Mechanical step variability during treadmill running.

The present study was designed to study intra-individual step variability measured both on vertical displacement of the body (delta Z) and on step time (delta t) parameters by means of a kinematic arm and during treadmill running. A group of 17 subjects ran successively at 60%, 80%, 100% and 140% of their maximal aerobic velocity (Vamax). The total number of steps analysed was 6116. The absolute delta Z step variability (sigma delta Z) ranged between 5 mm and 21 mm while the absolute delta t variability (sigma delta t) ranged between 6 ms and 40 ms. Step variabilities were due to step asymmetry (from 38.5% to 48.5% of the step variability) and to stride variability. For submaximal velocities (60%, 80%, and 100% Vamax) both sigma delta t and sigma delta Z were independent of velocity or body dimensions whereas differences between subjects were significant (P < 0.01) for sigma delta Z. On the other hand, variabilities were significantly increased when velocity was changed from submaximal to the 140% Vamax level. Furthermore, at submaximal levels sigma delta Z was linked to the subject's energy cost of running (P < 0.05). Therefore, the intra-individual step variability should not be neglected in future studies on mechanical efficiency of running and it is suggested that, to obtain a good accuracy (better than 1%, P < 0.05) on mean value and variability of the mechanical parameters, measurements should be performed on at least 32-64 consecutive steps, which corresponds to about 15 to 20s of running.