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 (Z) and on step time (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 (v_amax). The total number of steps analysed was 6116. The absolute Z step variability (Z) ranged between 5 mm and 21 mm while the absolute t variability (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%v_amax) both t and Z were independent of velocity or body dimensions whereas differences between subjects were significant (P < 0.01) for Z. On the other hand, variabilities were significantly increased when velocity was changed from submaximal to the 140%v_amax level. Furthermore, at submaximal levels 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 20 s of running.