Relationship between knee isokinetic muscle strength and countermovement jump performance in elite male football players

BACKGROUND: Countermovement jump (CMJ) performance is widely used to assess explosive lower-limb function in football players. Although knee isokinetic strength is frequently measured in elite sport environments, the extent to which it relates to CMJ performance remains unclear, particularly when CMJ is performed with free arm movement. Therefore, the aim of this study was to examine the relationship between knee isokinetic muscle strength characteristics and CMJ performance in elite male football players. METHODS: Twenty-four elite male football players (age 23.83 ± 5.98 years) participated in this cross-sectional study. CMJ height was assessed using an optical measurement system (Optojump Next). Concentric knee extensor and flexor peak torque was measured using an isokinetic dynamometer at angular velocities of 60°/s and 180°/s and expressed as peak torque/body weight% (PT/BW,%). Pearson correlation and linear regression analyses were used to examine associations between isokinetic strength variables and CMJ performance. Bilateral differences, hamstring-to-quadriceps (H/Q) ratios, and inter-limb asymmetries were also analyzed. RESULTS: Significant positive correlations were observed between CMJ height and knee extensor peak torque expressed as PT/BW (%) at both angular velocities. Stronger relationships were found at 180°/s (r = 0.558-0.642, p ≤ 0.005) compared with 60°/s (r = 0.483-0.500, p < 0.05). Regression analyses showed that knee extensor strength at 180°/s explained up to 41.2% of the variance in CMJ height. Hamstring strength demonstrated weaker and less consistent associations with CMJ performance, while H/Q ratios and inter-limb asymmetries were not significantly related to jump height. CONCLUSION: Quadriceps isokinetic strength expressed as PT/BW (%) was significantly associated with CMJ performance in elite male football players, with stronger relationships observed at higher angular velocity. These findings suggest that knee extensor strength assessed at higher angular velocity is meaningfully associated with explosive lower-limb performance and may provide useful complementary information within routine neuromuscular monitoring in professional football.