Changes in running mechanics and spring-mass behaviour induced by a 5-hour hilly running bout

The aim of this study was to investigate changes in running mechanics and spring-mass behaviour with fatigue induced by 5-hour hilly running (5HHR). Running mechanics were measured pre- and post-5HHR at 10, 12 and 14 km center dot h-1 on an instrumented treadmill in eight ultramarathon runners, and sampled at 1000 Hz for 10 consecutive steps. Contact (tc ) and aerial (ta ) times were determined from ground reaction force (GRF) signals and used to compute step frequency (f). Maximal GRF, loading rate, downward displacement of the centre of mass (z), and leg length change (L) during the support phase were determined and used to compute both vertical (Kvert ) and leg (Kleg ) stiffness. A significant decrease in tc was observed at 12 and 14 km center dot h-1 resulting in an increase of f at all speeds. Duty factor and Fmax significantly decreased at 10 km center dot h-1. A significant increase in Kvert and Kleg was observed at all running speeds with significant decreases in z and L. Despite the shorter duration, the changes in running mechanics appeared to be in the same direction (increased f and Kvert , decrease in z and Fmax ) but of lower amplitude compared with those obtained after an ultra-trail or an ultramarathon.