The role of plantar flexor function, isometric strength and task-specific kinetics, in sprint and jump performance

The role of isometric plantar flexion (PF) strength in sprinting and jumping is unclear and, in part, may be due to methodological issues with isometric dynamometry leading to inaccurate measures of PF strength. In addition, the relationship, and thus functional relevance, of isometric PF strength with PF kinetics during these athletic tasks remains unknown. The aim of this study was to assess the interrelationships of isometric PF strength, task-specific sprint and jump PF kinetics and overall performance during jumps and sprint acceleration. Three-dimensional kinematics and ground reaction forces were recorded in 38 healthy individuals of varying athletic ability (18 females; height 1.75 ± 0.09 m; Mass: 70.9 ± 10.0 kg, Age 23 ± 3 years) during: standing isometric PF dynamometry, countermovement jumps (CMJ), drop jumps (DJ) and sprints. Isometric PF strength and sprint/jump PF kinetics were both assessed via inverse dynamics. Sprint speed, CMJ height and DJ reactive strength index defined task performances. Specific sprint and jump PF kinetics showed moderate-large correlations to task performances in 7 out of 9 relationships with peak positive power explaining 19-50% of the variance in the performance of these tasks (r/ρ = 0.434-0.706; p ≤ 0.024). Isometric PF strength, however, was unrelated to performance (CMJ: r = 0.126, DJ: r = 0.301, Sprint: ρ = 0.142), or task-specific PF kinetics during sprinting and jumping (all r ≤ 0.329). Dynamic task-specific function of the PF was an important component of overall performance, but this was not the case for isometric PF strength. These results highlight the distinctiveness of dynamic stretch-shortening cycle specific PF function compared to standing isometric PF strength for athletic performance.