Fig. 3. Changes of vertical hip displacement (GT_y,left+GT_y,right)/2 (C), gravitational potential energy (E_p) (B) and kinetic energy (E_k) (A,B) of COM during walking in children of different age, and in a representative adult. The upper three traces show the forward (E_k,f), vertical (E_k,v) and lateral (E_k,l) kinetic energies of COM. E_k (B) refers to the kinetic energy in the sagittal plane (sum of E_k,f and E_k,v) that we used to characterize the classic pendulum behaviour in the sagittal plane. Dotted vertical lines correspond to foot contact (FC) of one leg. For older children and adults, the recordings of E_k and E_p do not extend to the whole cycle. Correlation coefficient between E_p and E_k is indicated for each trial.