Changes in spring-mass model characteristics during repeated running sprints

This study investigated fatigue-induced changes in spring-mass model characteristics during repeated running sprints. Sixteen active subjects performed 12 × 40 m sprints interspersed with 30 s of passive recovery. Vertical and anterior-posterior ground reaction forces were measured at 5-10 m and 30-35 m and used to determine spring-mass model characteristics. Contact (P 0.001), flight (P 0.05) and swing times (P 0.001) together with braking, push-off and total stride durations (P 0.001) lengthened across repetitions. Stride frequency (P 0.001) and push-off forces (P 0.05) decreased with fatigue, whereas stride length (P = 0.06), braking (P = 0.08) and peak vertical forces (P = 0.17) changes approached significance. Center of mass vertical displacement (P 0.001) but not leg compression (P 0.05) increased with time. As a result, vertical stiffness decreased (P 0.001) from the first to the last repetition, whereas leg stiffness changes across sprint trials were not significant (P 0.05). Changes in vertical stiffness were correlated (r 0.7; P 0.001) with changes in stride frequency. When compared to 5-10 m, most of ground reaction force-related parameters were higher (P 0.05) at 30-35 m, whereas contact time, stride frequency, vertical and leg stiffness were lower (P 0.05). Vertical stiffness deteriorates when 40 m run-based sprints are repeated, which alters impact parameters. Maintaining faster stride frequencies through retaining higher vertical stiffness is a prerequisite to improve performance during repeated sprinting.