This study was conducted under the hypothesis that voluntary movement involves a perturbation of body balance and that a counter-perturbation has to be developed to limit the perturbation effects, which is a condition necessary to perform the movement efficiently. The stabilising action is produced in body segments that constitute the "postural" chain, and the voluntary movement by the segments said to constitute the "focal" chain. In order to deepen the understanding of how the postural chain contributes to the motor act, isometric transient efforts were considered. Seven adults in a sitting posture were instructed to exert bilateral horizontal pushes on a dynamometric bar, as rapidly as possible, up to their maximal force (Fx). Two sitting conditions were considered: full ischio-femoral contact (100 BP) and one-third ischio-femoral contact (30 BP), the latter being known to yield greater pelvis and spine mobility, that is greater postural mobility. Each session consisted of ten maximal pushes for each sitting condition. In order to explore the influence of postural mobility on muscular control and push force, surface EMGs of 14 postural and focal muscles were recorded. In addition, reaction forces (Rx) and displacement (Xp) of the centre of pressure (along the anteroposterior axis) were measured, as well as iliac crest acceleration (xh and zh, along the anteroposterior and vertical axes, respectively). The results showed that push force varied abruptly during the task ramp effort. When the ischio-femoral contact was limited, push force was enhanced, as well as the rate of push force rise (Fx/Deltat, Deltat being the force rise duration), suggesting a greater perturbation to balance. Also, there were significant increases in the Rx reaction forces, indicating body segment acceleration: "dynamic" phenomena occurred in the articulated body chain in response to increases in Fx. In addition, even though muscular contraction was isometric, postural EMGs, as well as focal EMGs, were phasic, a feature which characterises transient force exertion. The Rx reaction forces were associated with backward displacement of the centre of pressure, Xp. The centre of pressure displacement was interpreted as a backward pelvis rotation, an interpretation which was confirmed by backward and upward iliac crest accelerations. When ischio-femoral contact was reduced, the backward pelvis rotation was significantly increased, resulting from an increased pelvis and spine mobility. Distinct focal and postural EMG sequences were found to be associated with the effort. Two different sets of muscles were observed when considering recruitment order, the focal and the postural muscles. The ankle muscles were activated before the pelvis, the back and the scapular girdle, with the upper limb muscles activated only after the onset of the primum movens of push action (serratus anterior): the activation process followed a distal to proximal progression order. Moreover, the postural EMG sequence was anticipatory, that is there were anticipatory postural adjustments (APAs). Modifying the ischio-femoral contact did not induce a change in either the postural muscle set or in the recruitment order. There were significant increases in the level of activation (integrated EMG) of the postural muscles when ischio-femoral contact was reduced. They did not result from an increase in EMG duration but only from a modulation of EMG amplitude, suggesting that postural control for different ischio-femoral contacts involves adapting the motor program according to the postural requirements, rather than changing the postural strategy. Moreover, as APA amplitude was increased when ischio-femoral contact was reduced, it could be assumed that the postural chain is programmed in relation to postural chain mobility. In addition, the increase in postural EMGs was interpreted as an increased counter-perturbation opposed to an increased push force. It is concluded that greater mobility of the postural chain favours a greater dynamic counter-perturbat chain favours a greater dynamic counter-perturbation, which, in turn, allows the development of a greater push force; the ability to develop such a counter-perturbation (termed PKC: posturo-kinetic capacity) is enhanced when postural chain mobility is greater. Postural chain mobility appears to be a task parameter, and postural control appears to involve adapting the motor program according to the postural requirements, rather than changing the postural strategy.