Fig. 7. Ground reaction force (top), and COM energy changes (below) over time for the step period for representative trials, comparing steady, spring-like dynamics of level running (A) to the three energy responses observed during U perturbations: (B) `E<sub>Kh</sub> mode', (C) `E_com mode' and (D) `E_Kv mode'. The time interval shown is that illustrated in Fig. 3B. Bold and dotted lines in top panels indicate instantaneous vertical (f_v) and horizontal (f_h) GRF, respectively. The dotted vertical lines indicate time of tissue paper contact, and gray bars indicate duration of ground contact. Gravitational potential (E_P), horizontal kinetic (E_Kh) and vertical kinetic (E_Kv) energies are shown over time, with dotted horizontal lines to indicate the initial energy. Two energy sums, total vertical energy (E_V=E_P+E_Kv) and total COM energy (E_com=E_P+E_Ktot), allow distinction between energy conversion (e.g. E_PE_Kh as in B and net energy absorption (as in C). Note that at the point of ground contact in U trials (B-D), E_Kv is greater than in level running (A) because the body falls during the perturbation.