Fig. 4. Summary of COM mechanics during C, U and V treatments (A,B,C, respectively). Silhouettes of the bird with corresponding limb stick figures at three points during the perturbed step: toe-down, midstance, and toe-off. The broken silhouette and stick figure represent the time of tissue paper contact in the U treatment. The COM path is overlaid for the time interval illustrated in Fig. 3B, along with the corresponding net change in height (s_v, blue), GRF impulse vector (J, red, summed over the stance phase), and initial and final velocity vectors (V_i and V_f, respectively, green) to illustrate differences among treatments.