Fig. 4. Average mechanical power. Bars are the mean (N=9) average muscle–tendon (MT) positive mechanical power delivered by the sum of the ankle, knee and hip (black bars) and the ankle muscle–tendon system only (white bars) during unpowered overground walking. Gray bars are average exoskeleton positive mechanical power during powered walking on the treadmill. Error bars are ±1 s.e.m. All mechanical power values are normalized by subject mass (W kg^–1). Speeds/step lengths increase from left 0.8 L^* (1.00 m s^–1) to right 1.4 L^* (1.75 m s^–1). Brackets indicate the percentage contribution of bars from right to left. For example, in the 0.8 L^* condition, the exoskeleton average positive mechanical power was 70% of the ankle muscle–tendon average positive mechanical power, ankle muscle–tendon positive mechanical power was 34% of the ankle + knee + hip positive mechanical power and the exoskeleton average positive mechanical power was 24% of the ankle + knee + hip positive average positive mechanical power over the stride.