Fig. 5. The phase angle between the peak in the forward push (peak in the F_f,back curve) and the beginning of the double contact (DC) phase is shown as a function of walking speed for the different age groups. The phase angle (rad) is calculated as 2(t/T), where t is the difference between the time at which F_f,back is maximal and the beginning of DC, and T is the duration of the step. A positive value means that peak F_f,back occurs during DC; a negative value means that peak F_f,back occurs during the single contact phase preceding the DC phase. Other indications as in Fig. 3.