Understanding the gestures used by operators at work is a major issue in the field of ergonomics. Voluntary movement forms the basis of gesture. Knowing the processes required to execute a voluntary movement is essential if we are to understand the gestures adopted by operators. This paper aims to explain both gesture and voluntary movement. It then presents the most recent scientific knowledge in the field of neurophysiology and, through a series of experiments, the biomechanics necessary to perform voluntary movement. A voluntary movement results from an interaction between body segments which are intentionally mobilized, constituting the focal chain, and the remaining body segments, called the postural chain. Voluntary movement obeys the laws of physics and physiology. Its execution induces an endogeneous disturbance of body balance. In order to maintain balance, a counter-perturbation must be produced, which results from dynamics developed from muscular activities of the postural chain. These dynamics precede and accompany the voluntary movement. They are modified according to the initial posture adopted by the subject, the contact area between the subject and his supports, and the location and slipperiness of the support surfaces. The dynamics of the focal chain are closely dependent on those dynamics developed in the postural chain. Thus, the basis of a gesture is the outcome of organisational control over the way each muscle is brought into play and the coordination between the activity of different muscles to maintain a posture which is compatible with body balance and the requirements of the task.