Supplementary material from "Move less, spend more: The metabolic demands of short walking bouts."

The metabolic cost of steady-state walking is well known; however, across legged animals, most walking bouts are too short to reach steady-state. Here we investigate how bout duration affects the metabolic cost of human walking with varying mechanical powers, metabolic intensity, and durations. Ten participants walked for 10- to 240-second bouts on a stair climber at 0.20, 0.25, and 0.36 m/s and on treadmill at 1.39 m/s. Oxygen uptake was time-integrated and divided by bout duration to get bout average uptake (V̇O2_(b)). Fitting of oxygen uptake kinetics allowed calculating non-metabolic oxygen uptake during phase-I transient, and hence non-steady-state metabolic cost (Cmet_(b)) and efficiency. For 240-second bouts, such variables were also calculated at steady-state. Across all conditions, shorter bouts had higher V̇O2_(b) and Cmet_(b), with proportionally greater non-metabolic oxygen exchange. As bout duration increased, V̇O2_(b), Cmet_(b), and efficiency approached steady-state values. Our findings show that the time-averaged oxygen uptake and metabolic cost are greater for shorter than longer bouts: 30-second bouts consume 20 to 60% more oxygen than steady-state extrapolations. This is partially explained by the proportionally greater non-metabolic oxygen uptake, and leads to lower efficiency for shorter bouts. Inferring metabolic cost from steady-state substantially underestimates energy expenditure for short bouts.