Background Eccentric hip adduction and abduction strength plays an important role in the treatment and prevention of groin injuries in soccer players. Lower extremity strength deficits of less than 10% on the injured side, compared to the uninjured side, have been suggested as the clinical milestone before returning to sports following injury.
Objective To examine whether a side-to-side eccentric hip adduction or abduction strength symmetry can be assumed in non-injured soccer players and matched controls.
Material and Methods Nine elite soccer players 19.4 (1.5) years and nine recreational athletes 19.5 (2.0) years matched for sex, height and weight were included. Eccentric hip adduction and abduction strength of the dominant and non-dominant leg was tested for all the participants using an eccentric break test with a handheld dynamometer.
Results The dominant leg was 14% stronger than the non-dominant leg for hip adduction in the soccer players (p<0.05). No other side-to-side strength differences existed in soccer players or controls. In soccer players, hip abduction strength was 17–31% greater than controls for the dominant (p<0.05) and non-dominant leg (p<0.001).
Conclusion Eccentric hip adduction strength was greater in the dominant leg than in the non-dominant leg in soccer players, but not in matched controls. Eccentric hip abduction strength was greater in soccer players than matched controls, but soccer does not seem to induce a similar eccentric strength adaptation in the hip adductors.
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Eccentric hip strength is important for fundamental skills in soccer, such as kicking, accelerating and sudden change of direction.3,–,5 In soccer pelvic rotation, hip flexion and knee extension are the main contributors to the kicking movement, during a proximal to distal movement sequence.6,–,9 The loading of the swing limb eccentrically loads the hip flexor muscles and the adductors, and the knee extensors in the preparation of the forward movement of the limb before striking the ball.10 Soccer relevant, diagonal kicking approach angles seem to increase the demands of the hip and hip adduction strength of the kicking leg, and hip abduction strength of the supporting leg has been correlated to ball velocity.3
Significant MRI signal intensity changes in the gracilis and adductor longus of the kicking leg exists and indicate that there is substantial muscle activity in these muscles due to kicking.11 In the supporting leg, MRI signal intensity changes in the gracilis and the adductor longus were 50% lower than in the kicking leg. It has been hypothesised that kicking dominance may lead to an asymmetry in the lower extremity strength,12 but to what extent such a side-to-side strength asymmetry can be related to an increased injury risk is unknown.
Strength training programs improve soccer performance13,–,16 and are a fundamental part of soccer conditioning at the elite level. Strength training is also an integrated part of the rehabilitation process following injury, and specific strength training programs have been developed for athletes with hip and groin injuries.2 17 18
It has been suggested that lower extremity strength deficits of less than 10% on the injured side, compared to the uninjured side, should be considered the clinical milestone before returning an athlete to sports following an injury and rehabilitation.19 20 Furthermore, the achievement of a hip adduction:abduction ratio of more than 90% has been recommended before returning to sport after an adductor strain.17
Whether a side-to-side eccentric hip adduction or hip abduction strength symmetry can be assumed in non-injured soccer players and controls is unknown. Therefore, we examined the eccentric hip adduction or hip abduction strength in the dominant and the non-dominant kicking leg, in healthy, non-injured elite soccer players and matched controls.
Eighteen participants gave their informed consent to participate in the study. Nine male elite soccer players were included, and they had a weekly training/match volume of 14 h on average. Nine male recreational athletes with a weekly training/match volume of 2 h on average were used for comparison. These participants were matched for sex, height and weight (table 1). The recreational athletes had no experience with soccer or other sport, where unilateral habitual loading of the hip muscles was included. A questionnaire was given to the included participants to ascertain that they had no past injury to the hip and/or groin region and no injury to the lower extremities during the past year.
The testing set-up included a portable hand-held dynamometer (HHD; PowerTrack II Commander) and an examination table. The HHD was calibrated before testing, and all test procedures were standardised. The dominant and non-dominant leg was tested for all the participants. The preferred leg for kicking a ball was defined as the dominant leg.
Peak force occurs following motion of the limb during eccentric strength testing.21 Eccentric hip adduction or hip abduction strength was measured in a break test in the side-lying position as introduced by Tyler et al.22
No exercise/training or match was allowed the day before the initial test. All the participants were introduced to a 10-min standardised warm-up protocol before the hip strength measurements. This warm-up protocol included standardised running, squatting and hip adduction and abduction muscle activation. The participants were placed in the side-lying position with the leg being tested in a straight position. The hip and knee in the leg not being tested was placed in 90° of flexion (fig 1). The participants were told to stabilise themselves by holding on to the side of the examination table with their hands. The examiner applied resistance in a fixed position 8 cm proximal to the most prominent point of the lateral malleol, and the person being tested exerted a 3–5-s isometric maximum voluntary contraction against the dynamometer before the break was performed by the examiner. After the participants were instructed in the procedures, they were asked to perform one practice trial. To reduce measurement variation in the strength test related to the side-lying position,23 the individual test was repeated until a force plateau of less than 5% between two consecutive sessions was reached and the mean of these values were reported.
There was a 60-s rest period between each trial. The rest period was introduced to avoid a decline in strength across trials due to fatigue.24 The standardised command by the examiner was “go ahead-push-push-push-push-push”.
Leg length was measured from the most prominent point on the anterior superior iliac spine in the supine position to 8 cm proximal to the most prominent point of the lateral malleolus. Leg length was used to calculate torque, and all force values were weight adjusted (Nm/kg). Force values are reported as Newton-meters per kg body weight (Nm/kg).
The difference between soccer players and controls was expressed as a percentage and calculated as: (soccer players - controls)/controls, and the difference between dominant and non-dominant leg was expressed as a percentage and calculated as: (dominant - non-dominant)/non-dominant.
A single examiner, blinded to leg dominance and strength values during testing, performed all measurements. To ensure the reliability of the measurement procedure, intra-tester, interday reliability was examined before the initial study. Eccentric hip adduction and abduction strength using the break test was performed in 10 separate healthy male participants, M (SD) (25 (3) years, 187 (6) cm, 83 (10) kg) that engaged in physical activity (≥6 h/week). The intra-tester, inter-day reliability study showed no systematic variation between test and retest. Intra-class correlation coefficient (ICC) and standard error of measurement (SEM) were, respectively, ICC=0.91 (0.70–0.98), SEM=6.3% for hip adduction and ICC=0.86 (0.53–0.96), SEM=5.1% for hip abduction, showing minimal absolute measurement variation using this procedure. The absolute reliability of this procedure, expressed as SEM%, has to our knowledge not previously been documented. The ICC was in accordance with previous findings.25
Ethical approval was obtained from The Danish Ethics Committee of the Capital Region and the Danish Data Protection Agency.
Distributions of variables are presented as mean ± 1 SD. All the dependent variables demonstrated a normal distribution (Shapiro–Wilk W test), and two-sided F test was not significant for any unpaired comparisons; thus, parametric statistics were applied. Paired and unpaired Student's t tests were used appropriately. A level of p<0.05 was chosen to indicate statistical significance.
Hip torque is shown in table 2. The dominant side was stronger than the non-dominant side for hip adduction in the soccer players (p<0.05; fig 1). No other side-to-side strength differences existed in soccer players or controls. In soccer players, the hip abduction strength was greater than controls for the dominant side (p<0.05) and non-dominant side (p<0.001). No other between group differences existed.
To our knowledge, this is the first study investigating the eccentric hip adduction and hip abduction strength in soccer players and matched controls. Eccentric hip adduction strength was greater on the dominant side than the non-dominant side in soccer players but not in matched controls. Interestingly, there was no difference between soccer players and controls in eccentric hip adduction strength.
Role of leg dominance
Leg dominance seem to affect eccentric hip adduction strength in elite soccer players, suggesting that unilateral habitual loading such as kicking provides a small but significant strength increasing stimulus resulting in a difference between the dominant and non-dominant eccentric hip adduction strength in soccer players. Unilateral habitual loading in elite sports seem to affect both motor function and structural muscle tendon properties unilaterally.26,–,28 A larger cross-sectional area in the psoas in the dominant versus the non-dominant kicking leg and in quadratus lumborum in the dominant versus the nondominant stabilising leg has been documented in Australian rules footballers,29 demonstrating that structural muscle properties adapt to kicking.
To what extent the eccentric strength difference observed in the present study is related to muscle hypertrophy or improved coordination of the motor system remains to be investigated.
The present study showed that eccentric hip adduction strength is greater on the dominant than the non-dominant side in soccer players; therefore, differences between dominant and non-dominant eccentric hip strength should be taken into consideration when screening for eccentric hip adduction strength in non-injured soccer players and when evaluating adductor strength recovery in soccer players undergoing rehabilitation, using the other leg as control. A finding of 10–15% eccentric hip adduction strength difference in soccer players undergoing rehabilitation does not necessarily indicate any injuryrelated strength difference but could simply be the result of a unilateral strength adaptation related to kicking dominance. Therefore, if the dominant leg is the injured leg in soccer players, the eccentric hip adduction strength of the dominant leg after rehabilitation should rather be 10–15% stronger than the uninjured leg, whereas if the non-dominant is the injured leg, a 10–15% lower eccentric hip adduction strength value after rehabilitation may be considered normal.
Our results also imply that the eccentric hip adduction/abduction ratio, which has been suggested to be at least 90% after an adductor strain before returning to sport,17 should be considered with caution in soccer players, since the present study shows that the dominant strength difference was only present in hip adduction and not hip abduction. The difference in hip adduction strength in the dominant versus the non-dominant leg consequently also affects the hip adduction:abduction ratio differently in the dominant versus the non-dominant leg.
In relation to the present study, Masuda et al30 did not find any relevant difference, measuring isokinetic concentric peak hip adduction strength, between the dominant and non-dominant leg in a comparable group of soccer players. A possible explanation for our contradictive finding could be that the present eccentric testing set-up might be necessary to develop “true” peak force in soccer players. Another possible explanation is that isokinetic testing might introduce larger measurement variation than HHD, as shown when testing shoulder abduction,31 which decreases the ability to detect small, but still relevant, strength differences. The reliability of the eccentric testing method used in the present study shows that measurement variation during eccentric hip adduction and hip abduction testing was minimal and comparable to the study by Magnusson et al31 increasing the chance of finding small but still relevant existing strength differences between legs and groups.
Adduction strength: no difference between soccer players and controls
The lack of eccentric hip adduction strength difference between soccer players and controls was surprising. This would suggest that soccer does not involve activities of adequate load and intensity to induce substantial eccentric hip adduction strength gains. A lack of general eccentric hip adduction strength adaptation in soccer players is an interesting finding since this could possibly be related to the high prevalence of adductor-related injuries in soccer players.2 32 Kicking motions, sprinting and sudden change of direction place great stress to the adductor longus muscle–tendinous complex,33,–,35 and an adequate eccentric hip adduction strength capacity may, therefore, protect the adductors during these stressful activities
Specific hip adduction strength training of significant load and intensity, incorporating eccentric loading of the adductors, might be useful when planning an adductor injury prevention program directed towards elite soccer players. A strength training program, with an aggressive eccentric emphasis for the adductors, could potentially be of prophylactic use in the prevention of adductor injuries in soccer players. Studies have shown that an eccentric emphasis seems effective in increasing eccentric hamstring strength and preventing hamstring injury and re-injury.36,–,39 The increased muscle activity and load demands for the hip adductors in the dominant leg due to habitual kicking3 11 also have implications when designing a strength training program for soccer players, and the strength training program should reflect these specific demands. Therefore, focus on eccentric hip adduction on the dominant side seems important to protect the dominant adductors during repetitive kicking.
A limitation of the present study is the small sample size, which affects the generalisability of our results. Furthermore, the control group was not matched for activity level and training volume. Therefore, it is not possible to conclude whether the observed strength differences between soccer players and controls can be generalised to a larger group of individuals or whether the observed difference is due to the specific activity of soccer or simply just reflects the difference in activity levels and training volume.
Future studies including a larger sample size should examine whether eccentric strength differences exist between hip adductors or abductors in soccer players and controls matched for activity level and training volume.
What is already known on this topic
▶ Eccentric hip adduction and abduction strength plays an important role in treatment and prevention of groin injuries in soccer players. Lower extremity strength deficits of less than 10% on the injured side, compared to the uninjured side, are often considered the clinical milestone before returning an athlete to sports following injury and rehabilitation
What this study adds
▶ A side-to-side eccentric hip adduction strength symmetry cannot be assumed in soccer players, since eccentric hip adduction is greater on the dominant side. Knowledge of a side-to-side eccentric hip adduction strength difference is relevant, when using the non-injured side as control in the strength assessment of injured soccer players.
Eccentric hip adduction strength was greater on the dominant than the non-dominant leg in soccer players. Eccentric hip abduction strength was greater in soccer players than matched controls, but soccer does not seem to induce a similar substantial eccentric strength adaptation in the hip adductors.
This study was supported by grants from Danish Regions, The Association of Danish Physiotherapist and the Lundbeck Foundation. Thanks to Rasmus Mortensen, Charlotte Andersen, Monica Johannesen, Camilla Kølbæk, Mia Lorentzen, Bo Schnegelsberg and Klaus Klinge for their helpful contributions.
Competing interests None.
Patient consent Obtained.
Ethics approval Ethical approval was obtained from The Danish ethics committee of the Capital Region and the Danish Data Protection Agency.
Provenance and peer review Not commissioned; externally peer reviewed
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