Fig. 5. Stiffness (A,C), resonant frequency of the bouncing system and freely chosen step frequency (B,D) as a function of running speed. (A,B) 1 g, (C,D) 1.3 g. In A and C, filled circles give the vertical stiffness of the bouncing system [k_vert=M_b(/t_ce)²], whereas open circles give the leg stiffness (k_leg), calculated as described in the Materials and methods. The k_vert,1g line in C is drawn for comparison to show the similarity of the two stiffness at intermediate speeds. In B and D, filled squares indicate the freely chosen step frequency (f) for comparison with open squares, the resonant frequency of the bouncing system [f_s=1/(2t_ce)], calculated assuming that the effective contact time corresponds to one half-period of the oscillation of the elastic system. Note that the increase in gravity increases the maximum speed where f=f_s. The two lower lines indicate the frequency f_k,leg=(k_leg/M_b)^0.5/(2), calculated from the leg stiffness and the frequency f_c=1/(2t_c), calculated assuming that the time of contact corresponds to one half-period of the oscillation of the elastic system: both give false indication in their relation with the actual step frequency. Lines are least-squares linear regressions or weighted mean of all the data (Kaleidagraph 3.6.4).