At moderate to fast walking speeds, the human ankle provides net positive work at high-mechanical-power output to propel the body upward and forward during the stance period. On the contrary, conventional ankle–foot prostheses exhibit a passiveelastic response during stance, and consequently, cannot provide net work. Clinical studies indicate that transtibial ampu-tees using conventional prostheses have higher gait metabolic rates than normal. Researchers believe that the main cause for these higher rates is due to the inability of conventional prostheses to provide suf-ficient positive power at terminal stance in the trailing leg to limit heel strike losses of the adjacent leading leg. In this investigation, we evaluate the hypothesis that a powered ankle–foot prosthesis, ca-pable of providing human-like ankle work and power during stance, can decrease the metabolic cost of trans-port (COT) compared to a conventional passive-elastic prosthesis. To test the hypothesis, a powered pros- thesis is built that comprises a unidirectional spring, configured in parallel with a force-controllable actuator with series elasticity. The prosthesis is shown to deliver the high mechanical power and net po-sitive work observed in normal human walking. The rate of oxygen consumption and carbon dioxide production is measured as a determinant of metabolic rate on three unilateral transtibial amputees walking at self- selected speeds. We find that the powered prosthesis decreases the amputee’s metabolic COT on average by 14% compared to the conventional passive-elastic prostheses evaluated (Flex-Foot Ceterus R
and Freedom In-novations Sierra), even though the powered system is over twofold heavier than the conventional devices. These results highlight the clinical importance of prosthetic interventions that closely mimic the mass di-stribution, kinetics, and kinematics of the missing limb.

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