The modifications occurring in the movement and muscle activity patterns of the leg when changing from forward to backward walking were studied in five healthy subjects during walking on a motor driven treadmill. Movements were recorded with a Selspot optoelectronic system and muscle activity with electromyography using surface electrodes. The movement trajectories of the leg in forward and backward walking essentially mirrored each other, even though the movements occurred in the reversed direction. The angular displacements at the hip, knee and ankle joints showed similar overall magnitude and pattern in the two situations. Most of the investigated muscles changed their pattern of activity in relation to the different movement phases. At the ankle, there was a switch between flexors and extensors with flexor activation during support in backward walking. The bursts of activity in knee extensors were prolonged and shifted to the main part of the support phase. In the hip extensors, the activity periods retained their positions relative to the leg movements, but changed function due to the reversed direction of movement. Thus, drastic changes occur in the normal locomotor program to produce a reversal of leg movements and propulsion backwards.