The metabolic cost of producing joint moments is greater at the hip than the ankle.

Older adults walk using their hips relatively more and their ankles relatively less than young adults. This "distal-to-proximal redistribution" in leg joint mechanics is thought to drive the age-related increase in metabolic rate during walking. However, many morphological differences between hip and ankle joints make it difficult to predict how - or if - the distal-to-proximal redistribution affects metabolic rate during walking. To address this uncertainty, we compared the metabolic rate of participants while they repeatedly produced isolated hip and ankle moment cycles on a dynamometer following biofeedback. Overall, participants produced greater joint moments at their ankle vs. hip (p=0.004) and correspondingly activated their largest ankle extensor muscle more than their largest hip extensor muscle (p=0.046). Cycle average muscle activation across other hip and ankle extensors was nondifferent (all p=0.080-0.848). Despite producing greater joint moments using slightly more relative muscle activation at the ankle, participants expended more net metabolic power while producing moments at the hip (p=0.002). Therefore, producing joint extension moments at the hip requires more metabolic energy than that at the ankle. Our results support the notion that the distal-to-proximal redistribution of joint mechanics contribute to greater metabolic rate during walking in older vs. young adults.