Neural mechanisms of human locomotion are difficult to study because of the interdependence between the musculoskeletal and nervous systems. However, based on a theoretical framework of muscle function during pedaling, experiments were designed to elucidate neural control mechanisms of muscle coordination during pedaling which may elucidate principles of neural control of locomotion. Predictions of muscle coordination during backward pedaling were compared to experimental results. The concept of functional muscle groups for the control of locomotion was expanded such that this simple control scheme could account for complex changes in muscle coordination measured during backward pedaling. This control scheme was also useful in interpreting the muscle coordination of backward and forward walking. Changes in muscle coordination between mechanically equivalent one- and two-legged pedaling tasks were also measured. Because the mechanics of the tasks were the same, no change in muscle coordination is predicted from biomechanics. Thus, the differences in muscle coordination are indicative of neuronal interlimb coupling constraints operant during human locomotion. It was found that during the flexion phase of pedaling, the sensorimotor state of the contralateral leg during extension affects ipsilateral muscle coordination in flexion. It appears that the control of locomotion involves an integration of bilateral sensorimotor signals consistent with the biomechanical interactions of the limbs normally present in two-legged pedaling but w hich no longer apply in the one-legged pedaling task. These results are not consistent with independent neuronal oscillators for the control of each limb, but instead suggest that common neuronal elements may be used to control the limbs together as a single locomotor apparatus.