Fig. 4. Step duration and displacements of the centre of mass in vertical and forward direction as a function of running speed. (A–C) 1 g, (D–F) 1.3 g. Diamonds (black) indicate, from top to bottom, the step period (), the vertical displacement of the centre of gravity of the body during the step (S_v) and the step length (L) as a function of the running speed (_f). Triangles (blue) indicate the duration (t_ae) of the effective aerial phase, and the displacement of the centre of gravity during this phase in the vertical direction (S_ae) and in the forward direction (L_ae). Similarly, squares (red) indicate the duration (t_ce) of the effective contact phase and the corresponding displacements in the vertical direction (S_ce) and in the forward direction (L_ce). The red broken line in each panel indicates the actual contact time (t_c), and the vertical (S_c) and forward (L_c) displacement of the centre of mass during it. The blue broken line in each panel indicates the actual aerial time (t_a), and the vertical (S_a) and forward (L_a) displacement of the centre of mass during it. The vertical bars indicate the standard deviation of the mean calculated in each velocity class; the figures near the symbols in the upper panels indicate the number of items in the mean. Note that the step divisions based on the effective contact time and aerial time correspond to about half of total duration and displacements of the step, whereas the fraction of the step occupied by the actual contact and aerial phases varies widely with speed. Note also that the speed beyond which t_ae=t_ce, S_ae=S_ce and L_ae=L_ce is greater at 1.3 g than at 1 g.