Minimization of muscle activation costs demanded from mechanical work and power accounts for selection of duty factor in human gaits.

Duty factor DF - the proportion of a stride a foot is in contact with the ground - is of fundamental mechanical importance, and is often viewed as a defining kinematic parameter distinguishing walking (DF>0.5) from running (DF<0.5). However, the mechanical and/or physiological considerations that determine duty factor are not well understood. Here, a model is proposed that focuses on the interaction between mechanical and muscle costs to account for duty factor in human gaits. It minimizes the activation costs associated with mechanical work or power demand during muscle contraction (whichever is the more demanding). Empirical observations match model predictions using initial muscle parameters over a range of speeds within gaits. However, a better match is achieved - and a better account for the walk-run transition - with tuned muscle parameters. The tuned model is validated with responses in duty factor to walking at a range of imposed, unnatural step frequencies.