Joint Flexibility and Isometric Strength Parameters Are Not Relevant Determinants for Countermovement Jump Performance.

Vertical jumps are of great importance as a performance predictor for many types of sports that require speed and agility. However, to date, it is not clear if flexibility and/or the strength of the different leg muscles are determinants for countermovement jump (CMJ) performance. Therefore, the purpose of this study was to relate isometric maximum voluntary contraction (MVC) torque and the flexibility of various muscle groups of the lower body with CMJ performance. Thirty-six healthy male volunteers participated in this study. The participants performed MVCs of the knee extensors, knee flexors, and plantar flexors on a dynamometer. Moreover, range of motion of the hip flexors and plantar flexors was assessed with 3D motion capture, and the range of motion of the knee flexors (hamstrings) was assessed with a Sit n' Reach® box. CMJs were assessed with a force platform. The correlation analysis revealed a significant moderate correlation of CMJ height with the flexibility of the hip flexors (rP = -0.39) and plantar flexors (rP = 0.47), but not the knee flexors. Moreover, we found that absolute MVC values are not related to CMJ height. However, we did find that knee extensor MVC relative to body mass is significantly related to CMJ height (rP = 0.33) with a moderate magnitude. Although we found significant correlations, the magnitudes of correlations vary between trivial and large according to a 90% confidence interval. Thus, this indicates that range of motion or strength of the assessed leg muscles can explain CMJ performance only to a limited extent.