Fig. 5

a Placement of reflective markers. b Experimental snapshot, (c) trajectories of the left wrist in X–Y–Z coordinates and the squared time derivative (\({\text{V}}^{2}\), unit of velocity: m/s) of Z coordinates, as well as (d) a schematic illustration how tangential velocities of joints (\({\text{V}}\)) are decomposed into kinematic synergies (\({\text{W}}\)) and corresponding activation coefficients (\({\text{C}}\)) for a TD infant during crawling. Vertical dotted lines separate the swing and stance phases during a crawling cycle. For \({\text{V}}\) of (d), the black solid lines represent the original tangential velocities of joints, and the red dotted lines represent the reconstructed tangential velocities of joints resulting from the four kinematic synergies (i.e., W1, W2, W3 and W4) and corresponding activation coefficients (C1, C2, C3 and C4)