Fig. 6. The mass-specific internal power (W_int) and total mechanical power (_tot) are shown as a function of walking speed. The speed is normalised using the dimensionless Froude number [(gl), where _f is the forward mean walking speed, g is the acceleration of gravity and l is the leg length], which allows different size subjects to be compared. Each age group is represented by a different symbol (circles, 3–4 years; squares, 5–6 years; diamonds, 7–8 years; triangles, 9–10 years; inverted triangles, 11–12 years). Broken lines represent the weighted mean of the adult data (KaleidaGraph 3.6). Other indications are as in Fig. 2. Note that normalisation of the speed reduces the differences between adults and children.