Fig. 3. The four phases of the bounce of the body. Same subjects and speeds as in Fig. 1. In (A–F) the trend of the E_p–E_k transduction, R_int(t) (black), is illustrated with the simultaneous changes in gravitational potential energy, E_p, and kinetic energy, E_k=E_kv+E_kf (green), normalized to oscillate between zero and one. The colors in the E_p curve distinguish the fractions of the step where the vertical force exerted on the ground is greater than body weight (red), and lower than body weight (blue). The continuous E_p line indicates the contact phase whereas the dotted E_p line (light-blue) indicates the aerial phase (not present in A and B). The four phases correspond to the vertical displacement during the upward acceleration S_ce,up (red) and deceleration S_ae,up (blue), and the downward acceleration S_ae,down (blue) and deceleration S_ce,down (red). The vertical dotted lines are drawn through the two peaks of E_k and encompass the fraction of the step where the E_p–E_k transduction occurs, as indicated by the increment of the R_int(t) curve. Note that, particularly in the old subject, the transduction of E_k into E_p during the lift (increment R_int,up of R_int(t), below crossing of the interrupted lines) is smaller than the transduction of E_p into E_k during the downward displacement (increment R_int,down of R_int(t), above crossing of the interrupted lines). This asymmetry is accompanied by a ballistic lift (almost nil in the old subject) smaller than the ballistic fall. In the horizontal tracts of the R_int(t) curve no transduction occurs between E_p and E_k and muscle-tendon units absorb E_p and E_k simultaneously (phase β), and increase E_p and E_k simultaneously (phase ). Whereas most of β is confined within S_ce,down, usually extends beyond S_ce,up within a large fraction of S_ae,up due to a continuing increase of E_k. Also this asymmetry is larger in the old subject. In summary, the following features of the landing–take-off asymmetry of the bounce of the body are shown: (i) a peak of E_k greater during the fall than during the lift, (ii) a transduction of E_p into E_k during the fall greater than the transduction of E_k into E_p during the lift (i.e. R_int,down>R_int,up), and (iii) a simultaneous decrement of E_k and E_p after landing shorter than the simultaneous increment of E_k and E_p before and after take-off (i.e. β<). All these deviations from an elastic bounce are larger in the old subject.