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

Objective The aim of this study is to examine the relationship between eccentric hamstring muscle strength asymmetry and landing phase force asymmetry following a vertical jump, as well as functional performance outcomes (lower extremity stiffness), in professional male soccer players. Methods Thirty-one male soccer players actively competing in a professional league voluntarily participated in the study. The athletes’ vertical jump height, lower-limb stiffness and Peak Landing Vertical Ground Reaction Force (vGRF) data were measured using ForceDecks dual force platforms, eccentric hamstring peak forces were measured using the NordBord testing apparatus. The percentages of asymmetry between the two legs were calculated using the Bilateral Asymmetry Index (BAI) formula. Based on the athletes’ individual foot dominance, the data were recoded as Dominant (D) and Non-Dominant (ND); t-tests for dependent groups, Spearman correlation analysis, and categorical cross-tabulation matrices were used in the analyses. Results Group-level functional limb comparisons revealed trivial and statistically non-significant differences between dominant and non-dominant limbs in normalized eccentric hamstring strength (4.59 ± 0.83 N/kg vs. 4.45 ± 0.84 N/kg; p = 0.056, d = 0.168) and peak landing vGRF (37.49 ± 10.33 N/kg vs. 36.13 ± 9.77 N/kg; p = 0.068, d = 0.135). Average dynamic lower-limb stiffness ( K vert ) was 31.42 ± 6.18 kN/m. In relative asymmetry analyses, no linear relationship was found between isolated hamstring asymmetry (8.95 ± 6.21%) and dynamic landing vGRF asymmetry (11.50 ± 7.84%) (ρ = 0.028, 95% CI [-0.330, 0.378], p = 0.881). According to categorical cross-tabulations: 53.8% (n=7) of the 13 athletes in the high-risk group (>10% asymmetry) in the Nordic test distributed the load evenly across their legs during landing; 22.2% (n=4) of the 18 athletes who were within the safe range (≤10%) in the Nordic test were found to exhibit high risk (>15% asymmetry) at the moment of landing. Conclusion Isolated open-chain eccentric muscle asymmetries in elite soccer players do not directly correlate with dynamic multi-joint landing asymmetries, likely reflecting distinct task-specific biomechanical demands and potential neuromuscular compensation strategies. Therefore, incorporating dynamic landing mechanics (vGRF) alongside open-chain strength tests provides a more comprehensive functional screening of athletic movement strategies.