Fig. 5 Exoskeleton performance. (A) Bars showing the nine subject mean change in net metabolic power (powered – unpowered) due to powered assistance from bilateral exoskeletons. Error bars are ±1 s.e.m. All metabolic power values are normalized by subject mass. Right axis indicates the change in net metabolic power as a percentage difference from unpowered walking during each session. (B) Bars indicating nine subject mean ± 1 s.e.m. exoskeleton performance index (unitless). Performance index indicates the fraction of average exoskeleton positive mechanical power that results in a reduction in net metabolic power, assuming that artificial muscle work directly replaces biological muscle work. A performance index of 1.0 would indicate that all of the exoskeleton pneumatic muscle work replaced underlying biological muscle performing positive mechanical work with ⁺_muscle. For both panels, practice sessions (1–3) are shown left to right with beginning period (minutes 3–5) in light gray and end period (minutes 27–29) in dark gray.