Human interlimb coordination and the adaptations in leg muscle activity were studied during walking on a treadmill with split belts. Four different belt speeds (0.5, 1.0, 1.5, 2.0 m/s) were offered in all possible combinations for the left and right leg. Subjects adapted automatically to a difference in belt speed within 10-20 stride cycles. This adaptation was achieved by a reorganization of the stride cycle with a relative shortening of the duration of the support and lengthening of the swing phase of the "fast" leg and, vice versa, in support and swing duration on the "slow" leg. The electromyogram EMG patterns were characterized by two basic observations: (1) onset and timing of EMG activity were influenced by biomechanical constraints. A shortening of the support phase on the faster side was related to an earlier onset and increase in gastrocnemius activity, while a coactivation pattern in the antagonistic leg muscles was predominant during a prolonged support phase on the slower side. (2) A differential modulation of the antagonistic leg muscles took place. An increase in ipsilateral belt speed in combination with a constant contralateral belt speed was associated with an almost linear increase in ipsilateral gastrocnemius and contralateral tibialis anterior EMG activity, while the contralateral gastrocnemius and ipsilateral tibialis anterior EMG activity were little affected. It is concluded that a modifiable timing within the stride cycle takes place with a coupling between ipsilateral support and contralateral swing phase. The neuronal control of this coupling is obviously based on ipsilateral modulation of leg extensor EMG by proprioceptive feedback and an appropriate central (e.g. spinal) modulation of contralateral tibialis anterior EMG activity.