Fig. 6. (A) External work W_ext is plotted as a function of walking speed for both adults (triangles) and toddlers (circles). (Adult data were reproduced from Willems et al., 1995.) W_ext shows a U-shaped relationship with walking speed, reaching a minimum at speeds around 0.6 m s^-1. Toddlers seem to produce a higher amount of external mechanical work to walk at the same speed than adults. (B) Differences between adults and toddlers do not disappear when W_ext is plotted as a function of the dimensionless froude number. This suggests that other factors, apart from their small stature, make toddlers consume more energy than adults. (C) Positive mechanical work W_int,k required to move the body segments relative to the centre of mass is plotted as a function of walking speed for both adults (triangles) and toddlers (circles). (Adult data were reproduced from Willems et al., 1995.) W_int,k seems to be larger in toddlers. (D) If W_int,k is plotted as a function of dimensionless froude number, W_int,k is smaller in toddlers. (E,F) Work during double contact. W_ext (black line), W_ILM (circles) and the sum of W_ext and W_int,dc (+) are compared as a function of walking speed and froude number. As expected, W_ILM is larger than the sum of W_ext and W_int,dc. However, the shape of the curves and the position of the optimum walking speed do not change, regardless of the method used. (G) Total mechanical work W_tot is plotted as a function of walking speed for both adults (triangles) and toddlers (circles). (Adult data were reproduced from Willems et al., 1995.) W_ext shows a U-shaped relationship with walking speed, reaching a minimum at speeds around 0.6 m s^-1. (H) If W_tot is plotted as a function of froude number (and differences in size between adults and toddlers are taken into account), W_tot is comparable between adults and toddlers around froude number 0.4. At lower and higher froude numbers sharp increases in W_tot are observed. r² values apply to toddlers only.