Fig. 9. Comparison of COM mechanics among different U perturbation responses (A-C). (D) The V perturbation response. On the left, silhouettes of the bird with corresponding limb stick figures are shown at three points: toe-down, mid-stance and toe-off. Broken silhouettes represent the time of tissue paper contact in U trials. The COM path is shown for the time interval illustrated in Fig. 3B, along with the corresponding net change in height (s_v, blue), initial and final velocity vectors (V_i and V_f, respectively, green), and GRF impulse vector (J, red, summed over the stance period). On the right, net changes in external energy shown as means ± s.e.m. for all instances of each response.