Fig. 3. Description of the double contact (DC) phase as a function of speed and age. (A) In each age group, the step length (L_step; measured in metres) and the forward displacement of the centre of mass taking place at each step when both feet contact the ground (L_dc; m) are given as a function of the walking speed. L_dc was calculated as the mean forward speed during DC multiplied by the fraction of the step during which both feet contact the ground. (B) The angle of contact with the ground during a step was computed from L_step and the leg length as shown in the Materials and methods. (C) The relative importance of DC (L_dc/L_step) is shown as a function of the walking speed. The symbols represent mean values of data grouped into the following intervals along the abscissa: 0.28 m s^-1 to <0.42 m s^-1, 0.42 m s^-1 to <0.56 m s^-1...2.5 m s^-1 to <2.64 m s^-1. N is from low to high speed classes for 3-4 years: 2, 1, 7, 8, 6, 13, 7, 9, 7, 2, 3; for 5-6 years: 4, 3, 11, 7, 13, 10, 9, 18, 18, 12, 2, 1; for 7-8 years: 1, 4, 1, 9, 14, 8, 16, 20, 10, 13, 10, 13, 2, 10, 5, 3, 1; for 9-10 years: 3, 9, 16, 22, 19, 23, 19, 12, 12, 11, 17, 10, 8, 3, 3 and for 11-12 years: 1, 4, 5, 6, 8, 13, 18, 18, 12, 13, 19, 14, 16, 8, 6, 3. Bars indicating the S.D. of the mean are drawn when they exceed the size of the symbol. The broken lines indicate the adult trend.