Muscle length (top, orange), force (bottom, black) and activation (rectified EMG, bottom, orange) are shown, with triangles indicating the time of foot-ground contact, a shaded box indicating an obstacle encounter stride (S 0), and vertical lines indicating the mid-swing cut points between stride cycles. Stride categories were identified from video. Grey silhouettes at the top illustrate the leg posture at the time of foot contact. Strides cycles were cut based on a minimum in muscle-tendon force after it was low-pass filtered with a 6th order Butterworth filter with a cutoff frequency of 3.4 Hz. This resulted in a sinusoidal trajectory with a mid-swing minimum, which was confirmed against video to correspond to when the swing leg crossed vertical. Note that between the last two strides, the contralateral leg stepped on the obstacle, leading to a downward step of the instrumented leg in the final stride. For simplicity, these strides are group with the ‘mid-flat’ strides S+2 (as in Daley and Biewener, 2011) because the focus of the current analysis is the direct response to the obstacle encounter (S 0).