Evaluating the "cost of generating force" hypothesis across frequency in human running and hopping

Abstract

The volume of active muscle and duration of extensor muscle force well-explain the associated metabolic energy expenditure across body mass and speed during level-ground running and hopping. However, if these parameters fundamentally drive metabolic energy expenditure, then they should pertain to multiple modes of locomotion and provide a simple framework for relating biomechanics to metabolic energy expenditure in bouncing gaits. Therefore, we evaluated the ability of the cost of generating force hypothesis to link biomechanics and metabolic energy expenditure during human running and hopping across step frequencies. We asked participants to run and hop at 0%, {+/-}8% and {+/-}15% of preferred step frequency. We calculated changes in active muscle volume, force duration, and metabolic energy expenditure. Overall, as step frequency increased, active muscle volume decreased due to postural changes via effective mechanical advantage (EMA) or duty factor. Accounting for changes in EMA and muscle volume better related to metabolic energy expenditure during running and hopping at different step frequencies than assuming a constant EMA and muscle volume. Thus, to ultimately develop muscle mechanics models that can explain metabolic energy expenditure across different modes of locomotion, we suggest more precise measures of muscle force production that include the effects of EMA.