We examined healthy human subjects wearing robotic ankle exoskeletons to study the metabolic cost of ankle muscle–tendon work during uphill walking. The exoskeletons were powered by artificial pneumatic muscles and controlled by the user’s soleus electromyography. We hypothesized that as the demand for net positive external mechanical work increased with surface gradient, the positive work delivered by ankle exoskeletons would produce greater reductions in users’ metabolic cost. Nine human subjects walked at 1.25ms–1 on gradients of 0%, 5%, 10% and 15%. We compared rates of O2 consumption and CO2 production, exoskeleton mechanics, joint kinematics, and surface electromyography between unpowered and powered exoskeleton conditions. On steeper inclines, ankle exoskeletons delivered more average positive mechanical power (P<0.0001; +0.37±0.03Wkg–1 at 15% grade and +0.23±0.02Wkg–1 at 0% grade) and reduced subjects’ net metabolic power by more (P<0.0001; –0.98±0.12Wkg–1 at 15% grade and –0.45±0.07Wkg–1 at 0% grade). Soleus muscle activity was reduced by 16–25% when wearing powered exoskeletons on all surface gradients (P<0.0008). The ‘apparent efficiency’ of ankle muscle–tendon mechanical work decreased from 0.53 on level ground to 0.38 on 15% grade. This suggests a decreased contribution from previously stored Achilles’ tendon elastic energy and an increased contribution from actively shortening ankle plantar flexor muscle fibers to ankle muscle–tendon positive work during walking on steep uphill inclines. Although exoskeletons delivered 61% more mechanical work at the ankle up a 15% grade compared with level walking, relative reductions in net metabolic power were similar across surface gradients (10–13%). These results suggest a shift in the relative distribution of mechanical power output to more proximal (knee and hip) joints during inclined walking.

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