The central nervous system in humans continuously controls the speed of walking by modulating muscle activities. The underlying mechanisms of this control process are not well understood. Recent studies have probed the neural control of walking using sensory and mechanical perturbations. It has been suggested that transient responses to perturbations show patterns in the modulation of muscle activations not previously observed. This dissertation aims to systematically investigate differences in modulations of muscle activations between transient responses and steady-state walking. Three studies were designed to explore these modulations using visual and mechanical perturbations. The first study compared the qualitative patterns from transient responses to visual perturbations to those observed during steady-state walking. Small changes in the average muscle activations between two steady-state speeds were compared to small transient changes due to perturbations. We demonstrated that the decrease in the plantarflexor activity during transient responses that potentially contributed to an increase in speed was unique to these responses and not reproducible in steady-state walking conditions. The second study quantified the effects of average walking speed on transient responses to visual perturbations and compared these effects to steady-state walking conditions. A scaling effect on the amplitude of responses was shown across different treadmill speeds. Finally, in the last study, we explored characteristics of transient responses to mechanical perturbations of the treadmill. We examined the effects of perturbations at two different amplitudes on both kinematics and muscle activations. The responses of the neurofeedback to kinematic deviations were quantified and it was shown that the local limit cycle approximation was reasonable to describe the system. Together these studies shed light on how modulations of muscle activity are utilized by the nervous system to regulate the key variable of walking speed, as well as other aspects of human locomotion.