The interplay between gastrocnemius medialis force-length and force-velocity potentials, cumulative EMG activity and energy cost at speeds above and below the walk to run transition speed.

Abstract

The aim of this study was to investigate the interplay between the force–length (F–L) and force–velocity (F–V) potentials of gastrocnemius medialis (GM) muscle fascicles, the cumulative muscle activity per distance travelled (CMAPD) of the lower limb muscles (GM, vastus lateralis, biceps femori, tibialis anterior) and net energy cost (Cnet) during walking and running at speeds above and below the walk-to-run transition speed( walking:2–8kmh−1;running:6-10kmh−1). A strong association was observed between Cnet and CMAPD: both changed significantly with walking speed but were unaffected by speed in running. The F–LandF–V potentials decreased with speed in both gaits and, at 6–8kmh−1, weres ignificantly larger in running. At low to moderate walking speeds (2–6kmh−1), the changes in GM force potentials were notas sociated with substantial changes in CMAPD (andCnet), where as at walking speeds of 7–8 km h−1, even small changes in force potentials were associated with steep increases in CMAPD (andCnet). These data suggest that:(i) the walk to run transition could be explained by an abrupt increase in Cnet driven by an upregulation of the EMG activity (e.g.,inCMAPD) at sustained walking speeds (>7kmh−1) and (ii) there duction inthe muscle’s ability to produce force (e.g.,intheF–LandF–V potentials) contributes to the increase in CMAPD (andCnet). Switching to running allows regaining of high force potentials, thus limiting the increase in CMAPD (and Cnet) that would otherwise occur to sustain the increase in locomotion speed.