Hamstrings Strength Imbalance in Professional Football (Soccer) Players in Australia.

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HAMSTRINGS STRENGTH IMBALANCE IN PROFESSIONAL FOOTBALL (SOCCER) PLAYERS IN AUSTRALIA CLARE L. ARDERN,1 TANIA PIZZARI,2 MARTIN R. WOLLIN,3

AND

KATE E. WEBSTER1

1

School of Allied Health, La Trobe University, Melbourne, Australia; 2Department of Physiotherapy, La Trobe University, Melbourne, Australia; and 3Australian Institute of Sport, Canberra, Australia

ABSTRACT Ardern, CL, Pizzari, T, Wollin, MR, and Webster, KE. Hamstrings strength imbalance in professional football (soccer) players in Australia. J Strength Cond Res 29(4): 997–1002, 2015—The aim of this study was to describe the isokinetic thigh muscle strength profile of professional male football players in Australia. Concentric (608 and 2408$s21) and eccentric (308 and 1208$s21) hamstrings and quadriceps isokinetic strength was measured with a HUMAC NORM dynamometer. The primary variables were bilateral concentric and eccentric hamstring and quadriceps peak torque ratios, concentric hamstring-quadriceps peak torque ratios, and mixed ratios (eccentric hamstring 308$s21 O concentric quadriceps 2408$s21). Hamstring strength imbalance was defined as deficits in any 2 of: bilateral concentric hamstring peak torque ratio ,0.86, bilateral eccentric hamstring peak torque ratio ,0.86, concentric hamstring-quadriceps ratio ,0.47, and mixed ratio ,0.80. Fifty-five strength tests involving 42 players were conducted. Ten players (24%) were identified as having hamstring strength imbalance. Athletes with strength imbalance had significantly reduced concentric and eccentric bilateral hamstring peak torque ratios at all angular velocities tested; and reduced eccentric quadriceps peak torque (308$s21) in their stance leg, compared with those without strength imbalance. Approximately, 1 in 4 players had preseason hamstring strength imbalance; and all strength deficits were observed in the stance leg. Concentric and eccentric hamstrings strength imbalance may impact in-season football performance and could have implications for the future risk of injury.

KEY WORDS muscle strength, muscle strength dynamometer, athletic performance, injury risk

Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal’s Web site (http:// journals.lww.com/nsca-jscr). Address correspondence to Dr. Clare L. Ardern, [email protected]. 29(4)/997–1002 Journal of Strength and Conditioning Research Ó 2015 National Strength and Conditioning Association

INTRODUCTION

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dequate thigh muscle strength is important for sprinting and jumping performance; and these activities are integral to sports, such as football (soccer) (4,24). Therefore, reduced muscle strength may contribute to reduced sports performance and may also predispose an athlete to sustaining a muscle strain injury (12). Preseason strength testing allows team strength and conditioning staff to identify athletes with strength deficits and facilitates the implementation of targeted strengthening programs well before the athlete engages in the competitive season, thereby maximizing performance and minimizing injury risk. Lower limb muscle strain injuries account for approximately one-third of all injuries in professional football (8); and hamstring strains are the most common of these. In European professional football, a typical squad of 25 senior team players sustains, on average, 5 hamstring injuries per season, resulting in a loss of approximately 80 playing days (2,8). Preseason thigh muscle strength imbalances in a cohort of professional European and South American football players have been shown to put an athlete at 4.6 times increased risk of sustaining an inseason hamstring strain (7); and the rate of hamstring strain injury recurrence in football has been reported to range from 16 to 25% (8,18). In their study, Croisier et al. (6) statistically determined the criteria for thigh muscle strength imbalance and included tests of bilateral hamstring and quadriceps ratios, a comparison of side-to-side differences in thigh muscle strength, and comparisons of the balance of strength between the hamstrings and quadriceps on the same leg (hamstring-quadriceps ratio and mixed ratio) (6). Studies of thigh muscle strength in French (4,7), Greek (10,17,24), Belgian (7,15), United Kingdom (5), Middle Eastern (21), Turkish (14), and South American (7,13) professional football players have reported quadriceps and hamstring strength obtained at angular velocities ranging from 12 to 3008$s21 and may provide a guide to the expected strength results and a comparative benchmark for thigh muscle strength for football players in Australia. In the largest study of preseason thigh muscle strength profiles of professional VOLUME 29 | NUMBER 4 | APRIL 2015 |

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Hamstrings Strength Imbalance European and South American football players, approximately 1 in 2 players had hamstring strength imbalance (7). The study by Croisier et al. (6) provided a validated protocol for assessing the thigh muscle strength profile of professional football players. To the best of our knowledge, there are currently no published data on thigh muscle strength from players engaged in professional football in Australia. It is unclear how the strength profile of players in Australia compares with those playing in other professional competitions, and whether the previous European, United Kingdom, Middle Eastern, and South American data can be generalized to the Australian football context. Knowledge of the strength profile of players in the Australian national competition may also help to provide information regarding strength performance benchmarks. The aim of this study was to describe the isokinetic strength profile of professional male football players in Australia by applying the criteria of Croisier et al. (6).

METHODS Experimental Approach to the Problem

The objective of this study was to evaluate the isokinetic hamstrings and quadriceps strength profile of professional male football players and identify whether any player had hamstring strength imbalance. Preseason performance testing data from 42 athletes were analyzed. Concentric (60 and 2408$s21) and eccentric (30 and 1208$s21) hamstrings and quadriceps isokinetic strength was measured with a HUMAC NORM dynamometer. The primary variables were bilateral concentric and eccentric hamstring and quadriceps peak torque ratios, concentric hamstring-quadriceps peak torque ratios, and mixed ratios (eccentric hamstring 308$s21 O concentric quadriceps 2408$s21). The choice of primary variables was made based on a protocol described by Croisier et al. (6). Strength imbalance was considered any 2 of: bilateral concentric hamstring peak torque ratio ,0.86, bilateral eccentric hamstring peak torque ratio ,0.86, concentric hamstring-quadriceps ratio ,0.47, and mixed ratio ,0.80, based on the criteria described by Croisier et al. (7).

Procedures

All strength measurements were obtained using an isokinetic dynamometer (HUMAC NORM; Computer Sports Medicine Inc., Stoughton, MA, USA). Assessments were completed during the preseason training period, before the commencement of the competitive season. All players were tested in the same training week. A standardized warm-up was completed before testing, which consisted of a 5-minute moderate-intensity bout of stationary cycling (rating of perceived exertion 10–14 (3)), followed by body weight squats, lunges, and tuck jumps (10 repetitions of each exercise). All strength measurements were completed in a seated position with the player positioned in 858 of hip flexion. Stabilization straps were placed over the thigh to be tested, chest, and waist to isolate sagittal plane knee movement. Isokinetic hamstring and quadriceps strength measurements were recorded at concentric (60 and 2408$s21) and eccentric (30 and 1208$s21) angular velocities, according to the protocol described by Croisier et al. (6). Two submaximal familiarization repetitions were completed for each of the 4 angular velocities immediately before each assessment at each angular velocity. All players completed the assessment sequence in the same order, but the leg tested first (skill or stance) varied. A summary of the protocol in order of test completion is provided in Table 1. Players were instructed that they were completing a maximal test, and verbal encouragement was provided during every repetition. For the concentric assessments, players were instructed to straighten and bend their knee as fast as possible. For the eccentric assessments, players were instructed to resist, as strongly as possible, the movement of the dynamometer arm throughout the full range of knee motion. The knee range of motion was set during dynamometer calibration to 1008 flexion from each player’s maximum active knee extension. Players received a 30-second rest between completing each set of test repetitions at each angular velocity.

Subjects

Forty-two professional male football players (age range is 17.7-35.6 years) on the senior playing roster of a single Australian national level football club completed preseason fitness testing for injury risk screening as part of routine training. To participate in the fitness testing, players must have been considered fit to play competitive football by team medical staff and have been participating in normal preseason training with the club. Strength and anthropometry data were analyzed for this study. Ethics approval was granted from the University Faculty Ethics Committee. As part of their professional playing contracts, players provided written consent (including parental consent for players younger than 18 years) for their data to be collected and analyzed.

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TABLE 1. Summary of isokinetic strength test protocol. Angular velocity Concentric 608$s21 Concentric 2408$s21 Eccentric 308$s21 Eccentric 1208$s21

Familiarization repetitions

Test repetitions

2

3

2

5

2

3

2

4

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Journal of Strength and Conditioning Research Statistical Analyses

Hamstring and quadriceps peak torque data from preseason assessments conducted between April 2011 and June 2013 were analyzed. For each angular velocity, the peak torque arithmetic mean was calculated from the test repetitions completed for each set, for the skill and stance legs, and was subsequently used to calculate the ratios used in the final analysis. The primary variables were the bilateral concentric and eccentric hamstring and quadriceps peak torque ratios, concentric hamstring-quadriceps peak torque ratios, and the mixed ratio, which is an alternative hamstring-quadriceps ratio. The bilateral peak torque ratios were calculated by dividing stance leg peak torque by skill leg peak torque. Concentric hamstring-quadriceps ratios were calculated for the skill and stance legs by dividing the hamstring peak torque by the quadriceps peak torque. The mixed ratio was calculated for the skill and stance legs by dividing the eccentric hamstring peak torque at 308$s21 by the concentric quadriceps peak torque at 2408$s21 angular velocities. The following criteria described by Croisier et al. (7) were used to define hamstring strength imbalance. In this study, strength imbalance was defined as the presence of deficits on at least 2 of these criteria.  Bilateral concentric hamstring peak torque ratio of ,0.86.  Bilateral eccentric hamstring peak torque ratio of ,0.86.  Concentric hamstring-quadriceps ratio of ,0.47.  Mixed ratio of ,0.80. Data were analyzed with SPSS 21 (SPSS, Inc., Chicago, IL, USA) for the overall player cohort; and comparisons were also made between players with and without hamstring strength imbalance. A p value of less than 0.05 was considered significant for all analyses. Players were tested once each preseason; for players who were at the club for more than 1 season, and therefore completed more than 1 preseason test, data from the first available test was included for analysis. Descriptive statistics were calculated for anthropometric data and compared between players with and without strength imbalance using independent samples t tests. Me-

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dians and interquartile ranges were calculated for all strength variables, as strength data were not normally distributed. Comparisons of strength variables between players with and without hamstring strength imbalance were made with Mann-Whitney U-tests.

RESULTS Participant Characteristics

There were 55 tests involving a total of 42 male athletes over 3 preseasons. Of these, 10 players (24%) were identified as having hamstring strength imbalance. In the first preseason, 22% (5 of 23 players) had hamstring strength imbalance; in the second preseason, 25% (4 of 16 players) had hamstring strength imbalance; in the third preseason, 19% (3 of 16 players) had hamstring strength imbalance. Two players had hamstring strength imbalance at their second test but not at their first test (the test that was included for analysis). There were no differences in mean age, height, and weight between players identified as having hamstring strength imbalance and those who did not have strength imbalance (Table 2). Hamstring and Quadriceps Peak Torque

Median concentric and eccentric quadriceps and hamstring peak torque results are presented in Supplemental Digital Content 1 (see Table). Players with hamstring strength imbalance recorded significantly reduced stance leg eccentric hamstring peak torque, when compared with those without strength imbalance. Players with strength imbalance also recorded significantly reduced stance leg eccentric quadriceps peak torque at 308$s21 when compared with players without strength imbalance. Bilateral Peak Torque Ratios

Players with hamstring strength imbalances had significantly reduced concentric and eccentric bilateral peak torque ratios for the hamstrings, at all angular velocities tested (see Table, Supplemental Digital Content 1; http://links.lww.com/ JSCR/A10). Players with hamstring strength imbalance also had significantly reduced concentric quadriceps bilateral peak torque ratio at the 608$s21 angular velocity (see Table,

TABLE 2. Athlete demographic data. Characteristics Age at testing, mean (SD), (y) Range Height, mean (SD), (cm) Range Weight, mean (SD), (kg) Range

Full cohort

Players with strength imbalance

Players without strength imbalance

24.9 (5.3) 17.7–35.6 180.6 (7.2) 163.5–194.0 76.4 (8.2) 61.4–95.0

24.3 (6.2) 18.0–33.2 184.3 (6.6) 174.0–194.0 78.7 (6.9) 68.0–87.4

25.1 (5.1) 17.7–35.6 179.5 (7.1) 163.5–193.0 75.7 (8.5) 61.4–95.0

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Hamstrings Strength Imbalance

TABLE 3. Number of players with asymmetries in tested strength parameters.* Rate of players (%) Bilateral peak torque ratio difference C608 $s21 C2408 $s21 E308 $s21 E1208 $s21 Hamstring-quadriceps ratio C608 $s21 C2408 $s21 E308 $s21 =C2408 $s21 (mixed ratio)

8/42 4/42 11/42 13/42

(19) (10) (26) (31)

0/42 (0) 1/42 (2) 0/42 (0)

*C = concentric; E = eccentric.

Supplemental Digital Content 1; http://links.lww.com/ JSCR/A10). The number of players with asymmetries on each strength parameter is presented in Table 3. Hamstrings-Quadriceps Ratios

The median mixed hamstring-quadriceps ratio for both legs was 1.3 (range, 0.8–2.1), and the median concentric hamstring-quadriceps ratios ranged from 0.46 to 1.17 (see Table, Supplemental Digital Content 2; http://links. lww.com/JSCR/A11). There were no differences in hamstring-quadriceps ratios between players with hamstring strength imbalance and those without hamstring strength imbalance (see Table, Supplemental Digital Content 2; http://links.lww.com/JSCR/A11).

DISCUSSION This study found that approximately 1 in 4 players of a single professional football club in Australia had preseason hamstring strength imbalance. The athletes with strength imbalance had significantly reduced concentric and eccentric hamstring peak torque and peak torque ratios at all angular velocities tested, and significantly reduced eccentric quadriceps peak torque (308$s21) in their stance leg, compared with those without strength imbalance. There was also a trend for lower stance leg quadriceps peak torque and bilateral peak torque ratios across the other angular velocities in the players with hamstring strength imbalance, although this was not statistically significant. Hamstring strength imbalance may suggest that a player has strength deficits that could impair football performance and increase a player’s risk of injury. Significant correlations between thigh muscle strength (measured using a 1 repetition maximum squatting task), and sprinting and jumping performance (r range from 0.7 to 0.9) have been demonstrated in elite Norwegian football players (22,23). In addi-

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tion, there were significant differences in strength and performance between the first- and last-ranked teams in the Norwegian competition (23). Therefore, to ensure performance is maximized, it may be important to assess players for strength imbalance and strength deficits during the preseason to guide the implementation of appropriate interventions that address these factors. Reductions in the hamstring bilateral peak torque ratio may also be associated with an increased risk of hamstring strain injury (12). Professional football players with reduced eccentric bilateral hamstring peak torque ratios have been shown to have approximately 4 times greater odds of hamstring strain (9). However, in the same study, there was no relationship between concentric ratios and hamstring strain. In this study, stance leg hamstring strength was reduced in all players with strength imbalance. There is evidence to suggest that athletes with a dominance of quadriceps activation relative to hamstring activation during such tasks may be at increased risk of a noncontact anterior cruciate ligament (ACL) injury (16,19). This is particularly important given that noncontact ACL injuries typically occur during a cutting or pivoting movement, which is likely to be performed on the stance leg. Because the strength deficits were consistently observed in the stance leg, it may also suggest that targeting this leg in injury prevention interventions could be beneficial. The number of players in this study with hamstring strength imbalance compares favorably with European and South American professional football players. In this study, approximately, 1 in 4 players were identified as having preseason hamstring strength imbalance, compared with approximately 1 in 2 European and South American players (7). In professional football players, previous reports show concentric quadriceps strength measurements to range from 130 to 250 N$m (13,14), and hamstring strength from 80 to 150 N$m (5,10,14). Eccentric quadriceps strength measurements range from 240 to 320 N$m (4,10,14), and hamstring strength from 140 to 190 N$m (4,10,14). Conventional hamstring-quadriceps ratios range from 0.5 to 0.8 (4,24), and mixed hamstring-quadriceps ratios (a comparison of eccentric hamstring strength and concentric quadriceps strength that aims to replicate the agonist-antagonist actions of the thigh muscles during knee movement (1)) range from 0.5 to 1.4 (4,14,15). The strength measurements obtained in this study are also largely comparable with these previous reports. The hamstring-quadriceps ratios, both conventional (0.6–0.9) and mixed ratios (range, 1.2–1.4), were similar to those reported in other professional football populations (0.5–1.4). Measures of absolute concentric and eccentric hamstring and quadriceps recorded in this study were also comparable with previously reported results. In this study, hamstring strength ranged from 93 to 164 N$m, and quadriceps strength ranged from 113 to 267 N$m. There are a number of factors that may influence hamstring strength imbalance. Age may reflect a player’s

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Journal of Strength and Conditioning Research level of neuromuscular maturation, which may, in turn, impact on muscle activation during dynamic tasks. The number of years of experience playing professional football may be a proxy for the amount of physical conditioning a player has and contribute to changes in symmetry in lower limb muscle strength (10). More experienced players may display less tendency for asymmetry due to “footedness,” or a preference for particular movement patterns that favor 1 leg over the other (10). The position a player typically plays (i.e., forward, midfielder, defender, goal keeper) may also influence muscle strength due to different physical performance demands. In this study, we did not compare players according to their usual playing position due to small numbers. The performance demands of the goalkeeper could conceivably be different to the demands of field players. In this study, the goalkeepers were not observed to have strength imbalance, so their data were retained in the overall analysis. Other factors related to injury history and previous training practices may also influence hamstring strength imbalance. Previous injury, insufficient rehabilitation from injury, and poor physical conditioning or training that inadequately addresses the performance aspects necessary for optimal performance in football may also contribute to strength imbalances. Therefore, strength and conditioning professionals working with football players may need to carefully consider these aspects when designing and implementing training programs. To the best of our knowledge, this is the first study to evaluate the isokinetic thigh muscle strength profiles of professional footballers playing in Australia. Therefore, this study provides important strength benchmarking data for an Australian football context and demonstrates that the strength profiles of players in Australia are similar to their European, United Kingdom, Middle Eastern, and South American counterparts. There are some limitations to consider in this study. Some athletes included in this study may have had a previous lower limb injury that could have affected their strength results. To account for the potential impact of symptomatic injuries on strength results at the time of testing, all players were assessed by club medical staff and declared fit to play competitive football. Therefore, no players with current injuries were included in the analysis. No players were injured during testing. Thigh muscle strength was evaluated isokinetically, and in a seated position. This is the gold standard method for strength evaluation (20), but it could be argued that this method may not adequately target functional neuromuscular performance. The order of strength testing was nonrandomized because we aimed to replicate the protocol of Croisier et al. (6). Given the relationship between thigh muscle strength and vertical jump and sprint performance, (15) strength and conditioning professionals may consider using a battery of muscle performance tests. Such a battery may include measures

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of function, such as jumping and sprinting, to complement the gold standard isokinetic testing. The single-leg hamstring bridge may also be a useful functional screening test because it targets hamstring endurance with the knee and hip joints in functional angles (11). There is also evidence that this test may be predictive of future hamstring injury (11). Strength and conditioning professionals in the training environment may also wish to quantify muscle strength; and hand-held dynamometry could represent a more cost-effective, portable, and time-efficient method of doing so in comparison with isokinetic dynamometry. Whiteley et al. (21) have described a novel and more functional method of testing isometric and eccentric hamstring strength in professional football players using a hand-held dynamometer; and this method was moderately to highly correlated (r = 0.3–0.6) with isokinetic strength measures (21). This study demonstrates that approximately 1 in 4 players at an Australian football club had preseason hamstring strength imbalance. Players with strength imbalance had significantly reduced eccentric hamstring peak torque, and concentric and eccentric peak torque ratios. They also had significantly reduced eccentric quadriceps peak torque (308$s21) on their stance leg. Players with strength imbalance had a reduced concentric hamstring-quadriceps ratio measured at 608$s21 when compared with athletes without strength imbalance. Overall, the strength profiles of professional football players in Australia are comparable with players in Europe, the United Kingdom, the Middle East, and South America, although the presence of concentric and eccentric hamstring strength imbalance may impact inseason football performance and could have implications for the risk of future hamstring strain injury.

PRACTICAL APPLICATIONS To the best of our knowledge, this is the first published report of thigh muscle strength in professional football players in Australia. Overall, the isokinetic strength profiles of football players in Australia were comparable with players in Europe, South America, and the Middle East, suggesting that strength results from other countries may be largely generalizable to an Australian football context. However, we found, based on statistically selected cutoffs, approximately 1 in 4 professional football players had preseason hamstring strength imbalance. Although this compares favorably with European and South American football players, where hamstring strength imbalance has been reported in 1 in 2 players, hamstring strength imbalances may impact football performance and could have implications for future injury. In addition, hamstring strength was reduced in the stance leg of all players with hamstring strength imbalance, which could have implications for an increased risk of ACL injury. Therefore, it may be important to assess players for lower limb strength imbalance and strength deficits during the preseason to guide the implementation of appropriate performance enhancement and injury prevention interventions. VOLUME 29 | NUMBER 4 | APRIL 2015 |

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Hamstrings Strength Imbalance ACKNOWLEDGMENTS No financial assistance was received for this project. The authors thank Tom Reddin and Belinda Pacella from Melbourne Heart Football Club’s medical staff for their assistance with participant recruitment.

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