## 3 Results  
It appears that all participants tolerated the VE well given average low group scores of 0.31/3 \(\pm\) 0.19 on the Simulator Sickness Questionnaire. In addition, average group scores of 5.51/7 \(\pm\) 0.77 were found on the Presence Questionnaire, and they were 4.54/7 \(\pm\) 0.68 on the SUS Questionnaire, showing that participants were relatively well immersed. Two participants contacted the platform without tripping when it appeared at trail foot toe-off (T2). One of them contacted it twice and the other only once. These trials were kept for analysis because the participants did not lose their balance and patterns prior to platform contact did not show any outlying behavior. Before presenting the dependent variables, the general pattern changes within the time series patterns for angular displacement, net muscle moment of force and net muscle power plots are described. Across the three joints, locomotor adjustments for the new VE were mostly seen around toe-off and during the swing phase (Figure 2). At the ankle, a greater dorsiflexion occurred at mid-swing when the obstacle changed to the platform at trailing foot toe-off (T2). The related kinetic and energetic changes were less evident in the plots because of the relatively small mass of a foot to control during swing phase compared to the whole body during the stance phase. Figure 2: Average plots of angular displacement (top), net muscle moment of force (middle) and net muscle power (bottom) at the ankle (left), knee (middle) and hip (right) joints of the trailing limb during unperturbed conditions (gray line), and perturbations of obstacle to platform at T1 (O-P-T1; dashed line) and T2 (O-P-T2; black line). Gait stride was normalized to 100% with toe-off fixed at 60%. At the knee joint, there was a decrease in knee flexion combined with a decreased knee flexor moment associated with decreased generation for the early change (T1) at toe-off. For the late perturbation (T2), kinetic and energetic curves were similar to those for the obstacle condition. Finally, at the hip joint, hip flexion was higher when the obstacle changed to a platform, regardless of the timing of perturbations (T1 or T2). The kinetic and energetic plots showed a larger hip flexor moment combined with energy generation around toe-off (H3) when the platform appeared early (T1). For the obstacle-only condition and late perturbation (T2), there was some hip flexor energy generation at toe-off that switched to a hip extensor moment with energy absorption before returning, at mid-swing, to hip flexor power generation (H3D). There did not appear to be any change in linear hip hiking power (HH) across conditions. To better understand how an obstacle avoidance strategy can change to level accommodation strategy, specific kinematic variables were calculated and presented in Figure 3. First, MJRA of dorsiflexion, knee and hip flexion during the obstacle crossing swing phase were all significantly different across conditions with a main effect of time (\(p<0.001\) for the three joints; ankle \(\eta^{2}=0.6811\) CI[0.3308, 0.7906]; knee \(\eta^{2}=0.8117\) CI[0.5647, 0.8761]; hip \(\eta^{2}=0.8340\) CI[0.6107, 0.8907]). More precisely, at the ankle, dorsiflexion was greater for late perturbation than O (\(p<0.001\)) and O-P-T1 (\(p=0.007\)) conditions, while O-P-T1 was similar to obstacle condition (\(p=0.715\)). At the knee, MJRA were only decreased for the early perturbation compared to the obstacle condition (\(p<0.001\)) and to late perturbation (\(p<0.001\)), but the late perturbation was not different from the obstacle condition (\(p=1.000\)). At the hip, MJRA increased for both changes to Pl (\(p<0.001\) for T1 and T2), and no differences were observed between the two perturbation timings (\(p=1.000\)).