Hamstring Strength Asymmetry Does Not Predict Landing Force Imbalance in Soccer

Isolated eccentric hamstring muscle strength asymmetry measured on specialized testing apparatus does not correlate with bilateral force asymmetry during dynamic vertical jump landings among professional male soccer players, according to a biomechanical study published in Frontiers in Physiology. The research, carried out across professional outfield players competing in an active league, evaluated whether single-joint strength imbalances between legs translate directly into uneven ground reaction forces when absorbing mechanical impact. The findings indicate that peripheral strength discrepancies observed during isolated laboratory testing fail to mirror the multi-joint loading mechanics exhibited during explosive jump landings, highlighting a fundamental distinction between isolated open-chain muscle capacity and closed-chain movement dynamics.

Hamstring strain injuries remain among the most frequent causes of prolonged competitive absence in professional soccer, where dynamic demands involve frequent high-intensity sprinting, rapid changes of direction, accelerations, and jumps. Biomechanical investigations demonstrate that the vast majority of these injuries occur during the late swing phase of high-speed running, a period during which the hamstring muscles lengthen under intense tension while contracting eccentrically to decelerate knee extension and counteract hip flexion. Deficits in eccentric strength and bilateral strength asymmetries have long been categorized as primary modifiable risk factors for hamstring strain injuries. Furthermore, soccer players routinely engage in unilateral and asymmetrical motor tasks, including kicking, cutting, and single-leg landings, which can foster strength discrepancies between dominant and non-dominant limbs.

During dynamic vertical landings, however, the lower-extremity kinetic chain is tasked with rapidly dissipating substantial ground reaction forces within a brief window of milliseconds. Although the quadriceps femoris acts as the primary agonist responsible for eccentric knee flexion deceleration and vertical shock absorption, the co-activation of the hamstring complex remains biomechanically necessary for secondary joint stabilization and dynamic knee control. Operating as dynamic antagonists, the bi-articular hamstrings exert posterior shear force on the proximal tibia to counteract anterior tibial translation driven by forceful quadriceps contractions, while simultaneously decelerating hip flexion and regulating dynamic vertical stiffness. Despite extensive research linking isolated hamstring capacity to athletic performance, the degree to which single-joint eccentric knee flexor asymmetries correspond to multi-joint kinetic distribution during landings has remained unclear.

To investigate this relationship, researchers examined 31 male soccer players actively competing on a professional first-team roster, with an average age of 23.13 years, an average height of 178.61 centimeters, and an average body mass of 76.11 kilograms. The participant group comprised six center backs, five fullbacks, twelve midfielders, four wingers, and four forwards, with 27 players identified as right-leg dominant and four as left-leg dominant based on their preferred kicking limb. Goalkeepers were excluded due to their distinct technical movement patterns and landing demands, as were any players with recent musculoskeletal injuries requiring surgery, absences exceeding two weeks within the prior six months, or acute joint pain on testing days. Assessments were conducted on a single in-season testing day scheduled precisely 72 hours after a competitive match and 48 hours before the subsequent match fixture, following a designated active recovery day to prevent acute match-induced neuromuscular fatigue.

The testing battery incorporated dual force platforms and an automated eccentric hamstring testing device following a standardized warm-up of stationary cycling and mobility exercises. Vertical jump height, lower-limb vertical stiffness, and peak landing vertical ground reaction force were captured at 1000 Hz using a dual-component ForceDecks platform system, during which athletes performed maximal countermovement jumps with hands fixed on their iliac crests to eliminate upper-body momentum. Eccentric hamstring peak forces were measured at 500 Hz using the NordBord testing apparatus, requiring athletes to execute controlled forward falls from a kneeling posture while maintaining full hip extension to isolate knee flexor braking without trunk flexion. Inter-limb asymmetries were determined using the Bilateral Asymmetry Index formula, recoded according to functional leg dominance rather than absolute anatomical orientation, and evaluated against established thresholds of 10 percent for isolated hamstring asymmetry and a tiered 10 to 15 percent cut-off for landing force asymmetry.

At the squad level, functional limb comparisons revealed trivial and statistically non-significant differences between dominant and non-dominant limbs for both normalized eccentric hamstring strength and peak landing impact forces. Dominant limbs averaged 4.59 Newtons per kilogram in eccentric hamstring strength compared to 4.45 Newtons per kilogram in non-dominant limbs, reflecting an average asymmetry of 8.95 percent and a non-significant paired difference. Similarly, peak landing vertical ground reaction force measured 37.49 Newtons per kilogram on the dominant leg versus 36.13 Newtons per kilogram on the non-dominant leg, yielding a squad average landing asymmetry of 11.50 percent. Across playing positions, one-way analysis of variance confirmed baseline anthropometric homogeneity as well as uniform asymmetry distributions for both isolated strength and dynamic impact absorption.

Relative asymmetry analyses demonstrated an absence of any linear relationship between isolated eccentric hamstring asymmetry and dynamic landing vertical ground reaction force asymmetry. Non-parametric Spearman correlation analysis revealed a correlation coefficient of 0.028 with a 95 percent confidence interval spanning minus 0.330 to positive 0.378, indicating that magnitude asymmetry in isolated knee flexion does not correlate with magnitude asymmetry during multi-joint landing. Furthermore, signed directional asymmetry demonstrated no association, with a correlation coefficient of 0.041. Directional concordance analysis showed that only 45.2 percent of the players favored the same limb across both isolated strength and dynamic landing tasks, whereas 54.8 percent exhibited divergent limb dominance between the two testing modalities. Additionally, lower-limb dynamic vertical stiffness averaged 31.42 kilonewtons per meter and exhibited no significant relationship with eccentric hamstring asymmetry.

Categorical cross-tabulation matrices based on clinical risk tiers further illustrated why linear correlations between the assessments were near zero. Of the 13 soccer players classified as high risk with greater than 10 percent asymmetry on the isolated Nordic hamstring assessment, 53.8 percent distributed their peak landing forces symmetrically across both limbs with 10 percent or less asymmetry during the countermovement jump. Conversely, among the 18 athletes categorized within the clinically safe range of 10 percent or less asymmetry on the Nordic test, 22.2 percent exhibited dynamic landing force asymmetries exceeding 15 percent, placing them directly into the high-risk dynamic landing quadrant. Statistical evaluation via Fisher’s exact test revealed no significant association between the categorical classifications of the two screening tools, and Cohen’s linear weighted Kappa confirmed a lack of inter-task agreement beyond chance.

These divergent outcomes suggest that elite soccer players utilize task-specific motor control strategies and neuromuscular compensation mechanisms to equalize impact forces across their lower extremities despite peripheral strength deficits. While the NordBord evaluates single-joint eccentric knee flexor capacity under strict hip stabilization, jump landing represents a closed-chain kinetic task governed by coordinated triple flexion at the hip, knee, and ankle. The dissipation of peak vertical ground reaction forces during double-leg landings is primarily driven by quadriceps eccentric braking and dynamic joint excursion rather than isolated posterior chain strength alone. As a consequence, high-level athletes may acutely modulate synergistic muscle recruitment and multi-joint kinematics to mask isolated strength deficiencies, while athletes possessing symmetrical hamstring strength may still demonstrate landing force asymmetry due to technical movement errors, poor spatial positioning, or stiff landing mechanics.

The researchers noted several methodological considerations, including the cross-sectional design without prospective injury surveillance and the absence of concurrent electromyography or quadriceps strength measurements. The sample of 31 professional outfield players represented the entire available first-team squad, providing high ecological validity among elite competitors but limiting statistical power to moderate-to-large associations, meaning subtle biological relationships cannot be fully excluded. Because isolated single-joint strength testing fails to identify kinetic landing imbalances, the authors concluded that athletic screening protocols should avoid relying exclusively on isolated devices such as the NordBord, recommending the addition of dynamic dual-force platform assessments to evaluate multi-joint movement strategies more comprehensively.

Subject of Research: Bilateral asymmetry and biomechanics in professional soccer players

Article Title: Comparative analysis of isolated eccentric hamstring asymmetry and dynamic landing force asymmetry in professional soccer players

Article References: Akyüz, F., Yıldırım, S., Özşengezer, N., Ekin, M. A., Buru, K., & Akyüz, M. (2026). Comparative analysis of isolated eccentric hamstring asymmetry and dynamic landing force asymmetry in professional soccer players. Frontiers in Physiology, 17, Article 1957845. https://doi.org/10.3389/fphys.2026.1957845

Image Credits: AI Generated

DOI: 10.3389/fphys.2026.1957845

Keywords: Soccer Biomechanics, Eccentric Hamstring Strength, Ground Reaction Force, NordBord, Injury Prevention, Countermovement Jump, ForceDecks, Sports Science, Comparative, analysis, isolated, eccentric