Fig. 4. The peak in kinetic energy of the centre of mass, E_k=E_k,f+E_k,v, attained because of gravity in its motion forwards and downwards is greater than the peak attained because of the muscular push in its motion forwards and upwards. Note that the downward/upward asymmetry decreases with speed due to the large increase of E_k,f relative to E_k,v (Fig. 1). The difference between the two peaks attained by E_k at each step also remains positive at the highest running speeds (inset). Statistics as in Fig. 2.