Potential prognostic factors for hamstring muscle injury in elite male soccer players: A prospective study.

Hamstring injuries remain the most common injury type across many professional sports. Despite a variety of intervention strategies, its incidence in soccer players playing in the UEFA Champions League has increased by 4% per year over the last decade. Test batteries trying to identify potential risk factors have produced inconclusive results. The purpose of the current study was to prospectively record hamstring injuries, to investigate the incidence and characteristics of the injuries, and to identify possible risk factors in elite male soccer players, playing in the Kosovo national premier league. A total of 143 soccer players from 11 teams in Kosovo were recruited. To identify possible prevalent musculoskeletal or medical conditions a widespread health and fitness assessment was performed including isokinetic strength testing, Nordic hamstring strength test, functional tests, and a comprehensive anamnesis surveying previous hamstring injuries. On average 27.9% of the players sustained at least one hamstring injury with three players suffering bilateral strains with the re-injury rate being 23%. Injured players were significantly older and heavier and had a higher body mass index compared to non-injured ones (p < 0.05). There was a lower passing rate in the Nordic hamstring strength test and a higher injury incidence among the previously injured players compared to non-injured ones (p < 0.05). Except for hamstring/quadriceps ratio and relative torque at 60°/sec (p < 0.05) for dominant and non-dominant leg, there were no other significant differences in isokinetic strength regardless of the angular velocity. No differences were observed for functional tests between cohorts. Regression analysis revealed that age, Nordic hamstring strength test, previous injury history, and isokinetic concentric torque at 240°/sec could determine hamstring injuries by 25.9%, with no other significant predicting risk factors. The battery of laboratory and field-based tests performed during preseason to determine performance related skills showed limited diagnostic conclusiveness, making it difficult to detect players at risk for future hamstring injuries.

Introduction

It is an international standard in elite soccer for players to undergo preseason medical examinations and fitness testing. The major aims of these health and performance screening measures are on the one hand to detect and prevent disease. On the other hand, a profile of strengths and weaknesses of motor abilities and skills across a variety of fitness and performance components of individual players is generated to provide baseline physical measurements for designing and implementing training programs, attempting to increase performance and potentially decrease the risk of injury. In fact, a vast number of studies, dealing with the overall injury incidence in soccer players have reported a high number of injuries, particularly for the lower limbs, regardless of the competing level [1, 2]. In this regard, country-specific differences and potential injury mediators have been reported [3]. The question however is, if the same set of laboratory and field-based tests performed to determine performance-related skills can be used to prospectively detect players at risk for future hamstring injuries and furthermore enable the selection of appropriate individualized injury prevention strategies to reduce the perceived risk [4-7].

Hamstring strain injuries (HSIs) are extremely common in soccer players resulting in essential losses in training and playing time [1, 8], impairing individual as well as team success and inducing huge financial costs for soccer clubs and federations [9-11]. Based on prospective studies performed in elite male soccer players in Europe, HSIs accounted for 12–16% of all injuries sustained [12, 13] and for 37% - 47% of all muscle injuries [1, 14], representing the most common non-contact injury. It has been calculated that approximately one in five players will suffer a hamstring injury in any given season. On average, for a professional player, 18 days and three matches are missed due to HSIs, whereas on a club level 90 days and 15 matches will be missed per season, corresponding to 25% of players’ absence in games [15]. A lower injury rate of HSIs has been reported among lower division soccer clubs [16]. Other studies show an exceptionally high recurrence rate of HSIs affecting approximately 30% of players [17]. However, the recurrence proportion differs based on the playing level of soccer players, with lower-level players suffering higher injury recurrence proportions [16]. It has to be noted that over the last decade, a considerable number of strategies to prevent HSIs have been designed [18-21]. Although there is distinct supportive evidence that specifically strategies which include eccentric hamstring exercises can prevent HSIs in elite soccer players [22], the incidence of HSIs in soccer players playing in the UEFA Champions League has increased by 4% per year over the last decade [23]. Based on recent research, 57–72% of all HSIs occurred during high-speed running activities with the long head of the biceps femoris, which is the muscle positioned laterally within the hamstring complex, being the most common injury site [24]. However, other high-speed and power activities such as acceleration, deceleration, kicking, and change of direction activities can also be a trigger for HSIs [25-27]. Although the mechanisms for HSIs are not fully understood, the majority of studies and modelling techniques suggest that the most likely timing of the injury is the late swing-phase of high-speed running. It has been shown that large passive torques affect the hamstring muscles during both the initial and the late swing phase and the mechanisms of injury may relate to the timing of these peak torques [24, 28–30].

A considerable number of researchers have analyzed modifiable and non-modifiable risk factors providing various conclusions. With respect to non-modifiable risk factors, previous HSIs and increasing age have shown to heighten the risk of suffering a HSI by 11.6, and 1.4 times, respectively [31]. Concerning modifiable risk factors, the number of factors potentially causing HSIs is decisively greater, including lower limb eccentric muscle strength, hamstring/quadriceps (H/Q) ratio, proprioception, muscle imbalances, flexibility, balance and agility [32-36]. With the exception of previously suffered HSIs [14, 37], meta-analytical and literature review research have reported inconclusive results for the aforementioned risk factors [14, 23, 38–40].

In light of the high incidence, high rates of recurrence, and financial burden, the identification of risk factors and injury mechanisms related to HSIs is essential to detect injury-prone players and to design measures to successfully prevent injuries. Such individualized conditioning programs must be based on a thorough analysis including assessments to identify areas of weakness within a player’s performance profile. Therefore, this study aimed to prospectively record HSIs in the Kosovo national premier soccer league to investigate the incidence and characteristics of HSIs in male elite soccer players during an entire soccer season. Additionally, this study aimed to identify possible risk factors for HSIs in elite Kosovar soccer players in an adequately powered prospective cohort study. For the present study, it was hypothesized that players with one or more risk factors would be more prone to suffer from HSIs.

Materials and methods

Design & participants

This observational prospective cohort study on physical and injury-related variables was approved by the Ethics Committee of the Medical Faculty of the University of Pristina (ref number 4738) and was completed during the 2013/2014 season of the Kosovo national premier soccer league. It is part of a long-term study consisting of different modules. A previous publication focused on the overall incidence, severity, and injury type in the highest competing level in Kosovo [3]. The same subjects participated in the current study focusing on hamstring injuries which represent the second most common injury subtype following knee injuries. In total 143 elite male soccer players from 11 teams of the Kosovo national premier soccer league fulfilled the inclusion criteria, provided written informed consent to participate, and completed all preseason anthropometric and functional performance test measures. Players suffering from acute lower limb injuries or recovering from recent surgical interventions (within the last 12 months) were excluded. To identify possible prevalent musculoskeletal or medical conditions as well as performance impairments that could possibly increase the risk of HSIs, a widespread health and fitness assessment was performed. As part of this assessment, a comprehensive anamnesis was carried out asking players to describe the nature and date of any previous hamstring strains using the Injury Report Form provided for by the Oslo Sports Trauma Research Centre [2] and to list any residual problems (number of previous hamstring strains; time since most recent injury; training and match time lost through injury). During the prospective study, all participants playing in the entire autumn and spring season were observed by the clubs’ medical staff (either physiotherapists or medical doctors) previously familiarized with the proper use of the questionnaires. To identify total body injury occurrence from where data collection concerning hamstring injuries were extracted, the questionnaire compiled by the Injury Consensus Group established by the FIFA Medical Assessment and Research Centre was used [41]. Continuous reporting, help, and feedback were provided by the medical staff on a weekly basis.

Anthropometric assessment/body composition

Prior to the physical fitness testing, anthropometric measurements were carried out according to international standards for anthropometric assessment [42] including measurements of body mass to the nearest 0.1 kg and body height to the nearest 0.5 cm, using a portable scale and stadiometer equipment (Seca, Hamburg, Germany). Thereafter, the body mass index was calculated [(BMI = weight (kg) / height (m2)].

Fitness assessment

All players underwent a comprehensive fitness test battery, which aimed to identify performance impairments that could be associated with an increased risk of suffering a HSI. Subjects were instructed to avoid any strenuous physical activity or training on the two days prior to testing to minimize the possible effect of fatigue. Prior to all the physical performance tests, except for the sit-and-reach test (SRT), a 15-minute warm-up protocol consisting of various running drills without the ball was conducted. Warm-up was not performed before the SRT to avoid any warm-up effect. This test battery is composed of a set of valid, reliable, and sensitive fitness tests, measuring performance characteristics and/or potentially modifiable risk factors, which might have an influence on HSIs. The same assessor, either physiotherapists or sports scientists trained in measurement techniques and protocols, assessed all players. All tests were completed within one week, beginning with the isokinetic torque measurements performed on one day, the SRT, and the Nordic hamstring strength test (NHST) carried out in the listed order on another day, and finally, the countermovement jump, speed, and agility tests performed on the last testing day. All tests were performed before season kick-off.

Muscle strength and power

To detect imbalances resulting in lowered H/Q ratios, all players were tested using the Biodex System 3 isokinetic dynamometry (Biodex Medical Systems, Shirley, NY, USA). Drawing on the recommendations by Croisier et al. (2008) [43], the testing protocol included several maximum concentric and eccentric exertions of both the hamstrings and quadriceps muscle groups over a range of motion of 100 degrees, interspersed by 2 minutes of passive recovery. Detailed information concerning the test protocol with the repetition number for each velocity is listed in Table 1.

Table 1

Crosier’s isokinetic test protocol.

Muscle group	Contraction mode	Angular velocity (°/s)	Repetitions
H / Q	Concentric	60	3
H / Q	Concentric	240	5
H	Eccentric	30	3
H	Eccentric	120	4

H (Hamstrings); Q (Quadriceps).

In addition, all participants were tested making use of the simple NHST [44]. For this test, the players are on their knees with their ankles fixated on the floor. The participants are instructed to lower their upper body toward the floor in a slow and controlled manner, keeping their backs and hips straight. Hamstring strength was classified as either failed or passed depending on whether the subjects could hold the position beyond 30 degrees from the vertical starting position.

Countermovement jump performance was additionally assessed as a representative measure of leg muscle power. Starting from an upright standing position with their feet shoulder-width apart and hands on their hips throughout the jumps, participants squat down to a knee angle of approximately 90 degrees before jumping up vertically as explosively as possible. Jump heights and indices of movement efficiency and symmetry were calculated automatically using the Leonardo Mechanograph® ground reaction force plate (Leonardo Mechanograph, Galileo Novotec Medical GmbH, Germany). The subjects performed the test three times, separated by 2 minutes between each trial, and the best result was used for further analysis [45].

Speed and agility

Performing change-of-direction maneuvers explosively and efficiently as quickly as possible are fundamental performance aspects in soccer, with these activities being associated with an increase in hamstrings injury susceptibility [18]. In the current study, the Illinois Agility Test (IAT), which has shown to be a reliable and valid test in team sports [46], was used to evaluate change-of-direction speed. This course is 10 meters long and 5 meters wide with four cones in the middle which are placed at a distance of 3.3 meters from the starting line to the end line. Starting from a laying position, the participants were instructed to accomplish the course as quickly as possible. The players had to sprint 10 meters, return to the starting line, make another turn and weave in and out of the 4 markers towards the end line as well as towards the base line, and then complete two 10 meter sprints to finish the agility course. Time was recorded using an electronic timing system (Brower Timing, USA) with infrared timing gates positioned at the start and the finish line. The best time of 3 trials was used for statistical analysis.

In addition to the IAT, a 20- and 40-meter sprint test was used to determine the players’ acceleration and speed. The tests were performed on a natural soccer field. Players started in a standing position, with the toes of the front foot placed on the starting line which was 50 cm behind the first photocell gates. The players were instructed to start at will but were not allowed to perform fluctuation movements before starting. The participants were timed using a wireless timing system (Brower Timing Systems, USA). The best result of three trials separated by 5 minutes of rest was used for analysis.

Flexibility

The SRT was chosen to assess flexibility, as it is a frequently used, valid, and sufficiently reliable field test to measure the flexibility of the lower extremities [47]. To perform the SRT, the athletes sat on the floor with their legs fully extended and the soles of their bare feet resting against a purpose-made sit-and-reach box. The athletes placed one hand on top of the other and were instructed to slowly lean forward as far as possible along the measuring line. The distance reached by the athletes’ fingertips (cm) was recorded. To eliminate warm-up effects, the SRT was only performed once.

Statistical analysis

Descriptive statistics, such as age, anthropometric data, and physical performance tests were used for baseline characteristics and means ± SD were calculated. Players with a previous hamstring injury as well as playing position are presented in percentages. Differences between groups are calculated by independent samples t-test. Comparisons in frequency distribution for non-parametric variables between groups as determined by the Chi2 test are presented in percentages and p-values. Relative isokinetic values are calculated by dividing absolute torque measurements by the body mass of the players. In order to explain the prediction contribution to the overall hamstring injury rate as the dependent variable, multiple linear regression models were developed. The effect size, acknowledged as the quantitative measure of the magnitude of an observed occurrence, was calculated and interpreted as follows: small (0.2–0.3), medium (0.5) or large (>0.8) [48]. The confidence intervals for training, match, and total exposure time with respect to the injury incidence were analyzed using the normal approximation model described by Sahai (1993) [49], whereas for the remaining parameters 95% confidence intervals (CIs) were applied. The exact P values are reported throughout the manuscript and values ≤0.05 were acknowledged as statistically significant. Data were analyzed using the Statistical Package for Social Sciences (SPSS version 21) and the effect size was calculated using G*Power (version 3.1).

Results

Player baseline characteristics

The total sample of 143 players was composed of 7 goalkeepers (4.9%), 27 internal (18.9%) and 20 external (14.0%) defenders, 18 central (12.6%) and 23 external (16.1%) midfielders, 20 wingers (14.0%) and 28 strikers (19.6%). The players’ age, body mass, height, and BMI were 23.2 ± 4.1 yrs, 74.2 ± 6.7 kg, 180.0 ± 5.3 cm, and 22.9 ± 1.7 kg∙m-2, respectively. 129 (90.2%) players specified their right leg, and 14 (9.8%) their left leg as the dominant leg, which was defined as the leg they preferably used to kick the ball.

Training and match exposure

Over the entire study period, a total of 36,833 hours of exposure time, consisting of 31,998 hours of training and 4,834 hours of match play, were registered. On average, players participated in 25.3 ± 4.0 matches and attended 149.2 ± 14.3 training sessions. This resulted in a mean exposure time of 257.6 ± 24.9 hours, including 33.8 ± 8.9 hours (13.1%) of match play and 223.8 ± 21.5 hours (86.9%) of training per player.

Hamstring injury incidence

There were 43 HSIs in total (16 training; 27 match) as shown in Table 2. The total injury incidence for the hamstring muscles was 1.17 injuries/1000 hours (95% CI, 0.84–1.57). The injury incidence out of a total of 36,833 exposure hours was higher in matches 5.59 injuries/1000 hours (95% CI, 3.68–8.13) which is 11.2 times higher compared to training injury incidence, at 0.50 injuries/1000 hours (95% CI, 0.29–0.81). There were no differences in injury incidence concerning the players’ position on the soccer field (p = 0.258). On average 27.9% of 143 players sustained at least one hamstring injury whilst three players suffered bilateral strains.

Table 2

Injury incidence per training and match time exposure hours.

	Exposure hours x 1000	Injury incidence (95% CI)
Total HSIs incidence	43 injuries / 36833 hours x 1000	1.17 (0.84–1.57)
Training	16 injuries / 31998 hours x 1000	0.50 (0.29–0.81)
Match	27 injuries / 4834 hours x 1000	5.59 (3.68–8.13)

Training and match days lost due to HSIs were 643 in total. Injuries occurring on match days accounted for 431 lost training days, whereas injuries sustained in training accounted for 212 days of training absence. The re-injury rate of HSIs was 23%.

An independent-samples t-test was conducted to compare injured and non-injured players for age, anthropometry, exposure time, isokinetic performance, and functional parameters. As presented in Table 3, there was a significant difference in age between injured and non-injured players, with the injured players being older (p < 0.001). The body mass and BMI values were significantly higher among the injured compared to non-injured players (body mass, p = 0.002; BMI, p = 0.002). Height on the other hand did not differ between categories (p = 0.505). Based on Pearson Chi-Square test results, it can be noticed that there is a statistically significant difference in the passing rate of the NHST with a lower passing rate among the injured compared to non-injured players (p = 0.001), and a higher previous hamstring injury rate (p = 0.023), respectively. Independent samples t-test revealed no significant differences with respect to training, match and total exposure time between groups [(training, p = 0.154); (match, p = 0.214); (total, p = 0.432)].

Table 3

Characteristics according to age, anthropometric and physical fitness parameters between HSIs injured and non-injured players.

	Not injured (n = 103)	Injured (n = 40)	η2 (95% CI)	p-Value (t-tests)
	Mean ± SD	Mean ±SD
Age (years)	22.2 ± 3.9	26.1 ± 3.4	1.06 (-6.45 to -3.96)	<0.001***
Body mass (kg)	73.2 ± 6.6	77.0 ± 6.1	0.60 (-4.79 to 0.06)	0.002**
BMI (kg/m2)	22.6 ± 1.7	23.6 ± 1.5	0.62 (-1.68 to -0.46)	0.002**
Height (cm)	179.8 ± 5.6	180.5 ± 4.7	0.13 (-0.62 to 3.27)	0.505
NHST (no/yes, % of total)	30/73 (21)	24/16 (16.8)	(no: 1.14 to 1.31/ yes: 1.62 to 1.89)	0.001**
Previous HSIs (no/yes, % of total)	24/79 (23.3)	17/23 (42.5)	(no: 0.15 to 0.32/ yes: 0.26 to 0.59)	0.023*
Exposure time training (hours)	225.4 ± 20.0	219.6 ± 24.9	0.26 (-6.92 to 8.88)	0.154
Exposure time match (hours)	33.2 ± 9.0	35.3 ± 8.5	0.24 (-3.97 to 2.57)	0.214
Total exposure time (hours)	258.6 ± 23.3	254.9 ± 28.7	0.14 (-8.85 to 9.42)	0.432
Absolute concentric torque (60°/s)
Hamstring torque, dominant (Nm)	129.4 ± 22.4	132.5 ± 23.3	0.14 (-6.10 to 10.45)	0.467
Hamstring torque, non-dominant (Nm)	126.4 ± 22.2	130.6 ± 17.1	0.21 (-4.88 to 10.48)	0.282
Quads torque, dominant (Nm)	224.3 ± 36.0	223.1 ± 30.2	0.04 (5.56 to 30.10)	0.820
Quads torque, non-dominant (Nm)	226.8 ± 35.7	224.3 ± 35.8	0.07 (-3.73 to 22.23)	0.707
H/Q ratio, dominant (%)	58.1 ± 7.7	59.9 ± 10.2	0.20 (7.03 to -0.95)	0.245
H/Q ratio, non-dominant (%)	56.1 ± 8.3	59.8 ± 13.3	0.33 (-4.51 to 2.83)	0.044*
Absolute concentric torque (240°/s)
Hamstring torque, dominant (Nm)	100.4 ± 20.2	105.9 ± 23.8	0.25 (-4.36 to 11.23)	0.162
Hamstring torque, non-dominant (Nm)	98.2 ± 19.9	98.4 ± 21.6	0.01 (-5.50 to 9.40)	0.969
Quads torque, dominant (Nm)	142. 2 ± 21.8	144.9 ± 23.8	0.12 (2.98 to 18.95)	0.509
Quads torque, non-dominant (Nm)	142.1 ± 23.1	141.9 ± 21.1	0.01 (-6.21 to 10.30)	0.945
H/Q ratio, dominant (%)	71.3 ± 12.9	73.4 ± 12.3	0.17 (-8.03 to 1.23)	0.393
H/Q ratio, non-dominant (%)	69.5 ± 10.6	70.4 ± 16.6	0.06 (-3.51 to 5.67)	0.711
Absolute eccentric torque
Hamstring torque (ecc, 30°/s) dominant (Nm)	121.2 ± 45.3	127.0 ± 43.4	0.13 (-16.02 to 16.81)	0.489
Hamstring torque (ecc, 30°/s) non-dominant (Nm)	114.8 ± 44.9	118.2 ± 43.6	0.08 (-15.06 to 17.52)	0.688
Hamstring torque (ecc, 120°/s) dominant (Nm)	94.1 ± 48.9	101.8 ± 49.7	0.16 (-26.61 to 9.11)	0.400
Hamstring torque (ecc,120°/s) non-dominant (Nm)	92.1 ± 42.9	102.6 ± 44.7	0.24 (-21.01 to 10.83)	0.195
Absolute eccentric torque
Hamstring torque (ecc, 30°/s) dominant (Nm)	121.2 ± 45.3	127.0 ± 43.4	0.13 (-16.02 to 16.81)	0.489
Hamstring torque (ecc, 30°/s) non-dominant (Nm)	114.8 ± 44.9	118.2 ± 43.6	0.08 (-15.06 to 17.52)	0.688
Hamstring torque (ecc, 120°/s) dominant (Nm)	94.1 ± 48.9	101.8 ± 49.7	0.16 (-26.61 to 9.11)	0.400
Hamstring torque (ecc,120°/s) non-dominant (Nm)	92.1 ± 42.9	102.6 ± 44.7	0.24 (-21.01 to 10.83)	0.195
Functional tests
Countermovement jump (cm)	44.9 ± 4.9	44.4 ± 4.9	0.10 (-0.96 to 2.62)	0.581
Illinois agility test (s)	15.64 ± 0.62	15.65 ± 0.56	0.02 (-0.38 to 0.06)	0.905
20 m dash (s)	3.12 ± 0.16	3.14 ± 0.12	0.14 (-0.11 to 0.00)	0.643
40 m dash (s)	5.51 ± 0.27	5.57 ± 0.26	0.23 (-0.18 to 0.01)	0.256
Sit & reach (cm)	28.3 ± 5.3	29.5 ± 5.5	0.22 (-0.70 to 3.20)	0.239

Data are shown as mean ± standard deviation; differences between groups are calculated by independent samples t-test. Significant differences between hamstring injury group and not injured category are marked by asterisks (*p<0.05, **p<0.01, ***p<0.001). Differences in frequency distribution between groups were determined by Chi2 test.

Abbreviations: Q (Quadriceps), H (Hamstrings); NHST (Nordic Hamstring Strength Test); “Dominant” refers to the kicking leg; η2 refers to partial eta squared; CI confidence intervals.

When comparing dominant and non-dominant legs in terms of absolute concentric hamstring torque at 60°/sec, no significant differences were observed between categories (dominant, p = 0.467; non-dominant, p = 0.282). Also, no cohort differences were revealed for the quadriceps torque at 60°/sec in both legs (dominant, p = 0.820; non-dominant, p = 0.707). In addition, the H/Q ratio for the dominant leg was not significantly different between injured and non-injured players (p = 0.245), whereas significantly higher H/Q ratio values were observed among the injured players for the non-dominant leg (p = 0.044). However, there was no difference in absolute concentric torque for hamstrings, quadriceps and H/Q ratio between injured and non-injured players at 240°/sec [(hamstrings dominant, p = 0.162); (hamstrings non-dominant, p = 0.969); (quadriceps dominant, p = 0.509); (quadriceps non-dominant, p = 0.945); (H/Q ratio dominant, p = 0.393); (H/Q ratio non-dominant, p = 0.711)]. Eccentric torque at 30°/sec [(dominant, p = 0.489); (non-dominant, p = 0.688)], and at 120°/sec [(dominant, p = 0.400); (non-dominant, p = 0.195)] was also not significantly different between categories. Further, no differences were found with respect to functional test parameters (countermovement jump, p = 0.581; Illinois agility test, p = 0.905, 20-meter dash, p = 0.643, 40-meter dash, p = 0.256, and sit-and-reach, p = 0.239).

Independent samples t-test did not reveal significant differences for relative concentric hamstring strength at 60°/sec (dominant, p = 0.460); (non-dominant, p = 0.649), whereas for the quadriceps torque, the injured players attained significantly lower values for both legs compared to non-injured ones (dominant, p = 0.039; non-dominant, p = 0.025) (Table 4). In addition, the level of relative concentric hamstring and quadriceps strength produced at 240°/sec was not significantly different between cohorts (p = 0.865). However, there was a tendency towards a better performance for the non-injured category (p = 0.055). On the other hand, the differences for relative eccentric hamstring strength were even smaller at 30°/sec [(dominant: injured, p = 0.903); (non-dominant: injured, p = 0.733)], and 120°/sec [(dominant: injured, p = 0.849); (non-dominant: injured, p = 0.509)], respectively.

Table 4

Isokinetic values relative to body mass.

	Not injured (n = 103) Mean ± SD	Injured (n = 40) Mean ± SD	η2 (95% CI)	p-Value (t-tests)
Relative concentric torque (60°/s)
Relative H torque, dominant (Nm/kg)	1.77 ± 0.27	1.73 ± 0.34	0.13 (-0.01 to 0.20)	0.460
Relative H torque, non-dominant (Nm/kg)	1.73 ± 0.27	1.70 ± 0.26	0.11 (0.01 to 0.20)	0.649
Relative Q torque, dominant (Nm/kg)	3.07 ± 0.42	2.91 ± 0.40	0.39 (0.20 to 0.50)	0.039*
Relative Q torque, non-dominant (Nm/kg)	3.10 ± 0.42	2.92 ± 0.46	0.41 (0.10 to 0.38)	0.025*
Relative concentric torque (240°/s)
Relative H torque, dominant (Nm/kg)	1.37 ± 0.24	1.38 ± 0.32	0.03 (-0.00 to 0.21)	0.865
Relative H torque, non-dominant (Nm/kg)	1.34 ± 0.24	1.28 ± 0.29	0.22 (-0.02 to 0.20)	0.220
Relative Q torque, dominant (Nm/kg)	1.94 ± 0.26	1.88 ± 0.32	0.21 (0.11 to 0.30)	0.278
Relative Q torque, non-dominant (Nm/kg)	1.94 ± 0.26	1.85 ± 0.27	0.34 (-0.01 to 0.18)	0.055
Relative eccentric torque
Relative H torque, dominant (30°/s, Nm/kg)	1.67 ± 0.62	1.65 ± 0.56	0.03 (-0.16 to 0.28)	0.903
Relative H torque, non-dominant (30°/s, Nm/kg)	1.58 ± 0.63	1.54 ± 0.58	0.07 (-0.16 to 0.28)	0.733
Relative H torque, dominant (120°/s, Nm/kg)	1.30 ± 0.68	1.32 ± 0.64	0.03 (-0.31 to 0.18)	0.849
Relative H torque, non-dominant (120°/s, Nm/kg)	1.26 ± 0.58	1.33 ± 0.57	0.12 (-0.23 to 0.19)	0.509

Data are shown as mean ± standard deviation; differences between groups are calculated by independent samples t-test. Significant differences between hamstring injury group and not injured category are marked by asterisks (*p<0.05, **p<0.01, ***p<0.001).

Abbreviations: Q (Quadriceps), H (Hamstrings); “Dominant” refers to the kicking leg

Table 5 presents the results derived from the multiple linear regression models, which partially explain the variation for injury incidence as the outcome variable. Model 1 shows that age alone accounted for 18.1% of variation. Adding the NHST to the model (model 2) resulted in improved predictability which accounted for 21.3% of the variation of injury incidence. Besides age and the NHST, the addition of the previous HSIs further improved the determination effect (model 3), accounting for 23.8%. Finally, the addition of concentric hamstring strength of the dominant leg at 240°/sec along with the parameters from the latter models increased the predictability of the hamstring injury to 25.9%. Body composition, isokinetic, functional tests (except for the dominant leg at 240°/sec), and playing position did not improve the predictability outcome of hamstring injuries and were not reported as models by the statistical analysis.

Table 5

Multiple linear regression models.

	Models	β ± SE	(95%`CI)	p-Value	Adjusted R2 (%)
Hamstring injury rate	1				18.1
	Constant	- 0.79 ± 0.20	-1.18 to -0.41	<0.001
	Age	0.05 ± 0.01	0.03 to 0.06	<0.001
	2				21.3
	Constant	-0.90 ± 0.2	-1.30 to -0.52	<0.001
	Age	0.04 ± 0.01	0.02 to 0.06	<0.001
	NHST	0.20 ± 0.07	0.03 to 0.31	0.017	23.8
	3
	Constant	-1.21 ± 0.24	-1.68 to 0.73	<0.001
	Age	0.05 ± 0.01	0.03 to 0.07	<0.001
	NHST	0.25 ± 0.08	0.09 to 0.40	0.002
	Previous hamstring injury	-0.21 ± 0.10	-0.41 to -0.01	0.035
	4				25.9
	Constant	-1.53 ± 0.29	-2.10 to -0.96	<0.001
	Age	0.05 ± 0.01	0.03 to 0.07	<0.001
	NHST	0.26 ± 0.08	0.11 to 0.42	0.001
	Previous hamstring injury	-0.21 ± 0.10	-0.40 to -.0.01	0.037
	Con. HS (240°/s–dominant leg)	0.01 ± 0.01	0.00 to 0.01	0.049

Covariates are listed with parameter estimates and standard errors (β±SE) and with p-values. Con. HS (Concentric Hamstring Strength); NHST (Nordic Hamstring Strength Test).

Discussion

The specific objectives of the present study were twofold. Firstly, it was attempted to prospectively investigate the hamstring injury profile and the incidence rate per 1000 hours exposure time during training and league matches in the Kosovo premier soccer league. The second aim was to assess if and which tests of a pre-season exercise test battery could be used to predict non-contact hamstring injuries.

In a previous epidemiological study among elite Kosovar soccer players, led by the group of the current study, it was shown that the overall injury incidence was approximately 20% lower compared to Western and Northern European soccer players [3]. However, when it comes to hamstring injuries in particular, the injury rate in the current investigation was 1.17 injuries per 1000 hours (95% CI, 0.84 to 1.57). This is more or less similar to those reported by Ekstrand et al. (2016) [8] for high-level European soccer players [(1.20), (95% CI 1.14 to 1.26)] over 13 years. The results are also comparable regarding training injury rates [(0.50 injuries/1000 hours (95% CI, 0.29–0.81)] for Kosovo players versus [(0.51), (95% CI, 0.47 to 0.55)] for UEFA elite players. However, in the present investigation, the match injury rate tended to be higher [(5.59 injuries/1000hours), (95% CI, 3.68 to 8.13)] compared to professional UEFA players [(4.77), (95% CI, 4.49 to 5.06)]. This fact is surprising as the overall training and match load is substantially smaller for Kosovo players [3]. However, Kosovo is an underdeveloped state and the socioeconomic situation differs substantially compared to more developed countries and proper soccer infrastructure is still missing. Therefore, factors such as poor field conditions, decreased strength and conditioning status, as well as the lack of medical and sufficient coaching staff, which are all due to the lower professionalism, might have contributed to a higher injury incidence. Indeed, Iacovelli and colleagues (2013) found that when surface conditions of the natural playing soccer fields were poor, lower extremity injury rate was 2.61 times higher in comparison to normal field conditions [50]. In addition, at the time when the study was conducted, Kosovo was not a member of the International Federation of Football Associations (FIFA) or the Union of European Football Associations (UEFA), therefore the playing level might be considered less professional. Notably, injury occurrence and recurrence among amateur players, particularly during training, was reported to be higher in comparison with professionals [16, 51]. Furthermore, the physical preparedness of Kosovo players may be more precarious since to date strength and conditioning coaches licensed by the Kosovo Football Federation, FIFA or UEFA are still missing. In fact, strength and conditioning coaches might act as observers and correctors of exercise techniques aiming to develop a strong and resilient athlete and can implement scientific driven prevention programs [52].

In line with previous findings [2, 14, 25, 53–55], the current study also found a significant difference in the injury incidence between older-aged and previously injured players compared to younger and non-injured ones. Although HSIs are also well known to the younger athletic population, functional and structural changes particular to aging athletes diminish the ability to adapt to high levels of loading and make them more susceptible to certain pathological conditions that affect both muscles and tendons [56]. Previous injuries to the fascial system and injury-related interruption of the training process cause a significant loss of performance both in younger and older individuals. Greater decrements, however, are observed in rapid muscle force capacity in older individuals, who also appear to have an impaired capability to fully recover following injuries [57, 58]. A possible reason could be a higher co-contraction time of the antagonist lower limb muscles, a lower efficiency, and an uneven transmission of the forces within the fascicle compared to younger players [59, 60]. In addition, the passing rate of the NHST in the current study was significantly lower among injured compared to non-injured players. However, studies analyzing eccentric knee flexor strength found no association with an increased risk of future HSIs [23, 61]. This could, to some extent, explain the absence of differences in isokinetic eccentric hamstring torque in the present study between injured and non-injured players. Therefore, the lower passing rate of NHST might be attributed to other confounding factors rather than the lack of eccentric strength of the hamstrings. Yet, the effect of age, passing rate of NHST, and the previous injury history were shown to have a deterministic effect on HSIs in the current and previous investigations [31, 62], whereas except for concentric hamstring strength at 240°/s for the dominant leg, the other isokinetic and functional risk factors did not show a significant predictability impact in the current study.

A study conducted by Bakken et al. (2018) revealed that greater quadriceps concentric peak torque at 60°/sec correlated with the risk of overuse injuries [7]. However, van Dyk and peers (2016) reported that quadriceps concentric and hamstring eccentric strength at 60°/sec were low risk factors for HSIs [40]. In addition, Lee et al. (2018) suggested that measurements at lower isokinetic speeds (concentric, 60°- and eccentric, 30°/sec) might be useful preseason screening protocols to detect players who might be at risk of sustaining HSIs [63]. In the current investigation, the absolute quadriceps concentric peak torque was not different between injured and non-injured players, whereas the relative quadriceps torque of the same testing velocity was significantly lower among the injured players. However, the discrepancies between our findings and other author’s findings may have occurred due to different protocols and testing methodology, rehabilitation programs after injury, and physical preparedness, which make it challenging for the results to be compared. Hamstring and quadriceps peak and relative torques at 240°/sec were not different between cohorts in the current study. Similar results were also reported by Lee (2018) [63] suggesting that high isokinetic testing speeds may not be good predictors of HSIs.

Except for absolute concentric torque H/Q ratio where the peak torque production of the non-dominant leg of the previously injured category was slightly but significantly higher than the non-injured category, there were no other differences observed in H/Q ratio between cohorts in the present study. It has to be noted that the scientific evidence is not consistent when it comes to differences in H/Q ratio between dominant and non-dominant legs [64, 65]. However, the authors consider that this difference might be attributed to the compensatory adaptation of the non-dominant leg (balancing leg) which acts as the weight-bearing limb during the execution of the technical tasks against the dominant one. Based on cut-off points established by Croisier et al. (2008) who suggest that an H/Q ratio lower than 0.55% would classify players at increased risk of suffering HSIs [43], both categories were not classified at increased risk in the current study, as the values were higher than Croisier’s cut-off points (all > 0.55%). This was also shown in other papers with authors reporting that H/Q ratio may not be considered a reliable risk factor at all [40, 66]. However, other investigations [43, 67, 68] have identified the H/Q ratio as a reliable risk factor for HSIs, yet with disputes concerning the interpretation of this parameter. Two of the largest studies concerning the HSIs prediction in soccer players found mismatching results concerning the H/Q ratio [40, 43] making it difficult to determine whether this parameter could be considered a valid and reliable risk factor.

The present study revealed no difference in functional and motoric tests between injured and non-injured players during acceleration, maximal running speed, agility, and countermovement jump tests. The ability to perform and to sustain multiple brief high-intensity intermittent activities and continuous changes of direction during match play is a key attribute for soccer performance. However, this performance profile has shown to result in temporary neuromuscular fatigue specifically, which might increase the risk of HSIs at the end of each half of soccer matches [15, 69]. In fact, it has been previously shown that fatigue can affect neuromuscular control and multi-muscle coordination patterns resulting in altered lower limb biomechanics as well as running and movement kinematics [70-73], factors which could be related to hamstring injuries.

Insufficient neuromuscular and kinetic chain control has been proposed as a risk factor for non-contact injuries [70]. Supporting earlier findings from Chumanov et al. (2007) [74-76], Schuermans et al. (2016, 2017) and more recently (2018) [20] observed that deficient core stability and lumbopelvic control during sprinting, specifically an inefficient posterior chain muscle recruitment pattern, might result in an increased hamstring injury risk. However, it has been criticized that prospective risk factor studies assessing the effect of lumbopelvic motion on hamstring injury rates are widely missing [19]. Additionally, previous injuries, either hamstring injuries or injuries to other body parts, might influence neuromuscular coordination and control. Malliaropoulos et al. (2018) [77] recently reported a statistically significant interdependence of HSIs with previous traumatic ankle ligament injuries in track and field athletes. Therefore, a prior injury to the lower limbs could be a predisposing factor for future HSIs. Our study population showed to have a high recurrence rate of HSIs, confirming this association.

At this point, it is important to note that testing in the current study was performed in a non-fatigued state. However, injury risk in general and hamstring risk more specifically cannot be captured by an assessment performed in a rested state but ideally should be conducted in a fatigued state creating an ecologically valid environment. This indicates, that injury risk assessment should not be performed simultaneously with pre-season exercise or physiological assessments or strength and conditioning screening. These tests aim to provide information about the actual fitness status of the players or to determine the progression and effectiveness of a training program. However, assessments focusing on identifying players at risk of HSIs need to have an entirely different emphasis which is specifically tailored to detecting the risk factors of HSIs.

Successful identification of injury predictors or dysfunction forms the basis of effective preventive measures. However, such appropriate injury prevention strategies can only be developed based on a comprehensive understanding of potential prognostic factors and based on valid and reliable tests for predicting hamstring injuries [6]. Despite a novel and comprehensive methodology including a homogenous study population, and a vast number of field and laboratory tests, some potential limitations are worth noting. Notably, a longer monitoring period could provide a better prediction depiction of HSIs. Further, different diagnostic, prevention, and rehabilitation protocols implemented by the teams’ staff could have influenced the results. Therefore, further research including standardized diagnostic, preventive, and load monitoring measures are of utmost importance in order to better predict HSIs in soccer players.

Conclusions

The main findings of this prospective study revealed that a battery of laboratory and field-based tests performed during pre-season to determine performance-related skills is of limited diagnostic conclusiveness, preventing the detection of players at risk of future hamstring injuries. Subsequently, appropriate individualized injury prevention strategies generated to reduce the perceived risk need to be created. Age, NHST, previous injury history, and concentric hamstring strength at 240°/sec resulted to be the best prediction factors of HSIs. Hence, it is obvious that hamstring injuries are a multifactorial problem involving the interaction of various confounding components that heighten the risk of hamstring injuries, posing the demands for further research in detecting the best predicting model of HSIs.

(SAV)

Click here for additional data file.

26 Nov 2019

PONE-D-19-20247

Potential prognostic factors for hamstring muscle injury in elite male soccer players: a prospective study

PLOS ONE

Dear Univ.-Prof. Dr. Tschan,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

We would appreciate receiving your revised manuscript by Jan 10 2020 11:59PM. When you are ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter.

To enhance the reproducibility of your results, we recommend that if applicable you deposit your laboratory protocols in protocols.io, where a protocol can be assigned its own identifier (DOI) such that it can be cited independently in the future. For instructions see: http://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols

Please include the following items when submitting your revised manuscript:

A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). This letter should be uploaded as separate file and labeled 'Response to Reviewers'.

A marked-up copy of your manuscript that highlights changes made to the original version. This file should be uploaded as separate file and labeled 'Revised Manuscript with Track Changes'.

An unmarked version of your revised paper without tracked changes. This file should be uploaded as separate file and labeled 'Manuscript'.

Please note while forming your response, if your article is accepted, you may have the opportunity to make the peer review history publicly available. The record will include editor decision letters (with reviews) and your responses to reviewer comments. If eligible, we will contact you to opt in or out.

We look forward to receiving your revised manuscript.

Kind regards,

Yumeng Li

Academic Editor

PLOS ONE

Journal Requirements:

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

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

http://www.journals.plos.org/plosone/s/file?id=wjVg/PLOSOne_formatting_sample_main_body.pdf and http://www.journals.plos.org/plosone/s/file?id=ba62/PLOSOne_formatting_sample_title_authors_affiliations.pdf

2. We note that your study is closely related to the following publication, on which you are an author:

https://bmcmusculoskeletdisord.biomedcentral.com/articles/10.1186/s12891-016-1202-9

Although you have briefly cited the above study in the methods section of your article, we feel that the scientific rationale of the current study and the contribution that it makes to the field should be justified better. Therefore, please cite and discuss the above study in the introduction, methods and discussion sections of your manuscript, clarifying how the present work is related to the previously published paper.

Please note that our second publication criterion states that "If a submitted study replicates or is very similar to previous work, authors must provide a sound scientific rationale for the submitted work and clearly reference and discuss the existing literature. Submissions that replicate or are derivative of existing work will likely be rejected if authors do not provide adequate justification." http://www.plosone.org/static/publication.action#results.

Thank you for your attention to this request.

3.  We note that you have indicated that data from this study are available upon request. PLOS only allows data to be available upon request if there are legal or ethical restrictions on sharing data publicly. For information on unacceptable data access restrictions, please see http://journals.plos.org/plosone/s/data-availability#loc-unacceptable-data-access-restrictions.

In your revised cover letter, please address the following prompts:

a) If there are ethical or legal restrictions on sharing a de-identified data set, please explain them in detail (e.g., data contain potentially identifying or sensitive patient information) and who has imposed them (e.g., an ethics committee). Please also provide contact information for a data access committee, ethics committee, or other institutional body to which data requests may be sent.

b) If there are no restrictions, please upload the minimal anonymized data set necessary to replicate your study findings as either Supporting Information files or to a stable, public repository and provide us with the relevant URLs, DOIs, or accession numbers. Please see http://www.bmj.com/content/340/bmj.c181.long for guidelines on how to de-identify and prepare clinical data for publication. For a list of acceptable repositories, please see http://journals.plos.org/plosone/s/data-availability#loc-recommended-repositories.

We will update your Data Availability statement on your behalf to reflect the information you provide.

Additional Editor Comments:

Based on reviewers' comments, some major revisions are needed before publication. Also, please check grammar/spelling throughout the paper.

[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

Reviewer #3: Partly

Reviewer #4: Yes

**********

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

Reviewer #1: Yes

Reviewer #2: N/A

Reviewer #3: No

Reviewer #4: Yes

**********

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

Reviewer #3: No

Reviewer #4: No

**********

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

Reviewer #3: Yes

Reviewer #4: Yes

**********

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: Summary

This is a comprehensive study that has investigated the incidence and characteristics of hamstrings injury in professional level soccer players. This is certainly an area that requires investigation given the incidence in hamstring injuries in this population and the impact on player health and wellness, and the revenue associated with players missing time on the field. Clearly, if definitive prognostic markers could be identified and prevention measures could be strategically employed to reduce the risk of hamstring injury, this would have enormous benefit in this field. Thus, the study and the overall area of research was definitely warranted and the authors did a comprehensive job at evaluating their data and attempting to identify these areas. Unfortunately, this study could not identify any battery of tests and field based tests that could conclusively diagnose increased risk of hamstring injury. Based on these findings, this study confirms that hamstring injuries are a multifaceted problem with numerous variables that interact to contribute towards injuries.

Minor Points

There was a difference in the H/Q absolute concentric torque on the non-dominant leg and this was not addressed in the discussion section. This reviewer suggests the authors attempt to explain this finding perhaps in the context of a compensatory adaptation of the non-dominant leg in an attempt to potentially protect against' injury (perhaps address at lines 387-394).

Grammatical (Gr) and Spelling (Sp)

line 116-117-Gr-"might be a safe treat?"

line 124-Gr-"ex ante"

line 126-Sp"suing" should be using

line 129-Gr-insert "a" before weekly

line 136- missing close bracket after m2

line 324-dash between two-fold

line 372-Sp-grater to greater

Table 4 Relative Q torque, dominant (60deg/s) should be 3.07 and not 30.7

Reviewer #2: General comment

The authors provided us with data about factors that can explain hamstring injuries in high level soccer teams. There is a good attempt to congregate in a single study a large number of field and lab tests that are able to predict hamstring injury. However, the time and the way how the intervention was implemented leave us some doubts about the real novelty of the study. I leave to the authors some comments for consideration.

Specific comments

The paper needs some work to improve the English sentence structure. There are several grammar errors. Please seek some further assistance on overall manuscript.

Are HIS determined by team level and division positioning? I think that you should refer to this in your introduction section.

Line 91: please provide the age threshold that you refer as being a non-modifiable risk factor for HSI.

Do you think that using a sample from Kosovo soccer league can explain HSI in a high standard level such as Champions League teams? Are Kosovo teams with good and enough technical teams and/or medical support to prescribe the most appropriate tests and/or methods for injury prevention?

I understand that you used data from 2013-2014 soccer season. But, from those previous days until now there was a large increase in scientific evidence regarding soccer injuries, with HSI are included. Don’t you think that such kind of experimental data can be outdated? I refer to this aspect because, during this time period, the training methods were changing as well. Please reflect on that.

Please add a hypothesis after the aim of your study.

What were the inclusion criteria of your subjects?

Please provide detailed information about the testing distribution. Where those tests performed in a single day? How many times where performed in a single season?

Authors should report the confidence limit for statistical significance. Also should consider to add an effect size measure to strengthen their statistical analysis.

From your results I may conclude that HSI are most determined by non-match factors (e.g. age, body mass, Nordic test and previous injuries). They cannot avoid the “age factor” at some point of their career. You didn’t dissected the different components of body mass, not allowing to understand how those 70 kg where attributed to lean and fat mass. At the end, the higher hamstring strength given by Nordic exercise, and the previous injuries were the factors with differences. But, from my perspective, this is not new when considering data from previous studies.

At the end I see your study as descriptive of the HSI occurrence in a single soccer league. In my opinion one more interesting approach would be to define a deterministic model that would identify the most important factors for HSI occurrence in the injury cohort, giving to us the exact contribution for HSI appearance.

Reviewer #3: This is a valuable observational prospective cohort study that examined 143 elite male soccer players. However, there are some issues that needs to be addressed:

Line 203-204: The sentence has a grammatical error, rewrite the sentence.

Line 249-254: Specify the units of the parameters

In the statistical analysis: the researchers only compared parameters between injured and uninjured soccer players and the factors that had significant differences between the two groups have reported as the main parameters for predicting injuries. However, the regression analysis can show the most important factors that can predict the occurrence of HIS. It recommends to do statistical analysis by this method (Regression analysis) and report the most important factors of HIS injuries and discuss these parameters.

In the results part: there are no need to rewrite all the results, as they are depicted in the table 3 and 4. It suggests to add one more column into this table and write the value of the t-test in this column and delete the repeated results.

In the results part: Specify the effect size and observed power of t- test analysis in two more column of tables.

Reviewer #4: The authors investigate the incidence and characteristics of hamstring injuries in elite male football players in the Kosovo, and found that laboratory and field-based tests are limited in prospectively determine the risk of hamstring injuries. The study design, perform, data preparation, and manuscript writing were good. However, I can’t get any new findings or from this paper, it seems that this manuscript just repeat the previous study. Although there was no investigation in the Kosovo football players for hamstring injuries risk and this study found passing rate of NHST, H/Q ratio for prospectively determine the risk of hamstring injuries were different from the previous studies, it’s hard to use these new findings such as for other players. Thus, it’s not recommened to publish this manuscript.

**********

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

Reviewer #3: No

Reviewer #4: Yes: Xi Chen

[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 to be viewed.]

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 us at figures@plos.org. Please note that Supporting Information files do not need this step.

14 Jun 2020

Journal Requirements:

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

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

http://www.journals.plos.org/plosone/s/file?id=wjVg/PLOSOne_formatting_sample_main_body.pdf and http://www.journals.plos.org/plosone/s/file?id=ba62/PLOSOne_formatting_sample_title_authors_affiliations.pdf

2. We note that your study is closely related to the following publication, on which you are an author:

https://bmcmusculoskeletdisord.biomedcentral.com/articles/10.1186/s12891-016-1202-9

Although you have briefly cited the above study in the methods section of your article, we feel that the scientific rationale of the current study and the contribution that it makes to the field should be justified better. Therefore, please cite and discuss the above study in the introduction, methods and discussion sections of your manuscript, clarifying how the present work is related to the previously published paper.

Please note that our second publication criterion states that "If a submitted study replicates or is very similar to previous work, authors must provide a sound scientific rationale for the submitted work and clearly reference and discuss the existing literature. Submissions that replicate or are derivative of existing work will likely be rejected if authors do not provide adequate justification." http://www.plosone.org/static/publication.action#results.

Thank you for your attention to this request.

The authors would like to thank the Editor and the Reviewers for their positive and thought-provoking comments, which have improved the overall quality of the paper. The manuscript has been changed based on the reviewer’s comment and our point-by-point responses, highlighting the changes made are outlined below. In the text of the revised manuscript the color highlights the modified sentences.

The paper that the Editor is referring to has dealt with the epidemiology of the overall injuries in soccer players with no particular emphasis neither on hamstring injuries (HSIs), nor in attempting to predict HSIs. The first publication did not provide data concerning the assessed physical fitness parameters, and the data collection typology was completely different as described in the methodology of both articles.

As requested by the Editor, a reasonable justification of the first publication can be found in the introduction (page 3, lines 64-65) as follows:

In this regard, country-specific differences and potential injury mediators have been reported.

3. We note that you have indicated that data from this study are available upon request. PLOS only allows data to be available upon request if there are legal or ethical restrictions on sharing data publicly. For information on unacceptable data access restrictions, please see http://journals.plos.org/plosone/s/data-availability#loc-unacceptable-data-access-restrictions.

In your revised cover letter, please address the following prompts:

a) If there are ethical or legal restrictions on sharing a de-identified data set, please explain them in detail (e.g., data contain potentially identifying or sensitive patient information) and who has imposed them (e.g., an ethics committee). Please also provide contact information for a data access committee, ethics committee, or other institutional body to which data requests may be sent.

b) If there are no restrictions, please upload the minimal anonymized data set necessary to replicate your study findings as either Supporting Information files or to a stable, public repository and provide us with the relevant URLs, DOIs, or accession numbers. Please see http://www.bmj.com/content/340/bmj.c181.long for guidelines on how to de-identify and prepare clinical data for publication. For a list of acceptable repositories, please see http://journals.plos.org/plosone/s/data-availability#loc-recommended-repositories.

We will update your Data Availability statement on your behalf to reflect the information you provide.

We thank the Editor for raising this issue and we have uploaded the anonymized data set entitled as Supporting Information.

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

Reviewer #3: Partly

Reviewer #4: Yes

________________________________________

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

Reviewer #1: Yes

Reviewer #2: N/A

Reviewer #3: No

Reviewer #4: Yes

________________________________________

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

Reviewer #3: No

Reviewer #4: No

________________________________________

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

Reviewer #3: Yes

Reviewer #4: Yes

________________________________________

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: Summary

This is a comprehensive study that has investigated the incidence and characteristics of hamstrings injury in professional level soccer players. This is certainly an area that requires investigation given the incidence in hamstring injuries in this population and the impact on player health and wellness, and the revenue associated with players missing time on the field. Clearly, if definitive prognostic markers could be identified and prevention measures could be strategically employed to reduce the risk of hamstring injury, this would have enormous benefit in this field. Thus, the study and the overall area of research was definitely warranted and the authors did a comprehensive job at evaluating their data and attempting to identify these areas. Unfortunately, this study could not identify any battery of tests and field based tests that could conclusively diagnose increased risk of hamstring injury. Based on these findings, this study confirms that hamstring injuries are a multifaceted problem with numerous variables that interact to contribute towards injuries.

Minor Points

There was a difference in the H/Q absolute concentric torque on the non-dominant leg and this was not addressed in the discussion section. This reviewer suggests the authors attempt to explain this finding perhaps in the context of a compensatory adaptation of the non-dominant leg in an attempt to potentially protect against' injury (perhaps address at lines 387-394).

The authors thank Reviewer 1 for his/her comments. Accordingly, we have inserted the discussion concerning the H/Q absolute concentric torque of the non-dominant leg (page 19, lines 388-395) as follows:

Except for absolute concentric torque H/Q ratio where the peak torque production of the non-dominant leg of the previously injured category was slightly but significantly higher, there were no other differences observed in H/Q ratio between cohorts in the present study. It has to be noted that the scientific evidence is not consistent when it comes to differences in H/Q ratio between dominant and non-dominant legs (64, 65). However, the authors consider that this difference might be attributed to the compensatory adaptation of the non-dominant leg (balancing leg) which acts as a weight-bearing limb during the execution of the technical tasks against the dominant one.

Grammatical (Gr) and Spelling (Sp)

line 116-117-Gr-"might be a safe treat?"

line 124-Gr-"ex ante"

line 126-Sp"suing" should be using

line 129-Gr-insert "a" before weekly

line 136- missing close bracket after m2

line 324-dash between two-fold

line 372-Sp-grater to greater

The authors thank the Reviewers 1 for his/her valuable comments and suggestions and we have made all Grammatical and Spelling corrections accordingly.

Table 4 Relative Q torque, dominant (60deg/s) should be 3.07 and not 30.7

The authors thank the Reviewers 1 for his/her valuable observation and the correction in Table 4 has been made.

Reviewer #2: General comment

The authors provided us with data about factors that can explain hamstring injuries in high level soccer teams. There is a good attempt to congregate in a single study a large number of field and lab tests that are able to predict hamstring injury. However, the time and the way how the intervention was implemented leave us some doubts about the real novelty of the study. I leave to the authors some comments for consideration.

Specific comments

The paper needs some work to improve the English sentence structure. There are several grammar errors. Please seek some further assistance on overall manuscript.

The authors thank the Reviewers 2 for his/her valuable comments and suggestions and we have checked the improved overall English sentence structure of the manuscript.

Are HIS determined by team level and division positioning? I think that you should refer to this in your introduction section.

The authors thank the Reviewers 2 for his/her valuable comment which have contributed to improve the quality of the manuscript, and have added the information required (page 3, lines 75-76) as follows:

On the other hand, a lower injury rate of HSIs has been reported among lower division soccer clubs (16).

Line 91: please provide the age threshold that you refer as being a non-modifiable risk factor for HSI.

Thank you Reviewer 2 for your suggestion. Unfortunately, the authors could not find a paper determining the age threshold. However, we have included the justification concerning the age factor (page 4, lines 94-96) as follows:

With respect to non-modifiable risk factors previous hamstring injury and increasing age have shown to increase the odds to suffer a HSI by 11.6, and 1.4, respectively (31).

Do you think that using a sample from Kosovo soccer league can explain HSI in a high standard level such as Champions League teams? Are Kosovo teams with good and enough technical teams and/or medical support to prescribe the most appropriate tests and/or methods for injury prevention?

Thank you Reviewer 2 for your comments concerning this issue. The authors think that we have extensively addressed this issue in the “discussion” section of the manuscript (pages 16-17, lines 334-350) as follows:

Nevertheless, Kosovo is a state under development and the socioeconomic situation differs substantially compared to developed countries and proper soccer infrastructure is still missing. Therefore, factors such as field conditions, professional level and strength and conditioning status might have contributed to a higher injury incidence. Indeed, Iacovelli and colleagues (2013) found that when surface conditions of the natural playing soccer fields were abnormal, lower extremity injury rate was 2.61 times higher in comparison to a normal field condition (50). In addition, at the time when the study was conducted, Kosovo was not a member of International Federation of Football Associations (FIFA) and Union of European Football Associations (UEFA), therefore the playing level might be considered less-professional. Notably, injury occurrence and recurrence among amateur players, particularly during training, was reported to be higher in comparison with professional ones (16, 51). Furthermore, the physical preparedness of Kosovo players may be more precarious since, to date, strength and conditioning coaches licensed by Kosovo Football Federation, FIFA or UEFA are still missing. In fact, strength and conditioning coaches might act as observers and correctors of exercise techniques aiming to develop a strong and resilient athlete and can implement scientific driven prevention programs (52).

I understand that you used data from 2013-2014 soccer season. But, from those previous days until now there was a large increase in scientific evidence regarding soccer injuries, with HSI are included. Don’t you think that such kind of experimental data can be outdated? I refer to this aspect because, during this time period, the training methods were changing as well. Please reflect on that.

The authors thank Reviewer 2 for this important comment. The authors believe that since the results of the present manuscript are in line with recent publications, the information and findings of the current paper could play a role when hamstring injury prediction is concerned. It is widely accepted that the time lag between evidence being produced and its use in practice takes almost 2 decades.

We leave to the Reviewer 2 for his/her discretion an article dealing with translational research that supports the author’s argument.

Morris ZS, Wooding S, Grant J. The answer is 17 years, what is the question: Understanding time lags in translational research. J R Soc Med. 2011;104(12):510–20.

Please add a hypothesis after the aim of your study.

Thank you Reviewer 2 for your suggestion. We have included the hypothesis after the aims of the study accordingly (page 5, lines 110-112) as follows:

For the present study, it was hypothesized that players having one or more risk factors would be more prone to suffer HSIs.

What were the inclusion criteria of your subjects?

The authors thank the Reviewers 2 for his/her valuable comment which have contributed to improve the quality of the manuscript, and have added the information required (page 5, lines 125-127) as follows:

Players suffering from acute lower limb injuries or recovering from recent surgical interventions (within the last 12 months) were excluded

Please provide detailed information about the testing distribution. Where those tests performed in a single day? How many times where performed in a single season?

Thank you Reviewer 2 for your suggestion. The authors have inserted the testing distribution information (page 7, lines 160-164) as follows:

Isokinetic torque measurements have been performed on a separate day within the same week as other tests of the test battery consisting of the sit and reach, and Nordic hamstring strength test performed in the listed order. Finally, the countermovement jump, speed, and agility tests were performed on the last testing day. All tests were performed only before season kick-off.

Authors should report the confidence limit for statistical significance. Also should consider to add an effect size measure to strengthen their statistical analysis.

Thank you Reviewer 2 and 3 for this important suggestion. The authors have inserted the confidence intervals and effect size accordingly which are inserted in Table 3 and 4.

From your results I may conclude that HSI are most determined by non-match factors (e.g. age, body mass, Nordic test and previous injuries). They cannot avoid the “age factor” at some point of their career. You didn’t dissected the different components of body mass, not allowing to understand how those 70 kg where attributed to lean and fat mass. At the end, the higher hamstring strength given by Nordic exercise, and the previous injuries were the factors with differences. But, from my perspective, this is not new when considering data from previous studies.

Thank you Reviewer 2 for this observation and suggestion. When we performed the regression analysis, body mass was not identified as a risk factor. Since we did not perform body composition analysis, but only calculated the BMI, we did not want to go into more detail concerning this issue.

At the end I see your study as descriptive of the HSI occurrence in a single soccer league. In my opinion one more interesting approach would be to define a deterministic model that would identify the most important factors for HSI occurrence in the injury cohort, giving to us the exact contribution for HSI appearance.

The authors thank Reviewer 2 and 3 for their valuable comments which have contributed to improve the quality of statistical analysis strengthening the meaning of the outcomes. The authors have performed the multiple-linear regression analysis consequently (page 15, lines 305-315) as follows:

Table 5 presents the results derived from the multiple linear regression models, which partially explain the variation for injury incidence as the outcome variable. Model 1 shows that age alone accounted for 18.1%. When Nordic hamstring test was added to the model (model 2) yielded improved predictability which accounted for 21.3% of the variation of injury incidence. Besides, age, Nordic hamstring test, and the addition of the previous hamstring strain injury further improved the determination effect (model 3) accounting for 23.8%. Finally, the addition of concentric hamstring strength of the dominant leg at 240º/sec along with the parameters from the latter model increased the predictability of the hamstring injury to 25.9%. Body composition, isokinetic, functional tests (except for the dominant leg at 240º/sec), and playing position, did not improve the predictability outcome of hamstring injuries and were not reported as models by the statistical analysis.

Reviewer #3: This is a valuable observational prospective cohort study that examined 143 elite male soccer players. However, there are some issues that needs to be addressed:

Line 203-204: The sentence has a grammatical error, rewrite the sentence.

Thank you Reviewer 2 for your suggestion. The authors have rewritten the sentence (page 9, lines 219-220) as follows:

Descriptive statistics, such as age, anthropometric, and physical performance tests were used for baseline characteristics and means ± SD were calculated.

Line 249-254: Specify the units of the parameters

Thank you Reviewer 2 for your suggestion. We have specified the units of the parameters accordingly as presented in Table 2.

In the results part: there are no need to rewrite all the results, as they are depicted in the table 3 and 4. It suggests to add one more column into this table and write the value of the t-test in this column and delete the repeated results.

The authors thank Reviewer 3 for his/her suggestion which has improved the “results” section of the manuscript. The authors have made the suggested changes accordingly (pages11-14, lines 266-303) as follows:

An independent-samples t-test was conducted to compare injured and non-injured players for age, anthropometry, exposure time, isokinetic performance and functional parameters. As presented in Table 3, there was a significant difference in age between injured and non-injured players (p < 0.001). The body mass and BMI values were significantly higher among the injured compared to non-injured players (body mass, p = 0.002; BMI, p = 0.002), respectively. Height on the other hand did not differ between categories (p = 0.505). Besides, based on Pearson Chi-Square test results, it can be noticed that there is a statistically significant difference in the passing rate of Nordic Hamstring test results with a lower passing rate among the injured compared to non-injured players (p = 0.001), and a higher previous hamstring injury rate (p = 0.023), respectively. On the other hand, independent samples t-test revealed no significant differences with respect to training, match and total exposure time between groups [(training, p = 0.154); (match, p = 0.214); (total, p = 0.432)].

When comparing dominant and non-dominant legs in absolute concentric hamstring torque at 60º/sec, no significant differences were observed between categories (dominant, p = 0.467; non-dominant, p = 0.282). No cohort differences were also revealed for the quadriceps torque at 60º/sec in both legs (dominant, p = 0.820); non-dominant, p = 0.707). Adding, the hamstring/quadriceps (H/Q) ratio for the dominant leg was not significantly different between injured and non-injured players (p = 0.245), whereas significantly higher H/Q ratio values were observed among the injured players for the non-dominant leg (p = 0.044). On the other hand, there was no difference for absolute concentric torque for hamstrings, quadriceps and H/Q ratio between injured and non-injured players at 240º/sec [(hamstrings dominant, p = 0.162); (hamstrings non-dominant, p = 0.969); (quadriceps dominant, p = 0.509); (quadriceps non-dominant, p = 0.945); (H/Q ratio dominant, p = 0.393); (H/Q ratio non-dominant, p = 0.711)]. Eccentric torque at 30º/sec [(dominant, p = 0.489); (non-dominant, p = 0.688)], and at 120º/sec [(dominant, p = 0.400); (non-dominant, p = 0.195)] was also not significantly different between categories. Further, no differences were found with respect to functional test parameters (countermovement jump, p = 0.581); Illinois agility test, p = 0.905, 20-meter dash: injured, p = 0.643, 40-meter dash, p = 0.256, and sit-and-reach, p = 0.239).

Independent samples t-test did not reveal significant differences for relative concentric hamstring strength at 60º/sec (dominant, p = 0.460); (non-dominant, p = 0.649), whereas for the quadriceps torque, the injured players attained significantly lower values for both legs compared to non-injured ones (dominant, p = 0.039); (non-dominant, p = 0.025), respectively (Table 4). In addition, the level of relative concentric hamstring and quadriceps strength produced at 240º/sec was not significantly different between cohorts (p = 0.865). However, there was a tendency for a better performance for the non-injured category (p = 0.055). On the other hand, the differences for relative eccentric hamstring strength were even narrower at 30º/sec [(dominant: injured, p = 0.903); (non-dominant: injured, p = 0.733)], and 120º/sec [(dominant: injured, p = 0.849); (non-dominant: injured, p = 0.509)], respectively.

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

31 Jul 2020

PONE-D-19-20247R1

Potential prognostic factors for hamstring muscle injury in elite male soccer players: a prospective study

PLOS ONE

Dear Dr. Tschan,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

Please submit your revised manuscript by Sep 14 2020 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:

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: http://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols

We look forward to receiving your revised manuscript.

Kind regards,

Yumeng Li

Academic Editor

PLOS ONE

Additional Editor Comments (if provided):

Most reviewers commented that this paper lacks of novelty and readers cannot get sufficient new information. The authors should provide more information about the novelty and significance of the present study.

[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: All comments have been addressed

Reviewer #3: 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 #2: Yes

Reviewer #3: Yes

**********

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

Reviewer #2: Yes

Reviewer #3: 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

Reviewer #3: 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

Reviewer #3: 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 #2: The authors provided reasonable changes according with the reviewers opinion. Despite that I still maintain my reserves about the novelty of your study. In this sense I just have minor suggestions.

- At the end you should add a limitations' section to better describe the flaus of the intervention;

- Please make your conclusions more clearly for the reader. If the Nordic exercise performance predicts better the hamstrings injury incidence, this should be one of your key points.

Reviewer #3: (No Response)

**********

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: No

Reviewer #3: Yes: Ali Abbasi

[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.

9 Sep 2020

Comments to the Author

The authors would like to thank the Editor and Reviewer 2 for his/her positive and thought-provoking comments, which have improved the overall quality of the paper. The manuscript has been changed based on the reviewer’s comments and our point-by-point responses, highlighting the changes made are outlined below. In the text of the revised manuscript the color highlights the modified sentences.

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.

The authors thank the Editor and Reviewer 2 for their request to further improve the English language in the manuscript and the corrections were made accordingly.

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: The authors provided reasonable changes according with the reviewers opinion. Despite that I still maintain my reserves about the novelty of your study. In this sense I just have minor suggestions.

- At the end you should add a limitations' section to better describe the flaus of the intervention;

- Please make your conclusions more clearly for the reader. If the Nordic exercise performance predicts better the hamstrings injury incidence, this should be one of your key points.

The authors thank Reviewer 2 for his/her valuable comments concerning the limitations of the study and the respective changes can be found in: page 21; lines 448-455.

Despite a novel and comprehensive methodology including a homogenous study population, and a vast number of field and laboratory tests, some potential limitations are worth noting. Notably, a longer monitoring period could provide a better prediction depiction of HSIs. Further, different, diagnostic, prevention, and rehabilitation protocols implemented by the teams’ staff could have influenced the results. Therefore, further research including standardized diagnostic and preventive, and load monitoring measures are of utmost importance in order to better predict HSIs in soccer players.

The authors thank Reviewer 2 for his/her valuable comments with respect to the conclusions section and have improved the section as follows: page 21-22; lines 462-463.

Age, NHST, previous injury history, and concentric hamstring strength at 240°/sec resulted to be the best prediction factors of HSIs.

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

9 Oct 2020

Potential prognostic factors for hamstring muscle injury in elite male soccer players: a prospective study

PONE-D-19-20247R2

Dear Dr. Tschan,

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,

Yumeng Li

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

All comments have been successfully addressed. The paper could be accepted in its current form.

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: 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 #2: Yes

**********

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

Reviewer #2: N/A

**********

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: 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 #2: Considering the changes made by the authors i have no further comments. Still I mantain my reservations about the novelty of your study and the extrapolation of the results for other teams.

**********

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: No

29 Oct 2020

PONE-D-19-20247R2

Potential prognostic factors for hamstring muscle injury in elite male soccer players: a prospective study

Dear Dr. Tschan:

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. Yumeng Li

Academic Editor

PLOS ONE