Fig. 4

Response of unperturbed leg: Altered Magnitude. Averaged muscle activity and joint kinematics of the unperturbed (right) leg for a representative subject. Plotted from top to bottom is the normalized TA EMG, normalized SOL EMG, hip flexion (+) - extension (-), knee flexion (-) - extension (+) and ankle dorsi (+) - plantar (-) flexion for gait cycles at each of four surface stiffness levels. Mean (darker lines) and standard deviations (lightly shaded areas) values are shown along with an indication of the timing of the perturbation. Statistically significant changes are indicated by colored bars (corresponding to each stiffness level, aligned vertically from highest to lowest stiffness) that are placed beneath a black asterisk. Heel-strike and toe-off of the right leg are indicated by HS and TO, respectively. The duration of the gait cycles shown is approximately 1.8 s