Interaction between step-to-step variability and metabolic cost of transport during human walking

Minimizing the metabolic cost of transport can affect selection of the preferred walking speed. While many factors can affect metabolic cost of transport during human walking, its interaction with step-tostep variability is unclear. Here, we aimed to determine the interaction between metabolic cost of transport and step length variability during human walking at different speeds. In particular, two aspects of step length variability were analyzed: the amount of variations ('variations') and the organization of the step-to-step fluctuations ('fluctuations'). Ten healthy, young participants walked on a treadmill at five speeds, ranging from 0.75 to 1.75 m s^-1. Metabolic cost of transport, step length variations (coefficient of variation) and step length fluctuations (quantified via detrended fluctuation analysis) were calculated. A mixed-model ANOVA revealed that variations and walking speed were strong predictors of metabolic cost of transport (R²=0.917, P<0.001), whereas fluctuations were not. Preferred walking speed (1.05±0.20 m s^-1) was not significantly different from the speed at which metabolic cost of transport was minimized (1.04 ±0.05 m s^-1; P=0.792), nor from the speed at which fluctuations were most persistent (1.00±0.41 m s^-1; P=0.698). The minimization of variations occurred at a faster speed (1.56±0.17 m s^-1) than the preferred walking speed (P<0.001). Step length variations likely affect metabolic cost of transport because greater variations are indicative of suboptimal, mechanically inefficient steps. Fluctuations have little or no effect on metabolic cost of transport, but still may relate to preferred walking speed.