Electromyographic (EMG) activity produced in the triceps surae (TS) and subsequent landing were examined under various visual conditions during stair descent with the following results: The amount of precontact TS EMG was reduced during each visual perturbation. Perturbations corresponded to no knowledge or visualization of stairs (B), no stair visualization during descent (A) and vertical movement of the surround during descent (M). Erroneous visual information was primarily responsible for altered EMG activity. The only known difference between the M data sets was that the surround moved up (U) or down (D) as the subject descended. However, TS EMG characteristics were different under these two conditions. Specific visual information appeared necessary for vision to override the other sensory systems. There was no difference in EMG when the room moved up (U) compared to the room not moving (NM). However, EMG activity was significantly different when the room moved down (D) compared to the room not moving (NM). The relationship between TS EMG activity and subsequent landing appeared related to landing strategy. Although the EMG was reduced during both the B and M test conditions compared to the control, the landing was "softer" for B and harder for M. The pre-contact EMG is apparently part of a preprogrammed movement pattern which can be modified by sensory information during task execution. Future studies should examine the neuronal mechanisms which provide the visual system access to the center controlling lower limb muscle activity during dynamic movement.