Fig. 3. Ankle exoskeleton mechanics. The thick lines are 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 1.25 m s^–1 on a single surface gradient (0%-level at top to 15%-uphill at bottom). In each subplot, bold lines are for unpowered (black circles), and powered (gray circles) walking, and 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 plantarflex the ankle is positive. Torque is the product of artificial muscle load 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 power indicates transfer of energy from exoskeletons to the user's biological ankle muscle–tendon system.