## 3 Results
Finally, MFC of the trailing foot over the obstacle had a main effect of time (\(p<0.001\); \(\eta^{2}=0.7874\) CI[0.3939, 0.8743]) with decreased MFC for early VE change compared to obstacle only and to the T2 perturbation (\(p<0.001\) for both), and late perturbation was not different than obstacle only (\(p=1.000\)). A main effect of time was also present for COMv (\(p=0.002\); \(\eta^{2}=0.4987\) CI[0.1064, 0.6677]). COMv was  
Figure 3: Average (standard deviation) of maximum joint relative angles (MJRA) during trailing limb swing phase (**A**). Minimal trailing limb foot clearance (MFC) of the obstacle—negative values correspond to the foot passing below obstacle level (**B**). Average pelvis center of mass (COM) velocity during trailing limb swing phase (**C**) across unperturbed (blue), early perturbation of obstacle to platform (O-P-T1, orange) and late perturbation (O-P-T2, gray) conditions. significantly slower for O-P-T2 condition than for the obstacle condition (\(p\) = 0.008) and for O-P-T1 (\(p\) = 0.025), but these latter two conditions were not different from each other (\(p\) = 0.673). We can see in the plot of COMv (Figure 4) that participants appeared to slow down only during the swing phase. For the accommodation strategies related to the muscle mechanical work for the power bursts around toe-off and during swing (Figure 5), ankle push-off (A2) remained unchanged across conditions (\(p\) = 0.755; \(\eta^{2}\) = 0.028 CI(0, 0.1939)). Knee extensor absorption at the end of the stance phase (K3), however, showed a main effect of time (\(p\) = 0.014; \(\eta^{2}\) = 0.4357 CI(0.0216, 0.6544)). Post-hoc analyses showed a tendency of K3 to be augmented for early change (\(p\) = 0.053), but not for late one (\(p\) = 0.146), and the O-P-T1 (obstacle-platform at T1) condition was not different than O-P-T2 conditions (\(p\) = 0.141). For knee flexor generation (K5) and the subsequent delayed knee extensor absorption (K3D), there were main effects of time for both power bursts (\(p\) < 0.001; \(\eta^{2}\) = 0.7894 CI[0.3962, 0.8756], \(p\) = 0.001; \(\eta^{2}\) = 0.6356 CI[0.1944, 0.7786] respectively). Post-hoc analyses showed that K5 significantly decreased when the obstacle switched to a platform early (O-P-T1; \(p\) < 0.001), but not late (O-P-T2; \(p\) = 1.000). Significant differences were also observed between the two perturbation timings (\(p\) < 0.001). The delayed knee extensor absorption right after K5 (K3D) was also significantly decreased for the early change compared to the obstacle only (\(p\) = 0.002) and late perturbation conditions (\(p\) = 0.019). There was no difference between late perturbated trials and obstacle only (\(p\) = 1.000). Hip flexor generation around toe-off (H3) showed a general effect of time (\(p\) < 0.001; \(\eta^{2}\) = 0.7530 CI[0.3192, 0.8555]) and was augmented in an early perturbation (O-P-T1; \(p\) < 0.001), but not for a late one (O-P-T2; \(p\) = 0.408). O-P-T1 and O-P-T2 conditions were also different from each other (\(p\) = 0.003). There was also a main effect of time for the hip flexor generation during mid-swing (H3D; \(p\) = 0.005; \(\eta^{2}\) = 0.4685 CI[0.0637, 0.6578]). Post-hoc analyses showed that it was increased for the late perturbation only (O-P-T2) compared to the obstacle condition (\(p\) = 0.037) and to the O-P-T1 condition (\(p\) = 0.034) with no significant difference observed between O-P-T1 and obstacle conditions (\(p\) = 0.379 ).The positive hip linear power burst at toe-off showed a main effect of time (\(p\) = 0.009; \(\eta^{2}\) = 0.4830 CI[0.0925, 0.6568]) with an increase of power burst for a late perturbation compared to the obstacle condition (\(p\) < 0.001). The obstacle condition was not different from O-P-T1 (\(p\) = 0.519). O-P-T1 was not different from O-P-T2 (\(p\) = 0.242). Figure 4: Average plots of pelvis COM velocity (COMv) across unperturbed (gray), early perturbation of obstacle to platform (O-P-T1, dashed line) and late perturbation (O-P-T2, black line) conditions.