Fig. 2. Energy fluctuations of the COM. (A) Out-of-phase (walking; red arrows) and in-phase oscillations (running; blue arrows) of kinetic energy and gravitational potential energy of the COM on a normalized time-basis with an indication of the efficiency of energy exchange (%=percentage of recovery pendulum). (B) Total kinetic energy (black line) is presented as a fraction of the total kinetic energy at transition (blue line). The red lines are the regressions for walking and running steps. (C) Kinetic energy due to vertical velocity of the COM, with horizontal regressions for walking and running steps. The black line indicates the mean of the average of all trials (N=3–5) with each subject (N=9). Grey lines indicate standard deviation (s.d.) between subjects. Grey and black bars on the x-axis represent contact with the ground by alternate feet.