Fig. 3. (A) COM paths over a step cycle for one individual during the three treatments: control (C), unexpected substrate drops (U) and visible substrate drops (V). All U trials for this individual are shown to illustrate within-subject variation (solid blue lines), with a representative V trial (dotted red line) and C step (broken green line) drawn for comparison. Thin gray lines represent the aerial phase of the step. In (B) the periods of analysis are schematically illustrated. In all cases, the COM trajectory was calculated between subsequent aerial phase peaks in COM height. During level running this corresponded to subsequent COM apexes, where E_Kv is zero and E_P is at a maximum. However, in the U and V trials the COM generally did not return to stable periodic motion within the perturbed step, and the COM was often moving downward at the beginning of the aerial phase following the perturbation. In these cases, there was a net gain in E_Kv, and the aerial phase peak in COM height occurred at the beginning of the flight phase. This E_Kv must be dealt with during the next stance phase because total vertical energy (E_V=E_P+E_Kv), total horizontal energy (E_H=E_Kh) and total mechanical energy (E_com) can change only during ground contact.