Fig. 3. The normalized ratio (see Materials and methods) between maximal downward and upward velocities attained at each step by the centre of mass of the body is plotted as a function of the running speed. The greater vertical velocity attained during the downward displacement indicates: (i) a lower vertical force exerted by the structures supporting the body at the lowest speeds when the aerial phase is nil, together with (ii) a ballistic fall greater than the ballistic lift at higher speeds (Fig. 2). Statistics as in Fig. 2.