Fig. 4 Lower limb joint kinetics. (A) Nine subject mean (thick black curve) + 1 s.d. (thin black curve) mechanical power delivered by each of the lower limb joints over the stride from heel-strike (0%) to heel-strike (100%). Left and right legs are averaged for each subject. Curves are for unpowered walking overground at 1.25 m s^–1. The mean exoskeleton mechanical power from the end of practice session 3 (thick dark gray curve) + 1 s.d. (thin dark gray curve) is overlaid on the bottom subplot for the ankle joint mechanical power. Mechanical power is computed as the product of exoskeleton torque and ankle joint angular velocity and is normalized by subject mass. Positive power indicates energy transferred to the user and negative power indicates energy absorbed from the user. (B) Bars showing the nine subject mean positive mechanical power delivered by the sum of the ankle, knee and hip joints (black) and ankle joint (white) during unpowered walking and the exoskeletons (gray) during powered walking. Error bars are ±1 s.e.m. All mechanical power values are normalized by subject mass. Braces indicate the percentage contribution of bars from right to left. For example, the exoskeleton average positive mechanical power was 63% of the ankle joint average positive mechanical power over the stride.