In locomotion, the capability to control and modulate intentionally the propulsive forces is fundamental for the adaptation of the body's progression, both in speed and direction. The purpose of this experiment was to determine how human beings can achieve such control on-line. To answer this question, four subjects walking steadily were faced with a linear increase in resistance (impeding forward displacement), lasting 3 s, once per minute. At the end of the variation, the new resistance was maintained. There were two tasks; in both tasks, in the initial steady state, the subjects had to walk steadily at 1.3 m s-1. As the resistance increased, subjects were either required to maintain their walking speed (compensation task) or to let the walking speed and amplitude adapt freely (no-intervention task). This provided an estimate of the effects of the perturbation alone. Throughout the experiment, the stride frequency (114 step min-1) was fixed by a metronome. Subjects maintained their stride frequency on both tasks. In the no-intervention task, walking speed was 1.3 and 1 m s-1 under normal and high resistance respectively. In the compensation task, under high steady resistance, walking speed was maintained by an increase in the activation gain of the neuromuscular synergy: all recorded muscles increased their EMG activity, but without any change in the shape of their activation profile throughout the cycle. During the transitional phases, however, as the resistance began to increase, the walking speed decreased temporarily (-2%) before returning rapidly to its initial value. By contrast, at the end of the resistance increase, no such changes in speed were observed. During the transitional phases, the on-line compensation for the resistance increase induced modifications in the shape of the activation burst in the medial gastrocnemius such that the transitional cycles clearly differed from the steady state cycles. The results observed in the compensation task suggest that the subjects used two different modes of control during steady states and transitional phases. In stable dynamic conditions, there appears to be an "intermittent control" mode, where propulsive forces are globally managed for the entire stance phase. As a result, no compensation occurred at the beginning of the perturbation. During the resistance increase, subjects appeared to switch to an "on-line control" mode in order to continuously adapt the propulsive forces to the time course of the external force, resulting in an observable compensation at the end of the resistance change.