## 3 Results
(**b**) Weighting curves (\(\textbf{C}_{i}\)) are shown as the mean (solid line) \(\pm\) standard deviation (shaded area) of three trials. The grey rectangular areas indicate the swing and stance phases of the left leg (\(L_{\text{L}}\)). Figure 5: Muscle synergies and weighting curves for outward-directed perturbed walking and unperturbed walking at (**a**) the lowest and (**b**) the fastest walking speed for a representative subject. The colors indicate unperturbed walking (grey) and perturbed walking, with a perturbation onset of 0% (orange), 30% (green), or 60% (purple) of the stance phase of the gait cycle. (**left**) Muscle synergies for unperturbed (wide grey bars) and outward-directed perturbed walking for different perturbation onsets (narrow colored bars). Muscle synergies for perturbed walking were only shown when similar to the corresponding muscle synergy for unperturbed walking (\(r_{p}\geq 0.623\)). (**right**) Weighting curves for unperturbed (black striped line) and perturbed walking with different perturbation onsets (colored solid lines). Weighting curves are displayed as the mean value (lines) \(\pm\) standard deviation (shaded area) of three repetitions. Figure 6: Muscle synergies and weighting curves for inward-directed perturbed walking and unperturbed walking at (**a**) the lowest and (**b**) the fastest walking speed for a representative subject. The colors indicate unperturbed walking (grey) and perturbed walking, with a perturbation onset of 0% (orange), 30% (green), or 60% (purple) of the stance phase of the gait cycle. (**left**) Muscle synergies for unperturbed (wide grey bars) and inward-directed perturbed walking for different perturbation onsets (narrow colored bars). Muscle synergies for perturbed walking were only shown when similar to the corresponding muscle synergy for unperturbed walking (\(r_{p}\geq 0.623\)). (**right**) Weighting curves for unperturbed (black striped line) and perturbed walking with different perturbation onsets (colored solid lines). Weighting curves are displayed as the mean value (lines) \(\pm\) standard deviation (shaded area) of three repetitions. These observations were consistent with the results across subjects. Figures for all subjects and walking speeds can be found in the Supplementary Materials. For walking at 0.8 m/s, all four muscle synergies for perturbed walking were similar to the four muscle synergies for unperturbed walking, regardless of perturbation condition (\(r_{p}\geq 0.623\)) (Figure 7). Significantly less than four muscle synergies for perturbed walking were similar to the four muscle synergies for unperturbed walking at the lowest walking speed (inwards: \(p=0.04\) and \(p=0.023\) for perturbation onsets 0% and 60% of the stance phase of the gait cycle, respectively; outwards: \(p=0.009\), \(p=0.024\), and \(p=0.003\) for perturbation onsets 0%, 30%, and 60% of the stance phase of the gait cycle, respectively). Further inspection of the synergies revealed which particular synergies were typically dissimilar to the corresponding unperturbed synergy (Figure 8). The summative count over all subjects of each individual synergy similar to the corresponding unperturbed synergy shows that mainly the push-off synergy was dissimilar at the lowest walking speed for both perturbation directions. Figure 7: The number of perturbed muscle synergies similar to those for unperturbed walking, averaged across subjects. Asterisks (*) indicate significant differences in the number of perturbed synergies similar to unperturbed synergies (\(p<0.05\), one-way ANOVAs followed by Bonferroni post hoc tests). IN, OUT; inward-, outward-directed perturbation; O0%, O30%, O60%: perturbation onset of 0%, 30%, 60% of stance phase of gait cycle; U, unperturbed.