Fig. 2. Joint kinematics. Thick lines are the mean ankle (left column), knee (middle column) and hip (right column) joint angles over the stride from heel strike (0%) to heel strike (100%) of nine subjects. Data are averages 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, curves 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%. For all joints 0 deg. is upright standing posture. Ankle joint plantarflexion, knee joint extension and hip joint extension are all positive.