Subjects walked on a treadmill in an immersive virtual environment while watching a hallway with and without continuous mediolateral (ML) optical flow perturbations of 3 amplitudes. We recorded electromyographic activity from the following 7 leg muscles: tibialis anterior (TA), medial gastrocnemius (MG), soleus (SOL), peroneus longus (PL), vastus lateralis (VL), medial hamstrings (MH), and gluteus medius (GM). hs, Heel-strike.