Fig. 4. Mechanical power, net metabolic power and efficiency of positive work production in walking children. The top row presents the mass-specific total positive mechanical power (_tot) as a function of speed in each age group. The second row shows the net energy consumption rate at steady state (_net); these data are taken from DeJaeger et al. (2001). The efficiency of positive work production during walking (bottom row) is calculated as W_tot divided by _net (equation 6). The values of efficiency are only presented at speeds above 0.75 m s^–1 where results are considered to be robust (see text). The continuous and broken lines are fitted through all the data of children and adults, respectively, using a second-order polynomial function (KaleidaGraph 3.6). Efficiency is computed from these polynomial functions. Other indications are as in Fig. 2. Note that in children younger than six, the efficiency of positive work production during walking is lower than in adults.