Intra- and Inter-Limb Strength Asymmetry in Soccer: A Comparison of Professional and Under-18 Players

Assessing the magnitude and direction of asymmetry in physical tests and morphological measurements in professional female soccer athletes

OBJECTIVES

To analyze the magnitude and direction of inter-limb asymmetries in vertical jump, T-test, isokinetic peak torque, and muscle volume in professional female soccer players.

METHODS

Sixteen female soccer players participated in the study (22.7 ± 3.5 years, 60.4 ± 12.7 kg, 164.2 ± 8.7 cm). The following tests were applied; the unilateral countermovement jump (CMJ), T-test (agility), peak torque of concentric knee extensor (PT_Con_Q), concentric and eccentric knee flexor (PT_Con_H; PT_Ecc_H), and muscle volume of quadriceps (MV_Q) and gastrocnemius (MV_G). The interlimb asymmetry magnitude was calculated as the percentage difference between the stronger and weaker limbs. For the direction of asymmetry, a negative sign (-) was assigned when the non-dominant lower limb was the stronger one, and a positive sign (+) when the dominant lower limb was the stronger one.

RESULTS

The asymmetry (%) values varied from 0.7 to 11.9%, with the highest values of asymmetry found in the CMJ, PT_Con_Q, PT_Con_H, and PT_Ecc_H tests. Moderate levels of agreement were observed for CMJ - Vol_G (K = 0.53; 75% agreement); PT_Con_Q - PT_Con_H (K = 0.48; 75% agreement); T-test - PT_Ecc_H (K = 0.43; 68.7% agreement). Fair levels of agreement were found between the T-test - Vol_Q (K = 0.21; 62.1% agreement); PT_Con_Q - PT_Ecc_H (K = 0.36; 68.8% agreement); and PT_Con_H - PT_Ecc_H (K = 0.31; 68.8% agreement).

CONCLUSIONS

We concluded that of greater asymmetries in the vertical jump and isokinetic torque tests. The direction of the asymmetry seems highly variable, however, the isokinetic tests showed greater agreements with the other tests.

Functional and Lateral Asymmetry of the Knee Joint Muscles Measured Using Tensiomyography (TMG) in Professional Football Players of Different Playing Positions

Background/Objectives: The aim of this study is to determine whether different playing positions in football influence muscle asymmetry, which is a common cause of injuries in football. This study aimed to determine the difference in the functional and lateral asymmetry of the knee joint muscles measured using tensiomyography (TMG) between football players of different playing positions. Methods: This study included 52 professional football players (25.1 ± 4.7 years) divided into three groups according to their playing position: defenders-DF (N = 18), midfielders-MF (N = 15), and forwards-FW (N = 19). Functional and lateral symmetry were investigated by applying TMG on the knee joint muscles Rectus femoris, Vastus medialis, Vastus lateralis, Biceps femoris, and Semitendinosus of the right and left leg, and using the algorithm proposed by the manufacturer and previous studies. Results: The results of one and two-way ANOVA revealed no significant differences in functional (F = 0.596, p = 0.555, on average) and lateral asymmetry (F = 0.888, p = 0.497, on average) from the aspect of muscle contraction velocity and muscle stiffness between football players of different playing positions. Conclusions: The study results show that the specificity of the playing position in football does not influence the functional and lateral asymmetry of the knee joint muscles. However, it has been shown that there is a high percentage of players with lateral (n = 14.5, 39.6%, on average) and specifically functional asymmetry (n = 27, 51.9%, on average).

Association between intra and inter-limb strength asymmetry with sprint kinematics and force-velocity profile in youth team athletes

BACKGROUND

Maximal force plays a pivotal role in enhancing performance across various dynamic sports, particularly in sprinting biomechanics. However, muscle strength asymmetry among major muscle groups like the hamstrings and quadriceps may hinder sprint performance and raise injury risks. This study examines how intra- and inter-limb strength asymmetry relates to sprint kinematics and the power-force-velocity profile in youth athletes.

METHODS

Seventy-four youth athletes from football, volleyball, and handball completed two testing sessions, 48 h apart. Assessments included anthropometric measurements, sprint tests, and isokinetic dynamometer strength evaluations at 60°.s-1 and 180°.s-1. Sprint kinematics were analyzed using the Optojump Next system, and force-velocity profile variables were determined via Samozino's method. Asymmetry analysis used a standard percentage difference equation.

RESULTS

Weak to moderate correlations (r = -0.46 to 0.45) were found between strength asymmetry and sprint parameters, suggesting limited impact of asymmetry on sprint kinematics and kinetics. Although most participants showed asymmetry levels below the 15% threshold deemed functionally significant, outliers had asymmetry values up to 42%, indicating considerable variability.

CONCLUSIONS

Lower extremity strength asymmetry moderately affects sprint kinematics and force-velocity profiles in young team athletes, challenging traditional views. Further research is needed to explore the mechanisms between strength asymmetry and sprint performance, aiming to inform targeted training interventions that optimize performance and reduce injury risks in youth athletes.

Evaluation of Lower Limb Asymmetry Index Based on the 30-Second Skater Squat Functional Test in Young Men

Introduction: Physical performance tests (PPTs) are used for the pre-season evaluation of athletes and to monitor and control the rehabilitation process. PPTs include single-leg jumps, single-leg squats, and balance tests. One of the physical fitness tests is the skater squat test. The 30 s skater squat functional test (SSFT) is used as one of the tests to assess fitness and symmetry in the lower limbs. The present study aimed to calculate and compare the asymmetry index using the 30 s skater squat functional test, the single-leg distance jump test, and the isometric measurement of knee joint extensor strength. Materials and Methods: The study examined 25 men aged 23 ± 3.17 years. The study used the 30 s SSFT, the single-leg long jump test (SLLJT), and an isometric dynamometer test to measure peak moment of force values for extensors of the knee using the JBA Zbigniew Staniak® measuring station ("JBA" Zb. Staniak, Poland). The statistical analysis of functional test results and iso-metric dynamometry results was based on correlation analysis. Results: There was a moderate correlation between 30 s SSFT and SLLJT (r = 0.540), and between SLLJT and measurements of peak moment of force of the knee joint extensors (r = 0.533). Conclusions: The asymmetry index calculated based on functional tests and peak moment of force of the knee extensors in a group of young men should not be used interchangeably. The asymmetry index calculated from the 30 s skater squat functional test detects greater differences in knee extensor strength than the ASI index calculated from the single-leg long jump test in a group of young male athletes. The practical significance of this study was that its results could play an important role in the training process and monitoring the return to sports after a possible injury.

The association between unilateral and bilateral performance-related measures in elite female soccer players: a multifaceted investigation

Purpose

The present study aimed to investigate a) the associations between bilateral performance utilizing countermovement jump (CMJ), squat jump (SJ), speed and unilateral CMJ, isokinetic peak torque in knee extension and flexion with angular velocities of 60°/s and 180°/s and tensiomyography (TMG) parameters; b) whether the asymmetries derived from unilateral tests are associated with bilateral CMJ, SJ and speed in elite female soccer players.

Methods

Thirty-five elite female soccer players (average age: 20 ± 5 years) completed CMJ, SJ, speed, isokinetic muscle strength and TMG tests.

Results

Compared to the non-dominant leg, the dominant leg demonstrated greater peak torque output in both knee flexion (7.4%) and knee extension (5.6%) isokinetic tasks, as well as m. vastus medialis contraction time (7.6%), and soccer-specific agility test (4.1%). Conversely, the hamstring to quadriceps peak torque ratio at 180°/s (8.5%) was significantly greater in the non-dominant leg. The associations between CMJ, SJ and speed performance were positive and ranged from weak (r = 0.350) to high (r = 0.710). For speed and TMG-derived variables, correlations were negative and ranged from weak (r = -0.345, p = 0.042, for vastus medialis contraction time) to moderate (r = -0.530, p = 0.001, for biceps femoris contraction time). Furthermore, both bilateral CMJ and SJ negatively correlated with TMG-derived variables, ranging from weak (r = -0.350, p = 0.039, for vastus lateralis contraction time) to moderate (r = -0.537, p = 0.003, for rectus femoris contraction time).

Conclusion

The overall significant, albeit inconsistent, correlations between the diverse performance scores obtained highlight the necessity for a multifaceted and thorough diagnostic strategy in female soccer players.

Intra- and inter-limb strength imbalance and asymmetry in soccer: A comparison of elite senior and junior players

Evaluation of muscle strength imbalance can be an important element in optimizing the training process of soccer players. The purpose of the study was to examine isokinetic peak torque (PT) and total work (TW) exerted by both knee extensors (quadriceps or Q) and flexors (hamstrings or H), intra-limb imbalance and the magnitude and direction of inter-limb asymmetry in top elite senior (n = 109) and junior (n = 74) soccer players. An isokinetic dynamometry was used to measure maximum peak torque of quadriceps (PT-Q) and hamstrings (PT-H) at an angular velocity of 60° ·s-1, as well as the total work for extensors (TW-Q) and flexors (TW-H) at an angular velocity of 240° ·s-1 in the dominant (DL) and non-dominant leg (NDL) during concentric muscle contraction. Intra-limb imbalance and inter-limb asymmetries were calculated using a standard equation. Statistical analysis using t-test and Mann-Whitney U-test revealed: (a) no differences (p > 0.05) between groups for PT-Q and PT-H, (b) greater strength levels (p < 0.05) for TW-Q and TW-H of senior players than juniors, and (c) no differences (p > 0.05) between groups for intra-limb imbalance and inter-limb asymmetry. Additionally, Pearson's chi-kwadrat (χ2) analysis showed no differences (p > 0.05) between groups for intra-limb imbalance and inter-limb asymmetry in relation to the 'normative' values accepted in the literature that indicate an increase in the risk of knee injury. This study shows that isokinetic assessment can be an important tool to identify imbalances/asymmetries and to develop strategies to reduce the risk of muscle injury.

Rationale and Practical Recommendations for Testing Protocols in Female Soccer: A Narrative Review

ABSTRACT

Beato, M, Datson, N, Anderson, L, Brownlee, T, Coates, A, and Hulton, A. Rationale and practical recommendations for testing protocols in female soccer: A narrative review. J Strength Cond Res 37(9): 1912-1922, 2023-The aim of this narrative review is to evaluate the presented literature on tests (aerobic, speed, changes of direction [COD], strength, power, jump, and anthropometry) of the varied components of female soccer and to draw attention to the most suitable protocols to allow practitioners to accurately track players' fitness status. The 2 most common field tests used to assess aerobic fitness are the Yo-Yo intermittent test (level 1 and level 2) and the 30-15 intermittent fitness test because of an ability to measure multiple players at once with a soccer-specific intermittent profile. The sprinting performance can be assessed on distances of <30 m; however, longer distances (e.g., 40 m) allow for achieving peak speed (flying sprint test), which can be assessed using global navigation satellite system. Changes-of-direction capacity has been found to be an important component of players testing and training programs, although there is no "gold standard" to assess COD or repeated sprint ability performance in female players. Lower-limb power can be assessed using jump tests that can use force platforms, jump mats, and optoelectronic devices, while maintaining a good reliability. Several in-direct tests are currently available for assessing anthropometry parameters, such as skinfold thickness, hydrodensitometry, and ultrasound. However, dual-energy x-ray absorptiometry is the most valid and reliable method for assessing body composition in team sport athletes, with the addition of bone health that is a key measure in female athletes. In conclusion, the evidence reported in this review will be able to aid practitioners, coaches, and researchers to decide which tests meet the requirements of their environment.

Measuring Interlimb Asymmetry for Strength and Power: A Brief Review of Assessment Methods, Data Analysis, Current Evidence, and Practical Recommendations

ABSTRACT

Bishop, C, de Keijzer, KL, Turner, AN, and Beato, M. Measuring interlimb asymmetry for strength and power: A brief review of assessment methods, data analysis, current evidence, and practical recommendations. J Strength Cond Res 37(3): 745-750, 2023-The aim of this brief narrative review is to summarize the present evidence, provide recommendations for data analysis, and provide appropriate training methods to reduce strength and power asymmetries within athlete populations. Present evidence shows that a strong interest in the assessment of asymmetry exists. Despite the perceived associated relationship between asymmetry and injury and performance, a clear link is still missing. Practitioners need to be aware of this when they decide to assess asymmetries and later design training interventions. Several bilateral and unilateral tests could be used to assess asymmetries, such as isokinetic dynamometry, the isometric mid-thigh pull, squat, and Nordic hamstring exercise. Based on the current evidence, future investigations require further standardization of methodology and analysis to optimize interpretation (e.g., within session and between session), adoption, and implementation of interlimb asymmetry testing and appropriate interventions. In this review, 3 training interventions have been proposed to reduce existing lower limb asymmetries in sport populations: traditional resistance training, flywheel resistance training, and combined training interventions, with some evidence suggesting that such interventions can reduce lower limb asymmetries. Nonetheless, the number and quality of articles currently available are too limited to draw firm conclusions; therefore, further research is needed to verify whether training interventions can achieve these aims. To develop an understanding and application of interventions addressing interlimb asymmetries within the sport, greater methodological rigor should be applied toward study design, data analysis, and interpretation of future investigations and when appraising the current literature.

Knee Isokinetic Profiles and Reference Values of Professional Female Soccer Players

Few studies have previously evaluated isokinetic parameters in female soccer players in comparison to those in males. The aim of this study was to describe normative quadriceps (Q) and hamstring (H) muscle strength values in professional female soccer players and to examine differences between dominant leg (DL) and nondominant leg (NDL). A standardized test protocol of concentric knee extension and flexion test protocol was conducted using the dynamometer isokinetic system (IsoMEd 2000). All the participants were healthy female professional soccer players from Spanish first and second division teams. Players were assessed for peak torque (PT) and maximum work (MW) values at 60°/s, 180°/s, and 240°/s. The mean difference was 7.17 (p-value = 0.0036), 4.4 (p-value = 0.0386), and 4.25 Nm (p-value = 0.0241) at speed 60°, 180°, and 240°/s, respectively. No statistically significant differences were detected for H-Q values between DL and NDL. This difference was 6.44 (p-value = 0.0449), and 5.87 J (p-value = 0.0266) at speed 60°, and 180°/s. The present study can be a tool that health professionals working with female professional soccer players in their care can use to assess and monitor a particular player.

Assessing Eccentric Hamstring Strength Using the NordBord: Between-Session Reliability and Interlimb Asymmetries in Professional Soccer Players

ABSTRACT

Bishop, C, Manuel, J, Drury, B, Beato, M, and Turner, A. Assessing eccentric hamstring strength using the NordBord: between-session reliability and interlimb asymmetries in professional soccer players. J Strength Cond Res 36(9): 2552-2557, 2022-The aims of the current study were to (a) determine between-session reliability of peak force during the Nordic hamstring exercise (NHE) in professional soccer players during the preseason and (b) determine the subsequent magnitude and direction of peak force asymmetry from the NHE between all test sessions. The NHE was used as a weekly monitoring tool in 14 professional soccer players, throughout a 6-week pre-season period to record peak force from the NordBord device (in Newtons-N) across an average of 6 repetitions for the dominant and nondominant limbs. The first 2 weeks were used as familiarization sessions, with 4 data collection sessions thereafter. When considering relative reliability, intraclass correlation coefficients (ICCs) showed moderate levels of between-session reliability across the 4 weeks on both limbs (dominant = 0.54-0.73 and nondominant = 0.68-0.73) and for the sum total (0.69-0.73). For absolute reliability, both limbs showed coefficient of variation (CV) values <10% across all 4 weeks, with the exception of the dominant limb between sessions 1-2 (10.06%). When determining systematic bias, no meaningful changes were evident in peak force on either limb ( g = -0.59 to 0.40) or for the magnitude of asymmetry ( g = -0.24 to 0.35). However, when assessing consistency in the direction of asymmetry, levels of agreement ranged from 43 to 57% across the data collection period, indicating substantial variation in the superior performing limb from session to session. The findings from this study indicate that 2 familiarization sessions is enough to elicit reliable peak force data during the NHE in professional soccer players during preseason, despite a very small learning effect across the testing period.

Is the Rotatory Knee Stability Immediately Decreased Following a Competitive Soccer Match?

Fatigue induced by soccer playing increases physical efforts, which might alter the transverse knee stability, a known factor that promotes knee injuries, particularly anterior cruciate ligament injury. Thereby, primarily, we aimed to determine whether rotatory knee stability decreases immediately following a competitive soccer match in amateur players. Furthermore, we assessed the role of the preferred and non-preferred limbs to kick a ball in rotatory knee stability and the correlation between performance parameters and rotatory knee stability. We hypothesized that the knee stability decreases immediately after a competitive soccer match in amateur players. Eight healthy amateur soccer players (aged 27.2 ± 4.7 years and with body mass index of 23.8 ± 1.2 kg m−2) were included immediately before and after a competitive soccer match. The rotatory knee stability was assessed in the preferred and non-preferred limbs through the acceleration and jerk of the pivot shift maneuver and by the internal knee rotation of a pivoting landing task. Two-way repeated-measures ANOVA for factors time (before and after the soccer match) and limb (preferred and non-preferred) and multiple comparisons were performed using α = 5%. There was a statistical significance for the main factor time in the acceleration (5.04 vs. 6.90 ms−2, Δ = 1.86 ms−2, p = 0.020, η2 = 0.331) and jerk (18.46 vs. 32.10 ms−2, Δ = 13.64 ms−2, p = 0.004, η2 = 0.456) of the pivot shift maneuver. Rotatory stability decreases following a competitive soccer match in amateur soccer players under fatigue. Both the acceleration and jerk during the pivot shift maneuver is increased without significant internal knee rotation changes during the pivoting landing task.

The Relationship between the Hamstring-to-Quadriceps Ratio and Jumping and Sprinting Abilities of Young Male Soccer Players

The correct torque ratio between the knee joint extensor and flexor muscle groups can effectively prevent injuries to the anterior cruciate ligament and hamstring strain. However, it is unclear whether a high torque ratio of the knee joint flexor muscles to the extensor muscles is beneficial for sport performance. Therefore, the aim of the study was to investigate the relationship between the hamstring-to-quadriceps (H/Q) ratio and sprint times (10- and 30-m) and jump heights (CMJ—countermovement jump and SJ—squat jump) in soccer players. The study examined 26 young elite soccer players (age: 18.1 ± 0.7 years; body height: 1.77 ± 0.05 m; body mass: 72.7 ± 5.7 kg). Knee joint flexor and extensor peak torques were assessed using the Cybex dynamometer (at 60°/s, 120°/s and 180°/s). Additionally, each participant performed the CMJ, SJ, and 30 m sprint. A significant relationship was obtained between the H/Q ratio (60°/s) and 30 m sprint time (r = 0.47). The positive direction of this relationship may indicate an important role of knee joint extensors in sprinting performance. Moreover, the H/Q ratio was not significantly associated with the CMJ, SJ or 10 m sprint performance. The H/Q ratio should be considered together with the peak torque values in terms of the assessment of sprinting and jumping performance.