Changes in spring-mass behavior and muscle activity during an exhaustive run at _ VO 2max

Abstract : Purpose: The aim of this study was to evaluate concomitantly the changes in leg-spring behavior and the associated modifications in the lower limb muscular activity during a constant pace run to exhaustion at severe intensity. Methods: Twelve trained runners performed a running test at the velocity associated with _ VO 2max (5.1 70.3 m s −1 ; mean time to exhaustion: 3537 69 s). Running step spatiotemporal parameters and spring-mass stiffness were calculated from vertical and horizontal components of ground reaction force measured by a 6.60 m long force platform system. The myoelectrical activity was measured by wireless surface electrodes on eight lower limb muscles. Results: The leg stiffness decreased significantly (−8.9%; Po 0.05) while the vertical stiffness did not change along the exhaustive exercise. Peak vertical force (−3.5%; Po 0.001) and aerial time (−9.7%; P o0.001) decreased and contact time significantly increased (+4.6%; Po 0.05). The myoelectrical activity decreased significantly for triceps surae but neither vastus medialis nor vastus lateralis presented significant change. Both rectus and biceps femoris increased in the early phase of swing (+14.7%; P o0.05) and during the pre-activation phase (+16.2%; Po 0.05). Conclusion: The decrease in leg spring-stiffness associated with the decrease in peak vertical ground reaction force was consistent with the decline in plantarflexor activity. The biarticular rectus femoris and biceps femoris seem to play a major role in the mechanical and spatiotemporal adjustments of stride pattern with the occurrence of fatigue during such exhaustive run.