Figure 8.

Five muscle synergies and their time-varying activation patterns extracted during forward, backward and combined locomotor conditions in three spinal cats together at 0.3 m/s. A, Synergy weights (mean ± SD) in five synergies (W₁–W₅) in the forward, backward and combined locomotor conditions, where a value of 1 indicates a maximal weight. The means and SDs are from 20 extractions of muscle synergies for each cycle, direction and cat (Cat 11, 4 forward and backward cycles; Cat 5, 8 forward and backward cycles; and Cat 3, 6 forward and backward cycles). For muscle abbreviations, see Figure 6 legend. B, Time-varying activation coefficients (C₁–C₅) for the five synergies extracted from forward EMG patterns (blue lines to the left from the vertical black line), from backward EMG patterns (red lines to the right of the black vertical line) and from combined forward-backward EMG patterns (green lines). The horizontal axis represents the normalized cycle times of forward (before the black vertical line) and backward locomotion (after the black line); 0.0 corresponds to stance onset. A value of 1 indicates a maximal activation within the normalized cycle. The thick lines and surrounding area represent the mean and SD, respectively. Vertical dashed lines separate stance (ST) and swing (SW) phases of forward and backward locomotion. C, The same time-varying activation coefficients (C₁–C₅) as in B plotted as a function of the normalized cycle time. Blue and red lines denote forward and backward locomotion, green dashed and dashed with dot lines denote the forward and backward components of the combined matrix of activation coefficients (C_{COM_FW} and C_{COM_BW}, respectively). Black dashed line corresponds to the mean relative time of swing onset in forward and backward locomotion. Coefficients R² computed between activation coefficients of forward locomotion (FW), backward locomotion (BW), and forward and backward components of the combined forward and backward locomotion (COM_FW and COM_BW).