Fig. 3. Ankle exoskeleton mechanics. Thick lines show the mean ankle joint angular velocity (left column), exoskeleton torque (middle column) and exoskeleton mechanical power (right column) over the stride from heel strike (0%) to heel strike (100%) of nine subjects. Data are average of left and right legs. Each row is walking data at a single speed/step length (0.8 L^* at top to 1.4 L^* at bottom). In each subplot, lines are for unpowered (black circles), and powered walking (dark gray circles). Thin lines are +1 s.d. Stance is 0–60% of the stride, swing 60–100%. Ankle joint angular velocity (deg. s^–1) is positive for ankle plantarflexion. Exoskeleton torque that acts to plantar flex the ankle is positive. Torque is the product of artificial muscle force and moment arm length and is normalized by subject mass (Nm kg^–1). Exoskeleton mechanical power is the product of exoskeleton torque and ankle joint angular velocity and is normalized by subject mass (W kg^–1). Positive exoskeleton mechanical power indicates transfer of energy from exoskeletons to the user's ankle muscle–tendon system. In the second and third columns, the ankle joint net muscle moment and the ankle joint mechanical power from unpowered walking overground (light gray circles) are overlaid.