The utilization of passive dynamics to control the swing trajectory is one mechanism which serves to minimize energy costs during locomotion, in addition to reducing the complexity of the neural control. In a reactive situation (e.g. trip or slip during walking), the energy cost may not be a major determinant of the locomotor activity as there is a need for quick corrective action under the threat of a fall. Therefore, we addressed the following question: does the nervous system utilize passive dynamics during the reactive control of locomotion? An unexpected mechanical perturbation was applied to the foot during early and late swing during walking. Video data were input into an inverse dynamics routine to obtain the joint moment and mechanical power profiles and to partition the joint moments into active and passive components. The nervous system still utilized the passive dynamics of the effector system; active control of a single joint, the knee joint, passively facilitated the flexor action at the proximal hip and distal ankle joint following the early swing perturbation. The minimization of the mechanical energy cost was not a major determinant for this task since the total mechanical work during the perturbed steps was greater than during normal steps. A neuromuscular constraint was observed following the late swing perturbation; the active control of the hip and knee joints were increased but the magnitude of the hip extensor/knee flexor moment was invariant and equal to 1.6. The intralimb dynamics identified during these responses may serve to simplify the complexity of the active control of the nervous system.