Fig. 7. The four phases of the bounce of the body during the running step. Each panel illustrates the changes in gravitational potential energy, E_p, and in kinetic energy of motion in the sagittal plane, E_k=E_kv+E_kf, of the centre of mass of the body simultaneously with the E_p–E_k energy transduction, R_int(t). The energy curves are normalized to oscillate between zero and one. Each panel begins and ends at the lowest value of the E_p curve. Same steps illustrated in Fig. 1. The different 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), with lighter blue indicating the aerial phase (not present in the upper left panel). 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 a transduction occurs between E_p and E_k as indicated by the increments of the R_int(t) curve below and above crossing the broken lines. Note that the transduction of E_k into E_p during the lift [lower-left increment of R_int(t)] is smaller than the transduction of E_p into E_k during the downward displacement (upper-right increment). In the horizontal tracts of the R_int(t) curve no transduction occurs between E_p and E_k and muscle–tendon units absorb simultaneously E_p and E_k (phase ß) and increase simultaneously E_p and E_k (phase ). Note that whereas most of ß is confined within S_ce,down, extends beyond S_ce,up within a large fraction of S_ae,up due to a continuing increase of E_k.