Fig. 5. Exoskeleton performance. Bars indicate nine subject means. Error bars are ±1 s.e.m. (A) Change in net metabolic power (powered–unpowered; W kg^–1) as a result of powered assistance from bilateral ankle exoskeletons. Values listed below bars indicate percentage difference in net metabolic power for powered versus unpowered walking in each condition. Asterisks indicate statistical significance for comparison of powered versus unpowered net metabolic power (P<0.05). (B) Exoskeleton average positive (black), negative (white) and net (dark gray) mechanical power (W kg^–1) over a stride for powered walking. (C) Exoskeleton performance index. Performance index (unitless) indicates the fraction of ankle muscle–tendon positive work performed by plantar flexor muscle shortening rather than Achilles' tendon recoil (i.e. muscle work fraction). Numbers listed above bars are equivalent ankle muscle–tendon `apparent efficiency' values (see Material and methods for details). For all panels, speeds/step lengths increase from left 0.8 L^* (1.00 m s^–1) to right 1.4 L^* (1.75 m s^–1).