Relationships between knee muscle strength and functional performance in professional soccer players

Functional tests such as the Modified Star Excursion Balance Test (mSEBT), hop tests, and jump tests are widely used to monitor performance, neuromuscular function, and readiness in soccer players; however, their relationship with knee muscle strength remains unclear.

This cross-sectional study investigated the associations between commonly used functional test outcomes and isokinetic quadriceps and hamstrings peak torque at 60°/s in professional male soccer players. Forty asymptomatic athletes completed the functional test battery, and isokinetic data were available for 38 players. The assessment included the mSEBT, single-leg hop for distance, bilateral hop, bilateral vertical jump, and single-leg vertical jump. Concentric isokinetic peak torque of the quadriceps and hamstrings was assessed at 60°/s using a Biodex System 4 dynamometer. Limb Symmetry Indices were calculated for strength and functional measures.

Bilateral vertical jump height showed moderate correlations with quadriceps peak torque, whereas single-leg vertical jump performance showed weak, side-specific associations with right hamstrings torque and left quadriceps torque. No meaningful correlations were found between single-leg hop performance and knee muscle strength. Strong inter-limb associations were observed for the mSEBT and single-leg hop test, but these tests were not significantly associated with one another. Functional symmetry indices were high, although they were not associated with quadriceps strength symmetry.

These findings indicate that functional tests reflect distinct neuromuscular attributes and should not be considered interchangeable with isolated measures of knee muscle strength. Therefore, return-to-sport and performance-monitoring decisions should integrate task-specific functional testing with objective strength assessment.