Changes in spring-mass model characteristics during repeated running sprints

Abstract

This study investigated fatigue-induced changes in spring-mass model characteristics during repeated running sprints. Sixteen active subjects performed 12×40m sprints interspersed with 30s of passive recovery. Vertical and anterior-posterior ground reaction forces were measured at 5-10m and 30-35m 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 5-10m, most of ground reaction force-related parameters were higher (P<0.05) at 30-35m, whereas contact time, stride frequency, vertical and leg stiffness were lower (P<0.05). Vertical stiffness deteriorates when 40m 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.