Fig. 1. Methods. (A) Ground reaction forces (F; measured in N), velocity (V; m s^-1), displacement (S; cm), energy of forward movement (E_k,f; J), energy of vertical motion (E_p+E_kv; J) and total mechanical energy (E_ext=E_kf+E_p+E_kv; J) of the centre of mass (COM) during a walking step. Vertical and forward components are indicated by the subscripts v and f, respectively. Bold continuous lines represent the back leg; broken lines represent the front leg, and thin continuous lines represent the sum of both legs. Back and front legs are indicated by the subscripts back and front, respectively. Increments of each energy curve are, respectively, ⁺E_f, ⁺E_v (a+b) and W_ext (c+d), representing the positive work done by the muscles to increase the level of each energy curve. The work done to maintain the motion of the COM is W_ext. (B) Work curves for each leg independently (W_f,front, W_f,back, W_v,front and W_v,back) and curve W_v (sum of W_v,front and W_v,back) during a step. Work curves are also presented during the double contact (DC) phase, W_back=W_f,back + `transfer from W<sub>v</sub>' (see equation 3); W<sub>front</sub>=W<sub>f,front</sub> + `transfer from W_v' (see equation 4); W_com=W_back+W_front. The work done by one leg against the other during DC (⁺W_int,dc) is equal to (⁺W_back+⁺W_front)-⁺W_com (see Materials and methods; equation 6). Data are from a 10.5-year-old boy walking at 1.32 m s^-1. The step duration is 0.485 s and DC lasts for 0.14 s. DC is delimited by the vertical dotted lines.