Fig. 4. Mechanical energy of COM. Mean changes (± S.D.) of selected parameters plotted as a function of the time after the onset of independent walking: correlation coefficient between E_p and E_k (A), phase shift between E_p and E_k (B), percentage of recovery of mechanical energy (C), external work per unit of body mass and per unit distance (W_ext; D) and internal work per unit of body mass and per unit distance (W_int; E). The time course of changes with age was fitted by an exponential function (r=0.93, 0.89, 0.87 and 0.92 for A, C, D and E, respectively). Data refer to walking at natural speed.