLOS journals require that the minimal data set be made fully available. For more information about our data policy, please see http://journals.plos.org/plosone/s/data-availability.

Upon re-submitting your revised manuscript, please upload your study’s minimal underlying data set as either Supporting Information files or to a stable, public repository and include the relevant URLs, DOIs, or accession numbers within your revised cover letter. For a list of acceptable repositories, please see http://journals.plos.org/plosone/s/data-availability#loc-recommended-repositories. Any potentially identifying patient information must be fully anonymized.

Important: If there are ethical or legal restrictions to sharing your data publicly, please explain these restrictions in detail. Please see our guidelines for more information on what we consider unacceptable restrictions to publicly sharing data: http://journals.plos.org/plosone/s/data-availability#loc-unacceptable-data-access-restrictions. Note that it is not acceptable for the authors to be the sole named individuals responsible for ensuring data access.

We will update your Data Availability statement to reflect the information you provide in your cover letter

6. PLOS requires an ORCID iD for the corresponding author in Editorial Manager on papers submitted after December 6th, 2016. Please ensure that you have an ORCID iD and that it is validated in Editorial Manager. To do this, go to ‘Update my Information’ (in the upper left-hand corner of the main menu), and click on the Fetch/Validate link next to the ORCID field. This will take you to the ORCID site and allow you to create a new iD or authenticate a pre-existing iD in Editorial Manager. Please see the following video for instructions on linking an ORCID iD to your Editorial Manager account: https://www.youtube.com/watch?v=_xcclfuvtxQ

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Partly

**********

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: I Don't Know

**********

3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

**********

4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: No

**********

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: The authors present a systematic review study on the effectiveness of interventions that aim to improve the functioning of the Intrinsic Foot Muscles. This work has potential relevance in the prevention of foot and ankle injuries and provides a better understanding of the functional anatomy of the foot and the effects of training thereon. The manuscript overall is of sound quality, however it needs some strict improvements in the methods section in order to become eligible for publication.

MAJOR ISSUES

Aims. The aim of the current study is somewhat unclear: “Nonetheless, current studies on IFM training are cross-sectional, and evidence to support this claim is lacking. Therefore, the aim of this systematic review is to identify and determine the effect of IFM training on foot function and dynamic postural balance” (line 48-50).

This needs to be further clarified. If all past studies are cross sectional, what will be the added value of this review? In fact, later you find 13 RCTs that have been done. Why do these not feature in the introduction?

This issue is further confused at the start of the discussion where the aim seems to have been to summarize the current studies. Furthermore, judging from the methods and the results, the aim seems to have been to assess the effect of IFM training on foot biomechanical outcomes using a meta-analysis. Please clarify.

Methods. The major issues of the current manuscript lie in the methods section. Although the authors provide an adequate description of the methods to see that their approach is well thought out, there is insufficient details, and sometimes inconsistencies, that limit the reproducibility of the approach. Relating to this, the following issues will need to be addressed:

1. In the description of the search (line 57-60), it would be good to add some specification of the BOOLEAN operators used in the search.

2. In the supplementary file specifying the search, it should be specified what type of search was performed (e.g. title/abstract/keywords, topic, mesh terms) and it should be specified how the search was adapted to the different search engines.

3. There is some confusion in the use of quotation marks. In the main text, single quotation marks are used, in the supplementary file these are double marks. Please specify what was used in the searches, as these have a different meaning in the search engines.

4. The authors note the inclusion criterium ‘Research specific to IFM training as an intervention’ (line 66). As the authors mention, these muscles have a function many whole-body activities (e.g. walking), so arguably can be trained with many whole body activities as well (e.g. walking, running, whole body vibration training). How was IFM training defined for inclusion in the review?

5. No limitations were introduced to the search dates and yet only articles from after 2013 were included. This might just result from the novelty within the research field, but it could also indicate a fatal flawy in the search strategy. That is, it might be that the field evolved and that now different terms are used compared to 20 years ago. On example might be the following study: DOI: 10.1123/jsr.21.4.327. Please comment on this issue.

6. The authors state ‘When possible, the results of the included studies were pooled for meta-analysis using RevMan 5.3.’ (line 84). How was it determined whether this was possible? It would be helpful to inform the reader on this process. How come only 6 of the 15 papers ended up in the meta-analysis?

7. The prospero registration introduces the usage of the PEDro scale, in the methods section a ‘Checklist for Measuring Quality’ is introduced and in the results the Modified Downs and Black Quality Index is reported. Please resolve these inconsistencies and please specify why the prospero protocol was not adhered to.

MINOR ISSUES

1. The use of abbreviations drastically limits the readability of the manuscript. Following PLOS one author guidelines, the use of abbreviations should be limited, and non-standard abbreviations should not be used unless they appear at least three times in the text. While I believe that most abbreviations are used more than thrice, this is mainly through their use in table 2. I would advise to limit abbreviating terms in the main text and only use abbreviations in the table.

2. Line 22. I would suggest to use the term ‘effector’ rather than ‘organ’

3. Line 27. With the introduction of IFMs, I would advise to name which muscles are meant specifically.

4. Line 39-40. With introduction of the training methods, it would be helpful if the authors could explain in a bit more detail what these training methods are about and in particular where they are different from each other.

5. Line 65-66. “The remaining studies were then reviewed as full texts on the basis of the following criteria: …” - The present wording makes it appear like the title and abstract screening were based on different criteria. Please clarify whether this was the case.

6. Line 91-92. “After full text screening, one article as clinical pearl was excluded” – personally I am unfamiliar with the term ‘clinical pearl’ and why this should lead to exclusion. This might be just me, but in any case a bit further clarification might be helpful.

7. Line 135. Descriptive stats are only provided for runners. How about the non-runner participants?

8. Line 148-149. The information about intervention duration is duplicated after section 3.2 and does not need to be mentioned here.

9. Line 153. Please define extrinsic foot muscles.

10. Line 169. Please define CSA

11. Line 199. Please provide a more extensive/descriptive caption for figure 4.

12. Line 216. The authors mention advance technologies. Can they provide some example of such technologies?

13. Line 238. I believe this should be ‘though’ instead of ‘through’.

Reviewer #2: The goal of this manuscript was to develop a systematic review and a meta-analysis systematic review to identify the effect of intrinsic foot muscles training on foot function and dynamic postural balance.

The manuscript is relatively well grounded, it deals with a relevant topic to be investigated and deserves to be published. In its current state, the level of English throughout your manuscript need careful revision. I suggest a fluent English speaker read and correct grammatical, word choice, and syntax errors throughout the manuscript.

Although paper is straightforward but there are many suggestions/comments that I addressed throughout the manuscript.

There is a critical point in the methodology that is pooling data from studies that used different intervention approaches and time of intervention, and thus, the results are not expected to be the same. Pooling the data into a meta-analysis, it groups studies even if the results are not coming from the same approach of IFM training. Pooling data that came from different interventions, exercises and times of intervention and populations lead to huge heterogeneity that will certainly compromise a meta-analysis. It is pivotal to choose same interventions to be included in a meta-analysis and only then we could have an unbiased statistical analysis. This should be acknowledged and maybe exclude the meta-analysis from the manuscript. it does not add much as the only two variables that could be pooled did not achieve power enough to conclude anything robust.

Language issues examples (there are other weird sentences and word choices that need revision):

1. Please, consider reviewing the English language and grammar by a native reviewer. The writing compromises the understanding of parts of the manuscript.

2. Introduction: “Whilst running, the feet act as the starting body parts…” – correct to body PART.

3. Introduction: “Given that the feet are the most proximal aspect of the lower limb…”- correct to most DISTAL. The feet are distal and not proximal part of LL.

4. Introduction: “ground impact force (GRF)…” – correct to ground REACTION force.

5. Introduction: “During the early stance phase in running…” – correct to OF REARFOOT STRIKE running

6. Introduction: “… and providing better propulsion for runners in the swing phase [12].” – it provides better propulsion in the PUSH OFF phase, not swing phase!

Abstract and Introduction

1. Please, provide the PROSPERO registry number in the abstract.

2. At the end of the second paragraph of the introduction, the authors provide a closing sentence concluding all previous sentences, however it does not make sense. “Therefore, strengthening of IFMs has been suggested for the prevention and treatment of foot and ankle sports injuries [16].” The previous sentences describe all important roles of the foot and its intrinsic muscles, but they do not provide any evidence that strengthening the intrinsic foot muscles would prevent injuries. It is a fallacy and must be amended.

3. Line 39: Please consider including other papers that presented different foot muscle exercises program, as Matias et al. BMC Musculoskelet Disord. 2016 doi: 10.1186/s12891-016-1016-9. and Ferreira et al. Trials. 2020 doi: 10.1186/s13063-019-4017-9.

4. Lines 43-44 : “These factors are believed to protect runners from common running-related injuries [21, 23]…” it is an incorrect affirmation. Reference 21 does not “believe”, it proved that foot-related exercises do prevent RRI and reference 23 did not study RRI. Consider rephrasing the sentence.

5. Lines 47-48: “… and transmission efficiency of GRF.” What did the authors mean by efficiency GRF?! It does not make sense the way it is written. Please, rephrase the sentence.

6. Lines 48-49: “Nonetheless, current studies on IFM training are cross-sectional, and evidence to support this claim is lacking.” This sentence is also a fallacy. There are many papers that as RCT and not cross-sectional and those RCTs have proven the efficacy of the foot muscles training for several conditions Just to name two: Mølgaard CM, J Sci Med Sport. 2018 doi: 10.1016/j.jsams.2017.05.019. And your references 10 and 21, among other that need to be reviewed.

7. It is not clear what is the aim of this systematic review: look for RCTs, since the authors criticized that the majority of current studies are cross-sectional, or focuses on all types of studies. This need to be clear in the introduction.

8. The main question investigated in the review must be clearly posted in the introduction and it is not clear at all.

Methods

1. The type of studies that were searched and included need to be described in the search strategy.

2. What is a “desired foot biomechanics outcome” itemized in (2) in the Study selection session.

3. Please, specify better what “outcome characteristics” are in the data extraction session.

4. There is a critical point in the methodology that is pooling data from studies that used different intervention approaches and time of intervention, and thus, the results are not expected to be the same. Pooling the data into a meta-analysis, it groups studies even if the results are not coming from the same approach of IFM training. This should be acknowledged and maybe exclude the meta-analysis of the manuscript. Or, at least, include as a limitation of the meta-analysis performed.

5. Figure 1 need to clearly describe the reasons for excluding the 204 papers from the review.

Results

1. “one article as clinical pearl…” What the authors mean by that?

2. When describing the meta-analysis, the muscle volume that changed in one study did not appear and should at least be mentioned, not only the CSA, but muscle volume. The way it looks, readers will not know IFM changes foot muscles volume.

3. Please, correct Weight to body mass, because you express in kg.

4. Table 3 – include descriptions of abbreviations in the caption: FPI, COM, INT and BMI.

Discussion

1. Overall, the discussion is straightforward and focused, but the English language that needs revision.

2. Lines 208-216: it is important to emphasizes that the studies did see differences after the intervention in CSA and volume of the foot muscles. But the heterogeneous and critical meta-analysis performed did not detect any difference probably due to pooling papers with completely different interventions, exercises and times of intervention and populations.

3. The same comment above can be applied to third, fourth, fifth paragraphs of the discussion.

**********

6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

29 Oct 2021

Review Comments to the Author

Reviewer #1:

The authors present a systematic review study on the effectiveness of interventions that aim to improve the functioning of the Intrinsic Foot Muscles. This work has potential relevance in the prevention of foot and ankle injuries and provides a better understanding of the functional anatomy of the foot and the effects of training thereon. The manuscript overall is of sound quality, however it needs some strict improvements in the methods section in order to become eligible for publication.

A: Thank you for your positive comments. Your comments were highly insightful and help us improve the quality of the manuscript. We have modified the methods section.

MAJOR ISSUES

Aims. The aim of the current study is somewhat unclear: “Nonetheless, current studies on IFM training are cross-sectional, and evidence to support this claim is lacking. Therefore, the aim of this systematic review is to identify and determine the effect of IFM training on foot function and dynamic postural balance” (line 48-50).

This needs to be further clarified. If all past studies are cross sectional, what will be the added value of this review? In fact, later you find 13 RCTs that have been done. Why do these not feature in the introduction?

A: Thank you for your suggestion. This sentence needs further consideration and we have rewritten it as follows:

“On the basis of previous findings, IFMs training can ameliorate foot biomechanic variables, resulting in the minimization of an accumulation of repetitive stresses and higher absorption of ground reaction force. Although numerous studies explore the effect of IFMs training, there is still lacking of evidence-based studies to review it systematically. Therefore, this systemic review with meta-analysis aimed to identify and determine the IFMs training on foot function and dynamic postural balance.” (Page 3 Line 51-55)

This issue is further confused at the start of the discussion where the aim seems to have been to summarize the current studies. Furthermore, judging from the methods and the results, the aim seems to have been to assess the effect of IFM training on foot biomechanical outcomes using a meta-analysis. Please clarify.

A: Thank you for your suggestion. We have modified it as follows:

“This systematic review summarized the current studies that explored the effect of IFMs training on foot biomechanical outcomes using a meta-analysis.” (Page 16 Line 229-230)

Methods. The major issues of the current manuscript lie in the methods section. Although the authors provide an adequate description of the methods to see that their approach is well thought out, there is insufficient details, and sometimes inconsistencies, that limit the reproducibility of the approach. Relating to this, the following issues will need to be addressed:

1. In the description of the search (line 57-60), it would be good to add some specification of the BOOLEAN operators used in the search.

A: We have added specification of the BOOLEAN operators in the manuscript. (Page 3 Line 62-67)

2. In the supplementary file specifying the search, it should be specified what type of search was performed (e.g. title/abstract/keywords, topic, mesh terms) and it should be specified how the search was adapted to the different search engines.

A: Thank you for your suggestion. We have revised it and given an example of search strategy for PubMed database.

3. There is some confusion in the use of quotation marks. In the main text, single quotation marks are used, in the supplementary file these are double marks. Please specify what was used in the searches, as these have a different meaning in the search engines.

A: Thank you for your suggestion. We have corrected the quotation marks. (Page 3 Line 62-67)

4. The authors note the inclusion criterium ‘Research specific to IFM training as an intervention’ (line 66). As the authors mention, these muscles have a function many whole-body activities (e.g. walking), so arguably can be trained with many whole body activities as well (e.g. walking, running, whole body vibration training). How was IFM training defined for inclusion in the review?

A: Thank you for your suggestion. IFMs training was defined as “training program emphasizing the neuromuscular recruitment of the plantar intrinsic foot muscles, like short foot exercise.” We have added it in the manuscript. (Page 4 Line 71-73)

5. No limitations were introduced to the search dates and yet only articles from after 2013 were included. This might just result from the novelty within the research field, but it could also indicate a fatal flaw in the search strategy. That is, it might be that the field evolved and that now different terms are used compared to 20 years ago. On example might be the following study: DOI: 10.1123/jsr.21.4.327. Please comment on this issue.

A: Thank you for your suggestion. In order to keep the novelty within the research field, we only included studies from 1 January 2011 to 14 February 2021. We have updated the search dates. (Page 3 Line 60-61)

For DOI: 10.1123/jsr.21.4.327, the main aim of this study was comparing the short-foot exercise and towel-curl exercise, rather than explore the effect of IFMs training on foot function or dynamic postural balance, so we excluded it.

6. The authors state ‘When possible, the results of the included studies were pooled for meta-analysis using RevMan 5.3.’ (line 84). How was it determined whether this was possible? It would be helpful to inform the reader on this process. How come only 6 of the 15 papers ended up in the meta-analysis?

A: Thank you for your suggestion. Because of the foot biomechanics are various, after reviewing all included articles, only some common parameters can be pooled.

We have modified the sentence to “The pooled mean difference of training effects was calculated and illustrated by forest plots with Review Manager version 5.3. Random effects models were used to calculate standardized mean differences and 95% confidence intervals (CIs) for the control and experimental groups.” (Page 5 Line 92-94)

7. The prospero registration introduces the usage of the PEDro scale, in the methods section a ‘Checklist for Measuring Quality’ is introduced and in the results the Modified Downs and Black Quality Index is reported. Please resolve these inconsistencies and please specify why the prospero protocol was not adhered to.

A: Thank you for your suggestion. ‘Checklist for Measuring Quality’ is refers to the ‘Modified Downs and Black Quality Index’. We have corrected it. Additionally, the study was registered in PROSPERO (CRD42021232984), we added it in the supporting information. (Page 4 Line 84)

MINOR ISSUES

1.The use of abbreviations drastically limits the readability of the manuscript. Following PLOS one author guidelines, the use of abbreviations should be limited, and non-standard abbreviations should not be used unless they appear at least three times in the text. While I believe that most abbreviations are used more than thrice, this is mainly through their use in table 2. I would advise to limit abbreviating terms in the main text and only use abbreviations in the table.

A: Thank you for your suggestion. We have modified the abbreviated terms in the main text.

2. Line 22. I would suggest to use the term ‘effector’ rather than ‘organ’

A: We have revised " organ " as " effector ".

3. Line 27. With the introduction of IFMs, I would advise to name which muscles are meant specifically.

A: We have named the potential muscles specifically. (Page 2 Line 29-30)

4. Line 39-40. With introduction of the training methods, it would be helpful if the authors could explain in a bit more detail what these training methods are about and in particular where they are different from each other.

A: Thank you for your suggestion. We have added details about the training methods as follows:

“Several methods can be used to train IFMs, such as short foot exercise (SFE), toe-posture exercises, towel curl exercises and metatarsophalangeal joint (MPJ) muscle training [17-21], among which, SFE was studied most because of it characteristics utilizing the IFMs to draw the metatarsal heads back towards the heel whilst minimizing distal interphalangeal flexion” (Page 2 Line 43-46)

5. Line 65-66. “The remaining studies were then reviewed as full texts on the basis of the following criteria: …” - The present wording makes it appear like the title and abstract screening were based on different criteria. Please clarify whether this was the case.

A: Thank you for your suggestion. Title, abstract and full texts screening all based on the same criteria.

We have revised the sentence to “After removing duplicate articles, the search results were screened independently by two authors according to titles, abstracts and full texts on the basis of the following criteria.” (Page 3 Line 70-71)

6. Line 91-92. “After full text screening, one article as clinical pearl was excluded” – personally I am unfamiliar with the term ‘clinical pearl’ and why this should lead to exclusion. This might be just me, but in any case a bit further clarification might be helpful.

A: Thank you for your suggestion. The relative references have been added in the text.

REF: Vincent KR, Vincent HK. Use of Foot Doming for Increasing Dynamic Stability and Injury Prevention in Runners and Athletes. Curr Sports Med Rep, 2018;17(10):320-321.

7. Line 135. Descriptive stats are only provided for runners. How about the non-runner participants?

A: Thank you for your suggestion. After re-reviewed the included studies, we added descriptive data of participants as follows:

“The 15 studies included a total of 610 participants. Six studies included elite long-distance runners (n=328) [17, 18, 22, 23, 37, 38]. Three studies explored IFMs on pes planus patients (n=75) [11, 35, 39], and six other studies only included healthy or asymptomatic subjects (n=207) [10, 16, 19, 25, 27, 36].” (Page 8 Line 152-154)

8. Line 148-149. The information about intervention duration is duplicated after section 3.2 and does not need to be mentioned here.

A: We have removed it.

9. Line 153. Please define extrinsic foot muscles.

A: Thank you for your suggestion. The extrinsic foot muscles are around the lower leg and act to plantarflexion dorsiflexion, invert and evert the foot. We have modified it. (Page 9 Line 171)

10. Line 169. Please define CSA

A: We have revised CSA as cross-sectional area

11. Line 199. Please provide a more extensive/descriptive caption for figure 4.

A: We have provided descriptive caption for figure 4.

Figure 4. Funnel plots showing publication bias among studies used to compare IFMs training and control groups (Page 11 Line 226-227)

12. Line 216. The authors mention advance technologies. Can they provide some example of such technologies?

A: We have provided example of advance technologies as follows:

“future studies should utilise more advanced technology, like magnetic resonance imaging to measure the IFMs fat infiltration and cross-sectional area after training.” (Page 17 Line 242-243)

13. Line 238. I believe this should be ‘though’ instead of ‘through’.

A: We have revised " through’" as " though".

Reviewer #2:

The goal of this manuscript was to develop a systematic review and a meta-analysis systematic review to identify the effect of intrinsic foot muscles training on foot function and dynamic postural balance.

The manuscript is relatively well grounded, it deals with a relevant topic to be investigated and deserves to be published. In its current state, the level of English throughout your manuscript need careful revision. I suggest a fluent English speaker read and correct grammatical, word choice, and syntax errors throughout the manuscript.

Although paper is straightforward but there are many suggestions/comments that I addressed throughout the manuscript.

A: Thank you for your positive comments. Your comments were highly insightful and help us improve the quality of the manuscript. The level of English throughout our manuscript have been carefully revised. A native speaker was invited to edit the manuscript.

There is a critical point in the methodology that is pooling data from studies that used different intervention approaches and time of intervention, and thus, the results are not expected to be the same. Pooling the data into a meta-analysis, it groups studies even if the results are not coming from the same approach of IFM training. Pooling data that came from different interventions, exercises and times of intervention and populations lead to huge heterogeneity that will certainly compromise a meta-analysis. It is pivotal to choose same interventions to be included in a meta-analysis and only then we could have an unbiased statistical analysis. This should be acknowledged and maybe exclude the meta-analysis from the manuscript. it does not add much as the only two variables that could be pooled did not achieve power enough to conclude anything robust.

A: Thank you for your suggestion. Given that pooling the data that came from different methods may lead to huge heterogeneity and compromise a meta-analysis, after review the included studies again, only two variables that could be pooled in the current studies.

Language issues examples (there are other weird sentences and word choices that need revision):

1. Please, consider reviewing the English language and grammar by a native reviewer. The writing compromises the understanding of parts of the manuscript.

A: Thank you for your suggestion. The manuscript language and grammar have been carefully revised by a native reviewer.

2. Introduction: “Whilst running, the feet act as the starting body parts…” – correct to body PART.

A: We have corrected it. (Page 2 Line 24)

3. Introduction: “Given that the feet are the most proximal aspect of the lower limb…”- correct to most DISTAL. The feet are distal and not proximal part of LL.

A: Thank you for your suggestion. We have corrected it. (Page 2 Line 27)

4. Introduction: “ground impact force (GRF)…” – correct to ground REACTION force.

A: We have corrected it. (Page 2 Line 28)

5. Introduction: “During the early stance phase in running…” – correct to OF REARFOOT STRIKE running

A: We have corrected it. (Page 2 Line 36)

6. Introduction: “… and providing better propulsion for runners in the swing phase [12].” – it provides better propulsion in the PUSH OFF phase, not swing phase!

A: Thank you for your suggestion. This is a typo. We have corrected it. (Page 2 Line 40)

Abstract and Introduction

1. Please, provide the PROSPERO registry number in the abstract.

A: Thank you for your suggestion. We have added registry number in the abstract. (Page 1 Line 15)

2. At the end of the second paragraph of the introduction, the authors provide a closing sentence concluding all previous sentences, however it does not make sense. “Therefore, strengthening of IFMs has been suggested for the prevention and treatment of foot and ankle sports injuries [16].” The previous sentences describe all important roles of the foot and its intrinsic muscles, but they do not provide any evidence that strengthening the intrinsic foot muscles would prevent injuries. It is a fallacy and must be amended.

A: Thank you for your suggestion. We have amended it.

3. Line 39: Please consider including other papers that presented different foot muscle exercises program, as Matias et al. BMC Musculoskelet Disord. 2016 doi: 10.1186/s12891-016-1016-9. and Ferreira et al. Trials. 2020 doi: 10.1186/s13063-019-4017-9.

A: Thank you for your suggestion. We have added the relative references in the text.

4. Lines 43-44: “These factors are believed to protect runners from common running-related injuries [21, 23]” it is an incorrect affirmation. Reference 21 does not “believe”, it proved that foot-related exercises do prevent RRI and reference 23 did not study RRI. Consider rephrasing the sentence.

A: Thank you for your suggestion. We have rephrased this sentence as follows:

“which proved that foot-related exercises do prevent running-related injuries [23]” (Page 3 Line 48)

5. Lines 47-48: “… and transmission efficiency of GRF.” What did the authors mean by efficiency GRF?! It does not make sense the way it is written. Please, rephrase the sentence.

A: Thank you for your suggestion. We have rephrased this sentence as follows:

“resulting in the minimization of an accumulation of repetitive stresses and higher absorption of ground reaction force.” (Page 3 Line 52-53)

6. Lines 48-49: “Nonetheless, current studies on IFM training are cross-sectional, and evidence to support this claim is lacking.” This sentence is also a fallacy. There are many papers that as RCT and not cross-sectional and those RCTs have proven the efficacy of the foot muscles training for several conditions Just to name two: Mølgaard CM, J Sci Med Sport. 2018 doi: 10.1016/j.jsams.2017.05.019. And your references 10 and 21, among other that need to be reviewed.

A: Thank you for your suggestion. This sentence needs further consideration and we have rewritten it as follows:

“On the basis of previous findings, IFMs training can ameliorate foot biomechanic variables, resulting in the minimization of an accumulation of repetitive stresses and higher absorption of ground reaction force. Although numerous studies explore the effect of IFMs training, there is still lacking of evidence-based studies to review it systematically. Therefore, this systemic review with meta-analysis aimed to identify and determine the IFMs training on foot function and dynamic postural balance.” (Page 3 Line 52-55)

7. It is not clear what is the aim of this systematic review: look for RCTs, since the authors criticized that the majority of current studies are cross-sectional, or focuses on all types of studies. This need to be clear in the introduction.

A: Thank you for your suggestion. Some sentence needs further consideration in the introduction and we have rephrased this part. (Page 3 Line 52-55)

8. The main question investigated in the review must be clearly posted in the introduction and it is not clear at all.

A: Thank you for your suggestion. We have rewritten the review question clearly.

Methods

1.The type of studies that were searched and included need to be described in the search strategy.

A: Thank you for your suggestion. The type of studies has been added in the search strategy. (Page 3 Line 66)

2. What is a “desired foot biomechanics outcome” itemized in (2) in the Study selection session.

A: Thank you for your suggestion. “biomechanics outcome” means common biomechanics parameters in the studies. We have corrected it. (Page 4 Line 73)

3. Please, specify better what “outcome characteristics” are in the data extraction session.

A: Thank you for your suggestion. In the “outcome characteristics”, we have modified it. (Page 4 Line 79)

4. There is a critical point in the methodology that is pooling data from studies that used different intervention approaches and time of intervention, and thus, the results are not expected to be the same. Pooling the data into a meta-analysis, it groups studies even if the results are not coming from the same approach of IFM training. This should be acknowledged and maybe exclude the meta-analysis of the manuscript. Or, at least, include as a limitation of the meta-analysis performed.

A: Thank you for your suggestion. We admitted that different intervention approaches and time of intervention may contribute to the results different from what we expected. We have modified it in the limitation. (Page 19 Line 281-282)

5. Figure 1 need to clearly describe the reasons for excluding the 204 papers from the review.

A: We have described the reasons for excluding papers clearly. (Page 6 Line 116-119)

Results

1.“one article as clinical pearl…” What the authors mean by that?

A: Thank you for your suggestion. The relative references have been added in the text.

REF: Vincent KR, Vincent HK. Use of Foot Doming for Increasing Dynamic Stability and Injury Prevention in Runners and Athletes. Curr Sports Med Rep, 2018;17(10):320-321. (Page 5 Line 100-101)

2.When describing the meta-analysis, the muscle volume that changed in one study did not appear and should at least be mentioned, not only the CSA, but muscle volume. The way it looks, readers will not know IFM changes foot muscles volume.

A: Thank you for your suggestion. We have added muscle volume change in the manuscript. (Page 9 Line 187)

3. Please, correct Weight to body mass, because you express in kg.

A: We have corrected descriptions Weight to body mass.

4. Table 3 – include descriptions of abbreviations in the caption: FPI, COM, INT and BMI.

A: Thank you for your suggestion. We have included descriptions of abbreviations.

Discussion

1.Overall, the discussion is straightforward and focused, but the English language that needs revision.

A: Thank you for your suggestion. We have asked for native English speaker to revise our manuscript.

2.Lines 208-216: it is important to emphasizes that the studies did see differences after the intervention in CSA and volume of the foot muscles. But the heterogeneous and critical meta-analysis performed did not detect any difference probably due to pooling papers with completely different interventions, exercises and times of intervention and populations.

A: Thank you for your suggestion. Some studies may be see differences after the intervention in CSA and volume of the foot muscles. But the heterogeneous and critical meta-analysis performed actually did not detect any difference.

3. The same comment above can be applied to third, fourth, fifth paragraphs of the discussion.

A: Thank you for your suggestion. IFMs training may bring some positive effect of foot biomechanics parameters, but after the pooled data analysis of the included studies, the actual effect of strength training on IFMs was various.

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

9 Dec 2021

A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.

A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.

An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

If applicable, we recommend that you deposit your laboratory protocols in protocols.io to enhance the reproducibility of your results. Protocols.io assigns your protocol its own identifier (DOI) so that it can be cited independently in the future. For instructions see: https://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols. Additionally, PLOS ONE offers an option for publishing peer-reviewed Lab Protocol articles, which describe protocols hosted on protocols.io. Read more information on sharing protocols at https://plos.org/protocols?utm_medium=editorial-email&utm_source=authorletters&utm_campaign=protocols.

We look forward to receiving your revised manuscript.

Kind regards,

Peter Andreas Federolf

Academic Editor

PLOS ONE

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #2: (No Response)

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #2: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #2: No

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #2: 1. Although most of the previous comments from this reviewer were addressed properly, the paper still needs improvement in language and flow. It is impossible to follow some new additions. I strongly recommend a professional service for proofreading, not only a colleague who is a native speaker, because it clearly did not work this time. The way the paper reads it is impossible to be fully understood.

2. I have used the search strategies now described in the paper and found some very important quality papers (PEDro scale 5 to 8) that were not included in this review. Additionally, I am familiar with many papers that used IFM training and it matched some of them that I found but is it no in this manuscript. I am listing below for the authors to consider including as it must not be out of your systematic review.

Missing relevant papers regarding you research question, your outcomes and your chosen study designs:

I. (PEDro score 7) Lee D-R, Choi Y-E. Effects of a 6-week intrinsic foot muscle exercise program on the functions of intrinsic foot muscle and dynamic balance in patients with chronic ankle instability. Journal of exercise rehabilitation. 2019;15(5):709.

Specifically, you cite this study but I did not understand why it was not included in your systematic review.

II. (PEDro score 5) Day EM, Hahn ME. Increased toe-flexor muscle strength does not alter metatarsophalangeal and ankle joint mechanics or running economy. Journal of sports sciences. 2019;37(23):2702-10.

III. (PEDro score 8) Jung D-Y, Koh E-K, Kwon O-Y. Effect of foot orthoses and short-foot exercise on the cross-sectional area of the abductor hallucis muscle in subjects with pes planus: a randomized controlled trial. Journal of back and musculoskeletal rehabilitation. 2011;24(4):225-31.

IV. (PEDro score 8) Kamonseki DH, Gonçalves GA, Liu CY, Júnior IL. Effect of stretching with and without muscle strengthening exercises for the foot and hip in patients with plantar fasciitis: a randomized controlled single-blind clinical trial. Manual therapy. 2016;23:76-82. (balance)

V. (PEDro score 6) Lynn SK, Padilla RA, Tsang KK. Differences in static-and dynamic-balance task performance after 4 weeks of intrinsic-foot-muscle training: the short-foot exercise versus the towel-curl exercise. Journal of sport rehabilitation. 2012;21(4):327-33.

3. There are two studies included that have a wrong year of publication. Please, correct it:

• Unver B, Erdem EU, Akbas E. Effects of Short-Foot Exercises on Fo 506 ot Posture, Pain, Disability and Plantar Pressure in Pes Planus. Journal of sport rehabilitation. 2019:1-16.

• Fraser JJ, Hertel J. Effects of a 4-week intrinsic foot muscle exercise program on motor function: a preliminary randomized control trial. Journal of sport rehabilitation. 2019;28(4):339-49.

4. In the PROSPERO registry, the risk of bias assessment is described as PEDro scale. However, in this manuscript the authors did no use this scale but the Modified Downs and Black Quality Index. Please resolve this inconsistency with your PROSPERO registry and specify why the PROSPERO protocol was not adhered to. The reviewer #1 had already pointed it out in the first review but it seems the authors did not understand it and have not explained this issue.

5. Ref 35 is listed as “Kim EK, Kim JS. 2016” As authors but in the evidence table, it is listed as “Eun-Kyung et al. (2016)” Please correct that.

6. Introduction Lines 53-54: ““Although numerous studies explore the effect of IFMs training, there is still lacking of evidence-based studies to review it systematically.” This sentence does not make sense. If there are many evidences, and it does indeed (you mentioned now in your paper) that proved the efficacy of the foot muscles training for several conditions, what exactly do you need? There is no “lacking evidenced-based studies”. On the contrary. What you really want is to review systematically the exiting evidences. Please, rephrase, again, the sentence.

7. Introduction Lines 66-67: “Randomized controlled trials, intervention studies, were eligible for inclusion.” What are intervention studies? Non-controlled studies? Case studies? Interrupted case series? Please, use the SIGN (Scottish Intercollegiate Guidelines Network) methodology to describe the type of studies that were searched and included in your review

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #2: Yes: Isabel C. N. Sacco

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

11 Jan 2022

Reviewer #2:

1. Although most of the previous comments from this reviewer were addressed properly, the paper still needs improvement in language and flow. It is impossible to follow some new additions. I strongly recommend a professional service for proofreading, not only a colleague who is a native speaker, because it clearly did not work this time. The way the paper reads it is impossible to be fully understood.

A: Thank you for your suggestion. We have modified the English language of the manuscript.

2. I have used the search strategies now described in the paper and found some very important quality papers (PEDro scale 5 to 8) that were not included in this review. Additionally, I am familiar with many papers that used IFM training and it matched some of them that I found but is it no in this manuscript. I am listing below for the authors to consider including as it must not be out of your systematic review.

Missing relevant papers regarding you research question, your outcomes and your chosen study designs:

I. (PEDro score 7) Lee D-R, Choi Y-E. Effects of a 6-week intrinsic foot muscle exercise program on the functions of intrinsic foot muscle and dynamic balance in patients with chronic ankle instability. Journal of exercise rehabilitation. 2019;15(5):709.

Specifically, you cite this study but I did not understand why it was not included in your systematic review.

A: Thank you for your suggestion. Given to the fact that patients with chronic ankle instability may have different foot biomechanics, and IFMs training has less effect on the ankle joint. So after negotiation between the two authors, we finally decided excluded this article.

II. (PEDro score 5) Day EM, Hahn ME. Increased toe-flexor muscle strength does not alter metatarsophalangeal and ankle joint mechanics or running economy. Journal of sports sciences. 2019;37(23):2702-10.

A: Thank you for your suggestion. After reexamined all articles, we found this article meets our inclusion criteria. We have added it and rewrite our manuscript.

III. (PEDro score 8) Jung D-Y, Koh E-K, Kwon O-Y. Effect of foot orthoses and short-foot exercise on the cross-sectional area of the abductor hallucis muscle in subjects with pes planus: a randomized controlled trial. Journal of back and musculoskeletal rehabilitation. 2011;24(4):225-31.

A: Thank you for your suggestion. Because of this article mainly compare foot orthosis (FO) group and the group combined foot orthosis and short-foot exercise (FOSF) after 8-week intervention. We cannot rule out the effect of foot orthosis on the cross-sectional area of the abductor hallucis muscle. So after reviewing the titles, abstracts and full texts, we excluded it.

IV. (PEDro score 8) Kamonseki DH, Gonçalves GA, Liu CY, Júnior IL. Effect of stretching with and without muscle strengthening exercises for the foot and hip in patients with plantar fasciitis: a randomized controlled single-blind clinical trial. Manual therapy. 2016;23:76-82. (balance)

A: Thank you for your suggestion. We only include research specific to intrinsic foot muscles training. The foot exercise group in this article utilizing extrinsic and intrinsic foot muscles. Given that the extrinsic foot muscles strengthening exercises may bring potential effect on the foot biomechanics outcome, so after reviewing the titles and abstracts, we excluded it.

V. (PEDro score 6) Lynn SK, Padilla RA, Tsang KK. Differences in static-and dynamic-balance task performance after 4 weeks of intrinsic-foot-muscle training: the short-foot exercise versus the towel-curl exercise. Journal of sport rehabilitation. 2012;21(4):327-33.

A: Thank you for your suggestion. The main aim of this study was to compare short-foot exercise and towel-curl exercise on foot function or dynamic postural balance, rather than explore the effect of IFMs training foot function and dynamic postural balance. So after reviewing the titles, abstracts and full texts, we excluded it.

3. There are two studies included that have a wrong year of publication. Please, correct it:

• Unver B, Erdem EU, Akbas E. Effects of Short-Foot Exercises on Fo 506 ot Posture, Pain, Disability and Plantar Pressure in Pes Planus. Journal of sport rehabilitation. 2019:1-16.

• Fraser JJ, Hertel J. Effects of a 4-week intrinsic foot muscle exercise program on motor function: a preliminary randomized control trial. Journal of sport rehabilitation. 2019;28(4):339-49.

A: Thank you for your suggestion. We have corrected it.

4. In the PROSPERO registry, the risk of bias assessment is described as PEDro scale. However, in this manuscript the authors did no use this scale but the Modified Downs and Black Quality Index. Please resolve this inconsistency with your PROSPERO registry and specify why the PROSPERO protocol was not adhered to. The reviewer #1 had already pointed it out in the first review but it seems the authors did not understand it and have not explained this issue.

A: Thank you for your suggestion. Even though we plan to utilize PEDro scale to assess the risk of bias, we finally decide to use Modified Downs and Black Quality Index to assess the methodological quality of the included studies based on the relative references, and we have added it to support our manuscript.

Ref:

1. Huffer D, Hing W, Newton R, Clair M. Strength training for plantar fasciitis and the intrinsic foot musculature: A systematic review. Phys Ther Sport, 2017; 24:44-52.

2. Downs SH, Black N. The feasibility of creating a checklist for the assessment of the methodological quality both of randomised and non-randomised studies of health care interventions. J Epidemiol Commun H, 1998.

5. Ref 35 is listed as “Kim EK, Kim JS. 2016” As authors but in the evidence table, it is listed as “Eun-Kyung et al. (2016)” Please correct that.

A: Thank you for your suggestion. We have corrected it.

6. Introduction Lines 53-54: ““Although numerous studies explore the effect of IFMs training, there is still lacking of evidence-based studies to review it systematically.” This sentence does not make sense. If there are many evidences, and it does indeed (you mentioned now in your paper) that proved the efficacy of the foot muscles training for several conditions, what exactly do you need? There is no “lacking evidenced-based studies”. On the contrary. What you really want is to review systematically the exiting evidences. Please, rephrase, again, the sentence.

A: Thank you for your suggestion. We have rephrased this sentence.

7. Introduction Lines 66-67: “Randomized controlled trials, intervention studies, were eligible for inclusion.” What are intervention studies? Non-controlled studies? Case studies? Interrupted case series? Please, use the SIGN (Scottish Intercollegiate Guidelines Network) methodology to describe the type of studies that were searched and included in your review.

A: Thank you for your suggestion. We have modified the description the type of studies according to the SIGN (Scottish Intercollegiate Guidelines Network) methodology, and added the relative reference to support our review.

Ref:

Scottish Intercollegiate Guidelines Network (SIGN) Methodology Review Group, editor. Report on the review of the method of grading guideline recommendations. Edinburgh: SIGN 1999.

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

10 Feb 2022

A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.

A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.

An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

If applicable, we recommend that you deposit your laboratory protocols in protocols.io to enhance the reproducibility of your results. Protocols.io assigns your protocol its own identifier (DOI) so that it can be cited independently in the future. For instructions see: https://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols. Additionally, PLOS ONE offers an option for publishing peer-reviewed Lab Protocol articles, which describe protocols hosted on protocols.io. Read more information on sharing protocols at https://plos.org/protocols?utm_medium=editorial-email&utm_source=authorletters&utm_campaign=protocols.

We look forward to receiving your revised manuscript.

Kind regards,

Peter Andreas Federolf

Academic Editor

PLOS ONE

Additional Editor Comments:

Comments of Reviewer 2:

The authors have written an interesting review paper that, through the peer-review process so far has made some obvious progress. Still, some major issues remain, as well as numerous minor issues. Major issues lie in the reproducibility and complexity of the methods and results. Still significant steps are required in these sections before they would be fit for publication. Further, there needs to be a clear match between the results collected here and the conclusions of the study, as the discussion section currently reads like a narrative literature review, without adding much analyses to the reviewed studies (the analyses seem isolated in the results and are not clearly linked to the discussion).

I appreciate the further specification of the authors decision for using the Modified Downs and Black quality assessment to the manuscript. However, I do not feel the authors have sufficiently explained why they have neglected an a-priori established part of their protocol and chosen to use a different method. This undermines the credibility of the manuscript as one of the major strengths of a systematic review lies in the protocol that is established a-priori. As such, I would again ask the authors to explain in their response to reviewers letter why they made this decision.

Further comments in order of appearance.

Line 3-5. This sentence does not flow well due to the double use of the word ‘relevant’. I would suggest the following sentence: ‘Keywords related to IFM training were used to search four databases for relevant studies published between January 2011 and February 2021.’

Line 34-36. This sentence does not flow well as it is unclear how ‘they’ should be interpreted. I would suggest rephrasing to something starting with: ‘The main IFMs are … ‘

Line 45. So far this paragraph has been about running, but this final sentence switches the focus to walking. This breaks up the flow in the introduction.

Line 49-52. There is a flaw in the logic here. It cannot be stated that the part up until the first reference ‘[24]’ is proof for the part until the second reference ‘[23]’. These findings might be related, but proof is not offered by this statement.

Line 55-56. The aims statement does not flow logically from the previous. First, you state that much about the positive effects of IFM is already known. This is not a reason to study the concept further. A better rationale could be something along the lines of the following: ‘while a number of isolated studies have shown benefits of IFM, the applicability of these findings is still limited because, to date, no previous study has systematically studied these effects nor has a meta-analysis been applied to get an overall estimate of the effect of IFM training. Therefore, the current study aims…’

Line 72. Consider changing ‘in accordance with’ to ‘based on’

Line 74-75. Please specify here what ‘desired foot biomechanical parameters’ were

Line 80. Consider ‘i.e.’ to ‘e.g.’

Line 87-91. This statement is confusing and needs to be rephrased.

Line 92-97. It is clear that the mean difference was taken as an outcome measure. However, it is unclear what this difference is based on, related to the different designs included in the study. For instance with an RCT. Was the difference of the experimental group between pre and post test used, or the difference between experimental and control group at post test?

Line 170. Please report exact p values and associated tests statistics.

Line 170-172. This sentence reads like a concluding sentence and therefore does not fit the results section.

Line 178-180. The authors conclude here that the funnel plot shows a ‘slight asymmetry’. What was the basis for this classification? That is, as a rule of thumb, 10 observations are required to statistically assess the asymmetry of a funnel plot. Unless the authors are aware of other guidelines, I think caution is warranted in this interpretation step. Especially considering that if one would scale figure 4B to have a X axis from -6 to 6, the asymmetry would appear way more drastic.

Line 184. The Eun-Kyung (2016) reference is not represented in table 1 and 2.

Line 184-187. This figure needs a clearer caption. What do the values indicate? Especially the ‘total’ column is not clear. I initially interpreted this as the N of the studies, but if this is true, then there are inconsistencies with table 2.

Table 3. Please specify the meaning of ‘NR’

Line 225. Which hypothesis is meant here?

Discussion in general: this section reach much like a narrative review and the link to the quantitative results of this study should be emphasized.

Line 179. Consider using ‘supervisor’ rather than ‘supervisors’

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: (No Response)

Reviewer #2: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Partly

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: No

Reviewer #2: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: (No Response)

Reviewer #2: The additions and corrections were properly executed and the paper sounds better in this version.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: Yes: Isabel C N Sacco

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

24 Feb 2022

Further comments in order of appearance.

Line 13-15. This sentence does not flow well due to the double use of the word ‘relevant’. I would suggest the following sentence: ‘Keywords related to IFM training were used to search four databases for relevant studies published between January 2011 and February 2021.’

A: Thank you for your suggestion. We have revised our manuscript follow your suggestion.

Line 34-36. This sentence does not flow well as it is unclear how ‘they’ should be interpreted. I would suggest rephrasing to something starting with: ‘The main IFMs are … ‘

A: Thank you for your suggestion. We have rephrased our manuscript and start with: ‘The main IFMs are …’

Line 45. So far this paragraph has been about running, but this final sentence switches the focus to walking. This breaks up the flow in the introduction.

A: Thank you for your suggestion. We have revised it.

Line 49-52. There is a flaw in the logic here. It cannot be stated that the part up until the first reference ‘[24]’ is proof for the part until the second reference ‘[23]’. These findings might be related, but proof is not offered by this statement.

A: Thank you for your suggestion. We have revised it.

Line 55-56. The aims statement does not flow logically from the previous. First, you state that much about the positive effects of IFM is already known. This is not a reason to study the concept further. A better rationale could be something along the lines of the following: ‘while a number of isolated studies have shown benefits of IFM, the applicability of these findings is still limited because, to date, no previous study has systematically studied these effects nor has a meta-analysis been applied to get an overall estimate of the effect of IFM training. Therefore, the current study aims…’

A: Thank you for your suggestion. We have revised our manuscript follow your suggestion.

Line 72. Consider changing ‘in accordance with’ to ‘based on’

A: We have corrected it.

Line 74-75. Please specify here what ‘desired foot biomechanical parameters’ were

A: Thank you for your suggestion. We have specified it.

Line 80. Consider ‘i.e.’ to ‘e.g.’

A: We have corrected it.

Line 87-91. This statement is confusing and needs to be rephrased.

A: Thank you for your suggestion. We have rewritten it.

Line 92-97. It is clear that the mean difference was taken as an outcome measure. However, it is unclear what this difference is based on, related to the different designs included in the study. For instance with an RCT. Was the difference of the experimental group between pre and post test used, or the difference between experimental and control group at post test?

A: Thank you for your suggestion. The mean difference is based on the difference between pre and post test used.

Line 170. Please report exact p values and associated tests statistics.

A: Thank you for your suggestion. We have reported exact p values.

Line 170-172. This sentence reads like a concluding sentence and therefore does not fit the results section.

A: We have deleted it.

Line 178-180. The authors conclude here that the funnel plot shows a ‘slight asymmetry’. What was the basis for this classification? That is, as a rule of thumb, 10 observations are required to statistically assess the asymmetry of a funnel plot. Unless the authors are aware of other guidelines, I think caution is warranted in this interpretation step. Especially considering that if one would scale figure 4B to have a X axis from -6 to 6, the asymmetry would appear way more drastic.

A: Thank you for your suggestion. We have modified our manuscript following your suggestion.

Line 184. The Eun-Kyung (2016) reference is not represented in table 1 and 2.

A: Thank you for your suggestion. Eun-Kyung (2016) has been revised as Kim et al [37] in table 1-3.

Line 184-187. This figure needs a clearer caption. What do the values indicate? Especially the ‘total’ column is not clear. I initially interpreted this as the N of the studies, but if this is true, then there are inconsistencies with table 2.

A: Thank you for your suggestion. We have modified the caption of figure, the ‘total’ column is the N of the studies, this typo has been corrected.

Table 3. Please specify the meaning of ‘NR’

A: We have added it.

Line 225. Which hypothesis is meant here? Discussion in general: this section reach much like a narrative review and the link to the quantitative results of this study should be emphasized.

A: Thank you for your suggestion. We have modified this section.

Line 279. Consider using ‘supervisor’ rather than ‘supervisors’

A: We have corrected it.

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

23 Mar 2022

Effect of intrinsic foot muscles training on foot function and dynamic postural balance: A systematic review and meta-analysis

PONE-D-21-23366R3

Dear Dr. Wang,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

An invoice for payment will follow shortly after the formal acceptance. To ensure an efficient process, please log into Editorial Manager at http://www.editorialmanager.com/pone/, click the 'Update My Information' link at the top of the page, and double check that your user information is up-to-date. If you have any billing related questions, please contact our Author Billing department directly at authorbilling@plos.org.

If your institution or institutions have a press office, please notify them about your upcoming paper to help maximize its impact. If they’ll be preparing press materials, please inform our press team as soon as possible -- no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact onepress@plos.org.

Kind regards,

Peter Andreas Federolf

Academic Editor

PLOS ONE

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: All of my previous comments have been addressed. To add one final comment: I would suggest adjusting the x-axes in figure 4 to have the same values in panel a and b. This will improve the readability and the reader's capability to compare both images.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: Yes: Steven van Andel

7 Apr 2022

PONE-D-21-23366R3

Effect of intrinsic foot muscles training on foot function and dynamic postural balance: A systematic review and meta-analysis

Dear Dr. Wang:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

If we can help with anything else, please email us at plosone@plos.org.

Thank you for submitting your work to PLOS ONE and supporting open access.

Kind regards,

PLOS ONE Editorial Office Staff

on behalf of

Dr. Peter Andreas Federolf

Academic Editor

PLOS ONE