Dissecting the metabolic costs of up- and down-hill walking

Work- and collision-based models of locomotion are often used to describe the relationship between the body’s mechanical work requirements and metabolic energy expenditure. While work- and collision-based models do a reasonable job of relating mechanical work to metabolic cost at a system level, these models may not map to the underlying force and work demands of muscle, which directly affects energy expenditure. We collected motion capture, force, electromyography and ultrasound data from the main power producing muscles during uphill and downhill walking between +/− 15% grade. These data were used to evaluate a musculoskeletal modelling approach to simulate muscle force- and work-related costs that could be compared to metabolic power that we measured using indirect calorimetry. Muscle force-related costs (activation heat rate + maintenance heat rate) increased at steeper up- and down-hill grades and were moderately correlated with mean joint moments. Muscle work-related costs (mechanical work rate + shortening / lengthening heat rate) increased as grade became more positive and were strongly correlated with net joint work. Compared to traditional models, the inclusion of a term to account for muscle force-related costs should lead to a more explanatory cost model that maps directly to the mechanical demands of muscle.