Effects of age and sex on association between toe muscular strength and vertical jump performance in adolescent populations.

Toe muscular strength plays an important role in enhancing athletic performance because the forefoot is the only part of the body touching the ground. In general, muscular strength increases with age throughout adolescence, and sex-related difference in muscular strength becomes evident during childhood and adolescence. However, toe muscular strength is known to be levelled off after late adolescence in both sexes. For adolescent populations, therefore, the association of toe muscular strength with physical performance might differ with age and/or sex. This study aimed to investigate differences in relationships between toe muscular strength and vertical jump performance across sex and age in adolescent populations. The maximum isometric strength of the toe muscles and vertical jump height (VJ) were assessed in 479 junior high school students (JH) aged 12-14 years (243 boys and 236 girls) and 465 high school students (HS) aged 15-18 years (265 boys and 200 girls). Two types of measurements were performed to evaluate the toe muscular strength: toe gripping strength (TGS) with the metatarsophalangeal joint in the plantar flexed position and toe push strength (TPS) with the metatarsophalangeal joint in the dorsiflexed position. TGS and TPS were normalized to body weight. Two-way ANOVA showed that TGS had significant main effects of sex (boys > girls) and age (HS > JH) while TPS only had a significant main effect of sex (boys > girls). When the effects of sex and age were separately analyzed, VJ was significantly correlated with TGS in JH girls, HS girls, and JH boys (r = 0.253-0.269, p < 0.05), but not in HS boys (r = 0.062, p = 0.3351). These results suggest that toe muscular strength is relatively weakly associated with vertical jump performance in adolescent boys and girls, but the association would not be established in high school boys.

Introduction

The forefoot is the only part of the body contacting with the ground at the propulsion phase of the walking or running [1, 2], and at the taking off phase of jumping [3]. Toe muscular strength has a role in controlling the forefoot motion, thus it could contribute to enhancing sport performance [4]. In fact, several studies have provided evidence indicating a significant association of toe muscular strength with sport performance [4-6].

In the growth stage, the development of sport performance during adolescence is known to be related to that in muscle strength [7]. The muscle force in the lower limb linearly increases from early childhood, and its sex-related difference becomes evident during childhood and adolescence [8, 9]. On the other hand, a different development pattern was recently reported for toe muscular strength [10]. Morita et al. (2018) [10] observed that the maximum toe muscular strength levelled off after late adolescence in both sexes, and thereafter sex difference existed. This finding tempts us to assume that the unique trend in development of toe muscular strength during adolescence could yield an age-related difference in the association of toe muscular strength with sport performance during the corresponding growth stage. For adolescent boys and girls, however, numerous previous studies have focused on the strength development of the knee flexor and extensor as well as the ankle plantar flexor and dorsiflexor muscles and their association with sport performance [11, 12].

The relationship between toe muscular strength and jump performance has been shown to be significantly correlated in junior high school girls but not in junior high school boys [5]. For young male adults, however, toe muscular strength was significantly related with the vertical jump performance [13], and the training of toe muscular strength enhanced the vertical jump performance [4]. It is known that age-related change in the jump height during the growth stage differs between boys and girls [14, 15]. The jump height in girls reaches a plateau at around 11–12 years of age while that in boys increases continuously [14, 15], and consequently the difference between boys and girls increases above 12 years old [14]. These studies suggest that the influence of toe muscular strength on jump performance in adolescents would differ in age and sex. As far as we know, no study has investigated the associations between toe muscular strength and jump performance in high school girls and boys. It is therefore unknown how toe muscular strength and its relationship with jump performance in the adolescent stage are influenced by age and sex.

The purpose of this study was to investigate the differences in relationships between toe muscular strength and vertical jump performance across sex and age in adolescent populations. We hypothesized that the age- and sex-related difference in jump performance during adolescence would be affected by the toe muscular strength, and the degree of its effect is greater in younger individuals with weaker muscular strength of the knee and ankle. In short, toe muscular strength would be related with jump performance more strongly in junior high school than in high school students and in girls than in boys. Additionally, the present study compared the relationship of jump performance with toe muscular strength measured at different Metatasophalangeal joint (MPJ) angles: the plantar flexed position versus dorsiflexed position.

Methods

Participants

Participants were 479 junior high school students aged 12–14 years {243 boys (JH boys) and 236 girls (JH girls)} and 465 high school students aged 15–18 years {265 boys (HS boys) and 200 girls (HS girls)}. They were recruited from the periodical physical examination organized by the Osaka Basketball Association Sport Injury Prevention Project in 2016–2019. All participants belonged to a basketball team of their school, and they practiced basketball for 2–3 hours per day almost all of the weekdays. They were deemed as well accustomed to jumping as high as possible, since the practice and competitive activities of basketball involve numbers of jumping actions. The participants with any severe musculoskeletal injuries or problems affecting physical performance were excluded. This study was approved by the Research Committee of Ritsumeikan University (Ethics for medical and health research involving human subjects, Ritsumeikan University, Japan, BKC-IRB-2017-013 and BKC-IRB-2018-037), and all the participants and their parents gave their written informed consent.

Measurements of toe muscular strength and vertical jump performance

Two types of apparatus were used to evaluate toe muscular strength in accordance with a previous study [6]. From the methodological viewpoint, several previous studies have demonstrated that toe muscular strength depends on the MPJ angles, which determine the force-length relationship of the extrinsic foot muscles [4, 16]. The angle of toe muscular strength development around the MPJ would therefore differ in its role for different motions in the forefoot; for that reason, toe gripping strength (TGS) was measured with the MPJ in the plantar flexed position and toe push strength (TPS) was measured with the MPJ in the dorsiflexed position. A commercially available dynamometer (T.K.K. 3361, Takei Scientific Instrument Co, Niigata, Japan) was used for measuring the TGS, and a custom-made dynamometer (T.K.K. 1268, Takei Scientific Instrument Co, Niigata, Japan) that is the same apparatus as used in previous studies [6, 17] was utilized for measuring the TPS.

TGS was measured while the participants sat with their hip, knee, and ankle joints at 90°. The participants were instructed to put their toes on a grip bar and pull it with maximum effort. TPS was measured while the participants sat on a chair with the same posture in the hip, knee, and ankle joints for the measurement of TGS. The participants were instructed to place their toes on an adjustable force plate with the MPJ at 45° dorsiflexed angle [17]. They were asked to use only the toe muscles and not to activate the calf muscles. Both TGS and TPS were measured three times for both the right and left feet in a randomized order with one-minute rest interval between trials.

The toe muscular strengths were strongly correlated between left and right feet for each of TGS (r = 0.84, p < 0.05) and TPS (r = 0.77, p < 0.05), and not significantly different in magnitude with a paired t-test between feet (TGS: t = 1.18, p = 0.24, and TPS: t = 0.90, p = 0.37). Therefore, the highest value from the six trials (three times in each foot) was divided by body weight and used for further analysis. A good repeatability of the TGS and TPS measurements of this study were confirmed by the calculation of intraclass correlation coefficients (ICC (95%CI): TGS = 0.830 (0.814–0.846), and TPS = 0.806 (0.788–0.824)).

The counter movement jump test has been extensively used to assess the leg extension power, as the assessment tool is simple and reliable [18]. In this study, a maximum vertical jump height (VJ) was measured by using a commercially available jump tester (Yardstick, Swift Performance Equipment, Australia) as a variant of the traditional sergeant jump [19]. Before the jump test, the participants were required to stand near the tester and raise their dominant hand to displace the plastic vanes for adjusting the effect of the participant’s height. The test required participants to use their dominant hand to displace the highest possible plastic vane with an overhead arm swinging motion at the apex of their jump. Jump height was determined as the number of vanes displaced above the metal pole. All jumps were performed from a standardized position with the participant standing and facing the vanes about a distance of 10 cm from the tester, with their dominant shoulder aligned with the end of the vanes. The magnitudes of the countermovement motion and arm swing before and during jumping were adjusted by the participant to attain the maximum jump height. The measurements were conducted in the gymnasium with the participants wearing their own basketball shoes. After 2–5 times submaximal jump practices until the participants felt satisfactory, the participants performed maximal jump tests twice. The higher jump height between the two measurements was used for further analysis.

Statistical analysis

All data are described as means ± standard deviations (SDs). Prior to the analysis, the normality of the data was tested by Kolmogorov-Smirnov test and confirmed as normal distribution. A separate paired t-test was performed to examine effects of MPJ positions on toe muscular strength (TGS vs. TPS) in each group. Two-way analysis of variance (ANOVA) (age × sex) with a Tukey-Kramer post hoc test was used to examine statistical differences in the measured variables (Body height, weight, TGS, TPS, and VJ). The relationships between TGS, TPS, and VJ were examined by the Pearson’s correlation coefficient in each group. The correlation coefficients (r) are roughly categorized by a previous paper [20], which are r <0.35 are generally considered to represent low or weak correlations, 0.36 to 0.67 modest or moderate correlations, and 0.68 to 1.0 strong or high correlations. The level of statistical significance was set at p < 0.05. All statistical analysis was performed using MATLAB software (v9.8.0.1323502, R2020a).

Results

Table 1 summarized the physical properties, toe muscular strengths, and jump performance of each group. Paired t-test revealed that the TPS was significantly greater than TGS in all groups (p < 0.05, Table 1). Two-way ANOVA for TGS resulted in significant main effects of sex (F = 10.84, p = 0.0010) and age (F = 14.91, p < 0.0001) without a significant interaction (F = 1.3, p = 0.2546). Post-hoc tests revealed significant differences between sexes without significant differences between age groups (Fig 1). TPS had a significant main effect of sex (F = 27.76, p < 0.00001) without a significant main effect of age (F = 0.16, p = 0.6853) and interaction (F = 0.09, p = 0.7599). Post-hoc tests revealed significant differences between sexes without significant differences between age groups (Fig 2). VJ had significant main effects of sex (F = 460.27, p < 0.0001) and age (F = 254.88, p < 0.0001) with a significant interaction (F = 24.37, p < 0.0001) (Table 1).

Table 1

Physical properties, toe muscular strengths, and jump performance of each group.

	Girls						Boys						Probability
	JH (N = 236)			HS (N = 200)			JH (N = 243)			HS (N = 265)			Age	Sex	Age *Sex
Age (years old)	13.0	±	0.7	16.1	±	0.8	13.1	±	0.7	16.1	±	0.7	―	―	―
Body height (cm)	157.0	±	6.6	161.4	±	12.1	162.5	±	10.2	173.0	±	7.1	*	*	*
Body weight (kg)	47.0	±	6.5	56.1	±	6.6	49.1	±	9.6	62.9	±	7.5	*	*	*
TGS (kg/BW)	0.216	±	0.067	0.232	±	0.063	0.237	±	0.065	0.252	±	0.072	*	*	0.255
TPS (kg/BW)	0.281	±	0.078	0.273	±	0.077	0.306	±	0.091	0.312	±	0.088	0.685	*	0.760
VJ (cm)	36.6	±	5.6	42.0	±	6.1	44.7	±	8.9	55.6	±	6.4	*	*	*

JH: junior high school, HS: high school, BW: body weight, TGS: toe gripping strength, TPS: toe push strength, VJ: vertical jump height

*: p<0.0001

Fig 1

Jitter plots of the toe gripping strength per body weight for each group.

The curves on the right side of the plots indicate the estimated gaussian distribution of each group. All brackets indicate significant difference between groups. Post-hoc tests revealed significant differences between sexes, but no significant differences between age groups. JH: junior high school, HS: high school, TGS: toe gripping strength.

Fig 2

Jitter plots of the toe push strength per body weight for each group.

The curves on the right side of the plots indicate the estimated gaussian distribution of each group. All brackets indicate significant difference between groups. Post-hoc tests revealed significant differences between sexes, but no significant differences between age groups. JH: junior high school, HS: high school, TPS: toe push strength.

Table 2 shows the correlation coefficients between TGS, TPS, and VJ in each group. Significant correlations between TGS and TPS were found in each group (p < 0.0001). VJ was significantly correlated with TGS in JH girls (r = 0.269, p = 0.0001), HS girls (r = 0.253, p = 0.0050), and JH boys (r = 0.258, p = 0.0001), but not in HS boys (r = 0.062, p = 0.3351). VJ was significantly correlated with TPS in JH boys (r = 0.228, p = 0.0002), but not in JH girls (r = 0.130, p = 0.0827), HS girls (r = 0.128, p = 0.1618), and HS boys (r = 0.006, p = 0.9102).

Table 2

Pearson’s correlation coefficients and the p-values between TGS, TPS, and VJ in each group.

	Girls				Boys
	JH (N = 236)		HS (N = 200)		JH (N = 243)		HS (N = 265)
	r	p	r	p	r	p	r	p
TGS vs. TPS	0.437	0.0000	0.475	0.0000	0.426	0.0000	0.368	0.0000
TGS vs. VJ	0.269	0.0001	0.253	0.0050	0.258	0.0001	0.062	0.3351
TPS vs. VJ	0.130	0.0827	0.128	0.1618	0.228	0.0002	0.006	0.9102

JH: junior high school, HS: high school, TGS: toe gripping strength, TPS: toe push strength, VJ: vertical jump height

Bold indicates a significant relationship between variables.

Discussion

The main finding obtained here were that TGS was significantly correlated with VJ in JH boys, JH girls, and HS girls, but not in HS boys. This result supports our hypothesis that toe muscular strength would be related with vertical jump performance more strongly in junior high school than in high school students and in girls than in boys.

TGS was significantly correlated with VJ in JH boys, JH girls, and HS girls although their correlation coefficients of these relationships were relatively weak (r = 0.253–0.269). No such significant relationship was found in HS boys (r = 0.062). These suggest that, for the adolescent populations, VJ could be affected by the TGS, but the degree of its association would vary with sex and age. It has been well known that the development of sport performance at the growth stage is different between boys and girls. In general, the appearance of peak height velocity is earlier in girls than boys (approximately 11 years for girls, and 12–14 years for boys, [21-23]), and the muscular strength and motor performance levels are higher in boys than girls after peak height velocity [24, 25]. The muscle force in the knee and ankle linearly increases from early childhood [9], and the strength is greater in boys compared to girls during childhood and adolescence [8, 9]. On the other hand, we found that the age-related development of the toe muscular strength showed a plateau during the adolescents, supporting the finding of Morita et al. (2018) [10]. The magnitude of toe muscular strength observed in this study was comparable to the normative data of adults in the previous studies (TGS: 0.228 kg/BW in male American-football players [6], 0.265 kg/BW in men and 0.200 kg/BW in women of community-dwelling individuals of the age of 20s [26]; TPS: 0.166kg/BW in female dancers [17], 0.235kg/BW in male American-football players [6]). Considering this aspect, therefore, it seems that the development of VJ in late adolescent boys would be caused by the increase in the strength capability of lower limb muscles other than foot muscles. In other words, the contribution of toe muscular strength to VJ in HS boys might have been diminished by the age-related development of the strength capability of lower limb muscles other than foot muscles (e.g. knee and ankle).

Furthermore, factors such as jumping technique, neural adaptation, and anthropometrics should also be considered as potential explanations for the lack of a significant association between TGS and VJ in HS boys. Lloyd et al. (2011) [27] showed that stretch shortening cycle may affect the increment of vertical jump height during adolescence. A previous study showed that the age of 15–16 years is a threshold of gender differentiation for efficiency in stretch shortening cycle [28]. However, all the participants in this study belonged to the basketball club of their school and practiced basketball on most of the weekdays. The game of basketball involves numbers of jumping situations. Therefore, it was supposed that the participants in this study were well accustomed to jumping as high as possible and would be able to use effective stretch shortening function. For anthropometric factors, a significant correlation exists between jump height and toe length in young male adults [29], between jump height and foot and toe length in young female adults [30]. For adolescent boys and girls examined here, we have no data on foot and/or toe length. Further research involving the foot anthropometric measurements is needed to elucidate the factors explaining the lack of the significant association between TGS and VJ in HS boys.

In the current study, the relationships between the TGS and VJ were significant, while the relationships between the TPS and VJ were not significant in girls (Table 2). The TPS was greater than the TGS in all groups, which suggests that the TPS rather than the TGS would represent the maximum toe muscular strength because of the force-length relationship in toe muscular strength as demonstrated by the previous study [16]. Also, a previous study showed that TPS was more strongly associated with the athletic performance than TGS [6]. Considering the kinematical interpretation that the MPJ is in the plantar flexed position at the taking-off [3], together with the magnitude of TPS and TGS, TPS was assumed to be more strongly correlated with the jump performance than TGS. However, the results of this study deny this assumption. The current results indicate that, at least in the adolescent girls, TGS may give a better reflection of toe muscular strength being well associated with jump performance, compared to TPS.

Jumping ability is one of the determinant factors for achieving high performance in various sports activities. It is known that for young adults, 6–7 weeks of the training on toe flexor muscles improved the jump performance [4, 31]. Taking this into account together with the observed significant correlations between toe muscular strength and vertical jump height, it is possible that strengthening toe muscular strength in adolescence boys and girls may enhance not only their vertical jumping ability but also various sport performance. However, the present study is a cross-sectional study. The relationship between toe muscular strength and jump performance could not make any inference about cause and effect. Further investigation of longitudinal or interventional study is needed to examine the causal effect of toe muscular strength on jump performance.

As the other limitations to this study, firstly, it should be remarked again that all the participants of this study belonged to the basketball club, and so they were accustomed to jumping actions. Thus, it is difficult to generalize the results of this study to sedentary adolescents. Furthermore, we did not evaluate the strength capacity of the other muscles located in the lower limbs. In general, the vertical jump performance is primarily determined by the ankle, knee, and hip joint moment [32]. In the adolescent stage, the lower limb muscle strengths linearly increase with increasing age [8, 9], while that of toe muscular strength reaches a plateau [10]. Thus, further investigation is needed to clarify the influence of the other lower limb muscles on the relationship between toe muscular strength and jump performance.

In conclusion, our results suggest that TGS is relatively weakly associated with vertical jump performance in adolescent boys and girls, but the association would not be established in high school boys.

10 Aug 2021

PONE-D-21-13635

Effects of age and sex on association between toe muscular strength and vertical jump performance in adolescent populations

PLOS ONE

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Reviewer #1: Thank you for asking me to review this paper on the effects of age and sex on the association between toe muscular strength and vertical jump performance in adolescent populations. The paper covers all important aspects of the study, but could be improved by shortening the introduction and clarifying some logical aspects of the introduction and discussion

General comment

Abbreviations. Consider spelling out some abbreviations that are non-standard which makes reading the paper difficult. For example TMS was confusing with your two groups TGS and TPS. As you don’t do any analysis with a TMS group you could spell this out in the paper. PHV and SCC could also remain spelt out as they are used infrequently.

Introduction

The introduction is very long and at times the logic is difficult to follow. For example, it makes sense that muscle force across muscles increases with growth, and that differences exist between boys and girls. It is unclear why toe muscular strength would be different given that you argue that there is a difference in TMS through adolescence, the same as other muscle groups, which levels off after late adolescence, and then you only investigate adolescents.

Justification for the study is not strong as you would expect jump performance to be significantly related to lower leg power and for toe muscles to contribute only a small amount if anything. Maybe the possible association is only important to those who require good jumping performance and therefore any differences in age and sex are important for improvements?

Line 62: Clarify why the forefoot is necessary for performance as it is in contact with the ground? What aspect of the forefoot is necessary? Do you mean the muscles controlling the forefoot?

Line 66 What do you mean by toe muscular strength appropriately adjusting forefoot motion?

Line 68 What sort of significant role? It could be to increase or decrease sport performance.

Line 72 Consider more recent references to support your argument- since the size and time of puberty has changed in the last 25 years and your statement here may no longer be true e.g. McKay M et al Normative reference values for strength and flexibility of 1,000 children and adults Neurology 2017 88:1-8 Line 75 remove the ‘was’ in ‘..thereafter sex differences was existed’

Line 80 change ‘its’ to ‘their’

Line 81ff the plateau in jump height may be population specific e.g. this is not seen in McKay et al Reference values for developing responsive functional outcome measures across the lifespan Neurology 2017 88: 1512-1519

Line 91 Suggest adding ‘ These studies suggest…”

Line 113 Maybe you could expand here to clarify the role of toe muscles in controlling the final propulsion and initial shock absorption in jumping?

Line 111 Aims. Take care to keep your aims to associations or relationships since you cant make any inference about cause or effect e.g You cant tell if the age and sex differences in jump performance were actually effected by toe muscle strength. Line 121 Suggest rewording to ‘..measured at different MPJ angles…’

Methods

Line 131 Was each participant data used only once? You say the data came from the project 2016-2019, was any participant measured on 3 occasions or only once? This will affect the independence of the data points.

Line 141 As you have mentioned your reasoning before you could delete the beginning of the sentence ‘As aforementioned..’ and just say ‘Toes gripping strength was….’

Line 154 Did you check whether the calf muscles were used rather than just asking the participants not to use them?

Line 162 which measurements were used for repeatability? Was this a within participant calculation? Did you do any inter-rater reliability for the measure? What was the 95% confidence interval?

Line 164ff Can you clarify how you adjusted the jump height for the participant height?

Line 174 How many were ‘several’ practices? Was there a criteria for how many were allowed?

Line 179 Statistical analysis. Did you test your data for normality?

Results

The first paragraph is a succinct clear reporting of the results. I found the correlation figures less helpful than the actual data so you could consider a table of the correlation results rather than the figures.

You do not report the strength of the correlations- which overall are weak. Consider adding this to the methods and results

Discussion

Line 240 You don’t need to spell out the abbreviations here. If you feel you do then write them out and remove the brackets.

Line 244 You can only say that TGS may give a better reflection of toe muscle strength than TPS, not that it represents maximum toe muscle strength.

Line 260 You can only say VJ could be affected by toe muscle strength as you only investigated associations.

Line 262 What do you mean by ‘drastically’? Can you only say ‘significantly’?

Paragraph from line 259 ff was difficult to follow. Why did you discuss the relationship of VJ to age and sex when this was not an aim of your study? I could not follow your argument very well. Consider reworking the paragraph.

Line 272 Can you say your results showed a plateau when you only investigated adolescents and not the age before or afterwards? Maybe there is no change in relationship, or maybe there is a plateau into young adults?

Conclusion

I don’t think you can conclude that TMS affects VJP as you have only shown an association not that the TMS affects the performance.

Table 1

Please indicate which are the results for the t-test and which from the ANOVA Aims to compare jump performance and toe muscular strength in 2 ways. Secondary was to directly compare the two types of measurement across age and sex.

Reviewer #2: Dear, Editor,

It was a great opportunity to review the manuscript. The authors investigated the correlations between TMS and physical performance across sex and age. The research questions are interesting; however some issues must be handled. In my opinion, the manuscript is suitable for publication in Plos One; however, some issues must be handled.

The points to be revised are as follows;

Page 10, Lines 159-161; Please give more information about normalization.

Page 10-11, lines 164-176; Please give reference about used methods.

Page 13; please add all abbreviation under the table such as HS, JH.

Page 14; it may be more methodologically correct to use real p-values instead of using > or <.

In results,

Page 14, line 213; What is corresponding association with TPS?

Page 14, lines 240-243; I am not sure that referring the table or figure in discussion section is suitable.

Page 18, lines 284-286; why did you not add the demographic data in regression analysis in your study?

Page 18, lines 280-281; I could not understand how the authors reached this conclusion, considering the above-mentioned information.

Page 18, lines 289-291; The obtained correlation values is quite low. If we were to do this study in sedentary adolescents, what will the results we obtained?

Do you have any limitations?

What do your results mean? Do you have any suggestions for using this information for clinical or sporting purposes?

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29 Sep 2021

First, according to the comment of reviewer #1, we reanalyzed the data of all samples and excluded the repeatedly measured subject and left only the first appearance. Then, the results were slightly changed. The major changes of the results were that the correlation coefficients between TGS and VJ in JHG and HSG became no significant. Therefore, we reorganized the manuscript.

Reviewer #1: Thank you for asking me to review this paper on the effects of age and sex on the association between toe muscular strength and vertical jump performance in adolescent populations. The paper covers all important aspects of the study, but could be improved by shortening the introduction and clarifying some logical aspects of the introduction and discussion

> We would like to thank you for your very constructive comments that have helped to improve our manuscript. We have attempted to address all of your comments in the revised manuscript. We hope that you find the revision satisfactory.

General comment

Abbreviations. Consider spelling out some abbreviations that are non-standard which makes reading the paper difficult. For example TMS was confusing with your two groups TGS and TPS. As you don’t do any analysis with a TMS group you could spell this out in the paper. PHV and SCC could also remain spelt out as they are used infrequently.

> Thank you for suggestion. We spelled out the abbreviations according to the reviewer’s comment.

Introduction

The introduction is very long and at times the logic is difficult to follow. For example, it makes sense that muscle force across muscles increases with growth, and that differences exist between boys and girls. It is unclear why toe muscular strength would be different given that you argue that there is a difference in TMS through adolescence, the same as other muscle groups, which levels off after late adolescence, and then you only investigate adolescents.

Justification for the study is not strong as you would expect jump performance to be significantly related to lower leg power and for toe muscles to contribute only a small amount if anything. Maybe the possible association is only important to those who require good jumping performance and therefore any differences in age and sex are important for improvements?

> According to the reviewer’s comment, we reorganized the introduction so that the logic is easy to follow.

Few previous studies investigated the relationship between toe muscular strength and jump performance. One study measured junior high school students (ref #5) and the other one measured young male adults over 20 years old (ref #13). Our study is filling the gap between them to measure junior high school and high school boys and girls aged 12-18 years.

Line 62: Clarify why the forefoot is necessary for performance as it is in contact with the ground? What aspect of the forefoot is necessary? Do you mean the muscles controlling the forefoot?

Line 66 What do you mean by toe muscular strength appropriately adjusting forefoot motion?

Line 68 What sort of significant role? It could be to increase or decrease sport performance.

Line 113 Maybe you could expand here to clarify the role of toe muscles in controlling the final propulsion and initial shock absorption in jumping?

> We consider the major role of the toe muscular strength is to control the forefoot motion. The foot is the only part of the body contacting the ground, and the angle of metatarsophalangeal joints could determine the direction of the movement; therefore, we believe the toe muscular strength is important for sport performance.

Line 72 Consider more recent references to support your argument- since the size and time of puberty has changed in the last 25 years and your statement here may no longer be true e.g. McKay M et al Normative reference values for strength and flexibility of 1,000 children and adults Neurology 2017 88:1-8

> We referred the above paper and reworded the sentences. McKay et al. reported muscle strength of 12 muscle groups - hand grip, ankle dorsiflexors and plantarflexors, knee flexors and extensors, hip abductors, internal and external rotators, elbow flexors and extensors, and shoulder internal and external rotators, but there was no measurement for toe muscular strength. Only one paper (ref #10 Morita et al.) reported the growth trend in toe muscular strength during children and adolescence.

Line 75 remove the ‘was’ in ‘..thereafter sex differences was existed’

> Removed.

Line 80 change ‘its’ to ‘their’

> Changed.

Line 81ff the plateau in jump height may be population specific e.g. this is not seen in McKay et al Reference values for developing responsive functional outcome measures across the lifespan Neurology 2017 88: 1512-1519

> We believe it is not population specific. McKay et al. divided the subjects into 3-9, 10-19, 20-59, and 60 over, while Focke et al. (ref #14) classified the age groups as 4-5, 6-7, 8-9, 10-11,12-14, 15-17, and Taylar et al. (ref #15) made yearly comparison between 10-15 years. The difference of these studies was that the former focuses on the age-related changed in lifespan, and the latter two focuses on the age-related changes in children and adolescents.

Line 91 Suggest adding ‘ These studies suggest…”

> Reworded.

Line 111 Aims. Take care to keep your aims to associations or relationships since you cant make any inference about cause or effect e.g You cant tell if the age and sex differences in jump performance were actually effected by toe muscle strength.

> We totally agree with the comment. This study investigated the relationship between toe muscular strength and jump performance; therefore, we could not conclude cause and effect.

We changed the purpose of this study and conclusion as follows,

“The purpose of this study was to investigate the differences in relationships between toe muscular strength and vertical jump performance across sex and age in adolescent populations.”

“In conclusion, our results suggest that TGS is associated with vertical jump performance in adolescent boys and girls, but the association would not be established in high school boys.”

Line 121 Suggest rewording to ‘..measured at different MPJ angles…’

> Reworded.

Methods

Line 131 Was each participant data used only once? You say the data came from the project 2016-2019, was any participant measured on 3 occasions or only once? This will affect the independence of the data points.

> Thank you for the suggestion. As aforementioned, we reanalyzed the data of all samples and excluded the repeatedly measured subjects and left only the first appearance. Relating parts in the manuscript, tables, and figures were renewed.

The number of subjects was changed as follows, JHG: 274 -> 236, HSG: 273 -> 200, JHB: 278 -> 243, HSB: 360 -> 265.

The values of each parameter in table 1 were also changed, but the results of the ANOVA between groups did not change.

The correlation coefficients between TGS and VJ in JHG and HSG became no significant, but significant correlations between TPS and VJ remained the same.

Figure 1 & 2, data plots changed. The result of post-hoc test in TGS changed.

Figure 3,4 and 5 were merged to Table 2.

Line 141 As you have mentioned your reasoning before you could delete the beginning of the sentence ‘As aforementioned..’ and just say ‘Toes gripping strength was….’

> Retouched.

Line 154 Did you check whether the calf muscles were used rather than just asking the participants not to use them?

> We asked the participants not to use calf muscles and checked visually whether the calf muscles were not bulging and the heel was kept on the foot plate during the measurement. When considered inappropriate, the measurement was discarded.

Line 162 which measurements were used for repeatability? Was this a within participant calculation? Did you do any inter-rater reliability for the measure? What was the 95% confidence interval?

> Repeatability of TGS and TPS were checked within participant. ICC is intra-rater reliability not the inter-rater reliability. We added 95% CI.

Line 164ff Can you clarify how you adjusted the jump height for the participant height?

> We did not normalize the jump height by participant height.

Jump height depends on the initial take-off velocity, which is determined by the impulse of ground reaction force and body weight. Therefore, jump height is not related to the body height.

Line 174 How many were ‘several’ practices? Was there a criteria for how many were allowed?

> We asked participants to practice for 2-5 times until the participants felt satisfactory. The number of repetitions of the practice is depending on the subjects.

Line 179 Statistical analysis. Did you test your data for normality?

> Yes. All the data set were confirmed as normal distribution. We added the description for confirming the normality of the data as follows,

“Prior to the analysis, the normality of the data was tested by Kolmogorov-Smirnov test and confirmed as normal distribution.”

Results

The first paragraph is a succinct clear reporting of the results. I found the correlation figures less helpful than the actual data so you could consider a table of the correlation results rather than the figures.

You do not report the strength of the correlations- which overall are weak. Consider adding this to the methods and results

> According to the comment, Figure 3,4 and 5 were deleted and merged to Table 2.

We added the description about the strength of the correlation coefficients in the discussion.

Discussion

Line 240 You don’t need to spell out the abbreviations here. If you feel you do then write them out and remove the brackets.

> Thank you for the suggestion. We removed the explanation of them.

Line 244 You can only say that TGS may give a better reflection of toe muscle strength than TPS, not that it represents maximum toe muscle strength.

> We agree with the comment.

Line 260 You can only say VJ could be affected by toe muscle strength as you only investigated associations.

> Changed the word “would” to “could”.

Line 262 What do you mean by ‘drastically’? Can you only say ‘significantly’?

Paragraph from line 259 ff was difficult to follow. Why did you discuss the relationship of VJ to age and sex when this was not an aim of your study? I could not follow your argument very well. Consider reworking the paragraph.

> We deleted the discussion of development of VJ.

Line 272 Can you say your results showed a plateau when you only investigated adolescents and not the age before or afterwards? Maybe there is no change in relationship, or maybe there is a plateau into young adults?

> Two-way ANOVA for TPS showed no significant main effect of age. Moreover, comparing with the data of adults in the previous studies, the magnitude of TPS in this study were comparable or larger. So, we could say TPS reached a plateau. For TGS, the previous study of Morita et al. (ref#10) measured TGS throughout children, adolescent, and young adults, and showed a plateau in the late adolescent in both sexes.

Conclusion

I don’t think you can conclude that TMS affects VJP as you have only shown an association not that the TMS affects the performance.

> Same as the previous comment of aims of this study, we cannot conclude the cause and effect between TMS and VJP, but only the association. So, we reworded the conclusion that we suggest that the toe muscular strength is associated with the jump performance in adolescent boys and girls.

Table 1

Please indicate which are the results for the t-test and which from the ANOVA

Aims to compare jump performance and toe muscular strength in 2 ways. Secondary was to directly compare the two types of measurement across age and sex.

> Paired t-test was used to compare TGS and TPS in each group and between left and right feet in each participant.

Reviewer #2: Dear, Editor,

It was a great opportunity to review the manuscript. The authors investigated the correlations between TMS and physical performance across sex and age. The research questions are interesting; however some issues must be handled. In my opinion, the manuscript is suitable for publication in Plos One; however, some issues must be handled.

> Thank you for the suggestive and constructive comments that have helped to improve our manuscript. We hope that you find the revision satisfactory.

The points to be revised are as follows;

Page 10, Lines 159-161; Please give more information about normalization.

> Normalization was conducted as dividing strength by body weight.

%TGS(%TPS) = Max values of TGS(TPS)/Body weight

We changed “normalized” to “divided”.

Page 10-11, lines 164-176; Please give reference about used methods.

> We added the reference.

Page 13; please add all abbreviation under the table such as HS, JH.

> Added.

Page 14; it may be more methodologically correct to use real p-values instead of using > or <.

> Thank you for the suggestion. We put the real p-values as far as possible.

In results,

Page 14, line 213; What is corresponding association with TPS?

> Reworded.

Page 14, lines 240-243; I am not sure that referring the table or figure in discussion section is suitable.

> Changed the referred table or figure appropriately.

Page 18, lines 284-286; why did you not add the demographic data in regression analysis in your study?

> Our aim was to investigate differences in the relationships between toe muscular strength and jump performance across sex and age in adolescent populations. Thus, we divided the participants into four groups depending on the sex (boys and girls) and age (junior high school and high school).

Page 18, lines 280-281; I could not understand how the authors reached this conclusion, considering the above-mentioned information.

> We reworded the conclusion as follows; “In conclusion, our results suggest that TGS is associated with vertical jump performance in adolescent boys and girls, but the association would not be established in high school boys.”

Page 18, lines 289-291; The obtained correlation values is quite low. If we were to do this study in sedentary adolescents, what will the results we obtained?

> The participants of this study were accustomed to jumping actions; therefore, they were considered to be able to maximally utilize maximum their muscle strength for jump performance.

Thus, it is difficult to generalize the results of this study to sedentary adolescents.

We mentioned it on the limitations of this study.

Do you have any limitations?

> We added the limitation.

What do your results mean? Do you have any suggestions for using this information for clinical or sporting purposes?

> We added the suggestion for using this information as follows,

“Jumping ability is one of the determinant factors for achieving high performance in various sports activities. It is known that for young adults, 6-7 weeks of the training on toe flexor muscles improved the jump performance [4,30]. Taking this into account together with the observed significant correlations between toe muscular strength and vertical jump height, it is possible that strengthening toe muscular strength in adolescence boys and girls may enhance not only their vertical jumping ability but also various sport performance.”.

Submitted filename: Reply PONE-D-21-13635 final.docx

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1 Nov 2021

PONE-D-21-13635R1Effects of age and sex on association between toe muscular strength and vertical jump performance in adolescent populationsPLOS ONE

Dear Dr. Kurihara,

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Reviewer #1: Thank you for asking me to re-review this paper. I congratulate the authors on reworking the analysis to ensure independence of data, and reworking sections of the manuscript accordingly. I have just a few comments

1. Jump height I assume was normalised to body height by the statement on page 10 line 158 that the height recorded was that ‘above the metal pole’. Could you clarify if the metal pole was set at the participants stretched arm height, or something else?

2. Please include the interpretation you used for the correlation, and reference it. The criteria used should be reported in the methods statistical section, the corresponding category used in your discussion line 216 (currently ‘relatively weak’), and inserted in your conclusion line 286 to describe the strength of the association. E.g TGS is relatively weakly associated with….

Reviewer #2: Thank you again for your effort on this manuscript! I appreciate the time and consideration taken to make these changes.

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9 Nov 2021

Reviewer #1: Thank you for asking me to re-review this paper. I congratulate the authors on reworking the analysis to ensure independence of data, and reworking sections of the manuscript accordingly.

Thank you very much for your constructive comments We believe that the manuscript has improved significantly through the current revision.

I have just a few comments

1. Jump height I assume was normalised to body height by the statement on page 10 line 158 that the height recorded was that ‘above the metal pole’. Could you clarify if the metal pole was set at the participants stretched arm height, or something else?

We are very sorry that we misunderstood your previous comments. According to your comments, we added the sentence as follows,

“Before the jump test, the participants were required to stand near the tester and raise their dominant hand to displace the plastic vanes for adjusting the effect of the participant’s height.”

2. Please include the interpretation you used for the correlation, and reference it. The criteria used should be reported in the methods statistical section, the corresponding category used in your discussion line 216 (currently ‘relatively weak’), and inserted in your conclusion line 286 to describe the strength of the association. E.g TGS is relatively weakly associated with….

We added the reference and explanation of the criteria used in the statistical section.

We added the term of “relatively weakly” in the conclusion.

Reviewer #2: Thank you again for your effort on this manuscript! I appreciate the time and consideration taken to make these changes.

Thank you very much for your constructive comments. We are thankful for the time and energy you expended.

Submitted filename: Reply PONE-D-21-13635 2nd.docx

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19 Dec 2021

Effects of age and sex on association between toe muscular strength and vertical jump performance in adolescent populations

PONE-D-21-13635R2

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Reviewer #1: All comments have been addressed

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Reviewer #1: Yes

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Reviewer #1: Yes

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23 Dec 2021

PONE-D-21-13635R2

Effects of age and sex on association between toe muscular strength and vertical jump performance in adolescent populations

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