Fig. 4. Exoskeleton performance. Bar graphs show the mean (A) change in net metabolic power (powered–unpowered; W kg^–1) as a result of powered assistance from bilateral ankle exoskeletons, (B) exoskeleton average positive (black), negative (white) and net (dark gray) mechanical power (W kg^–1) over a stride for powered walking and (C) exoskeleton performance index (unitless) of the nine subjects. Performance index 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 above bars are equivalent ankle muscle–tendon `apparent efficiency' values (see Materials and methods for details). For all panels, surface inclines increase from left (0%-level) to right (15%-uphill). Error bars are ±1 s.e.m. Asterisks (in A) indicate statistical significance for comparison of powered versus unpowered net metabolic power (P<0.05).