On the estimation of total muscle work done in human walking

In many whole-body exercise forms, e.g. cycling, the relationship between metabolic rate (MR) and mechanical power output during exercise is linear and unique. Such linearity is not seen for walking. We investigated whether total muscle power (Ptot), i.e. power required for the inverted pendulum motion in walking plus power against net external resistance, demonstrated such a single relationship between Ptot and MR, independent of walking conditions. We studied walking under conditions in which considerable net external work against the environment was done (walking uphill and when resistive forces are imposed) as well as on the level, i.e. without net external work done. Fifteen adults walked freely on a large treadmill at 27 combinations of three speeds (3, 4.5 and 6 kmh-1), three inclines (0%, 3% and 6%) and three resistive forces (0.1×9.81, 2.5×9.81 and 5×9.81 N). Kinematics were recorded by motion capturing. MR was estimated from gas exchange recordings. Required Ptot generated by skeletal muscle was estimated as the power associated with step-to-step transitions (Psst) in addition to net external power (Pext), with Ptot=Pext+Psst. The Pext-MR relationship was not entirely unique (R2=0.883) and was strongly affected by speed (P=0.004). The Ptot-MR relationship was stronger (R2=0.972) and the influence of walking speed was almost cancelled out. The Ptot-MR relationship resembles that found for cycling. On that basis, we conclude that Ptot seems to incorporate the major amount of work done during walking.