Learning to be economical: the energy cost of walking tracks motor adaptation

Key points center dot Neuroscientists often suggest that we adapt our movements to minimize energy use; however, recent studies have provided conflicting evidence in this regard. center dot In the present study, we show that motor learning robustly increases the economy of locomotion during split-belt treadmill adaptation. center dot We also demonstrate that reductions in metabolic power scale with the magnitude of adaptation and are also associated with a reduction in muscle activity throughout the lower limbs. center dot Our results provide strong evidence that increasing economy may be a key criterion driving the systematic changes in co-ordination during locomotor adaptation. center dot These findings may also facilitate the design of novel interventions to improve locomotor learning in stroke survivors. Abstract Many theories of motor control suggest that we select our movements to reduce energy use. However, it is unclear whether this process underlies short-term motor adaptation to novel environments. Here we asked whether adaptation to walking on a split-belt treadmill leads to a more economical walking pattern. We hypothesized that adaptation would be accompanied by a reduction in metabolic power and muscle activity and that these reductions would be temporally correlated. Eleven individuals performed a split-belt adaptation task where the belt speeds were set at 0.5 and 1.5 m s-1. Adaptation was characterized by step length symmetry, which is the normalized difference in step length between the legs. Metabolic power was calculated based on expired gas analysis, and surface EMG was used to record the activity of four bilateral leg muscles (tibialis anterior, lateral gastrocnemius, vastus lateralis and biceps femoris). All participants initially walked with unequal step lengths when the belts moved at different speeds, but gradually adapted to take steps of equal length. Additionally, net metabolic power was reduced from early adaptation to late adaptation (early, 3.78 +/- 1.05 W kg1; and late, 3.05 +/- 0.79 W kg1; P < 0.001). This reduction in power was also accompanied by a bilateral reduction in EMG throughout the gait cycle. Furthermore, the reductions in metabolic power occurred over the same time scale as the improvements in step length symmetry, and the magnitude of these improvements predicted the size of the reduction in metabolic power. Our results suggest that increasing economy may be a key criterion driving locomotor adaptation.