Fig. 5. Contribution of external and internal power to the total mechanical power spent during walking. In each age group, the mass-specific external power (W_ext), internal power (_int) and total power (_tot) are presented as a function of the walking speed. For _tot, the solid line represents the total mechanical power based upon equation 4, which allows reasonable energy transfers (see Materials and methods). The lower broken line shows the total mechanical power computed as the sum of _ext and _int,k (as has been done in the past). The upper broken line shows the total mechanical power based upon equation 5, which does not allow any energy transfers between _ext, _int,kand _int,dc. Lines represent the weighted mean of the data (KaleidaGraph 3.6). Note that at low speeds, _ext is greater than _int.