Dynamic postural adjustments associated with the development of isometric forces in sitting subjects

The present study was undertaken to determine whether dynamic postural adjustments in sitting subjects occur during the development of isometric forces and whether the performance, ie, the maximal isometric force, is related to the surface of contact of the ischio-femoral region with the seat. The subjects were asked to exert maximal horizontal two-handed isometric pushes on a bar. External force (Fx), antero-posterior reaction force (Rx) and antero-posterior displacement of the center of pressure (Xp) were measured continuously during the development of isometric forces, the subsequent hold state lasting 5 s. Two postural conditions were studied that differed by the ischio-femoral contact with the seat (100 or 30%), the foot support being bipodal (BP). It was observed that: a) Fx and Rx increased during the development of isometric forces while Xp moved backwards; b) the instantaneous variations in Fx, in relation to Rx and Xp were linearly related; c) the maximal force reached at the end of the ramp effort and the maximal values of Rx and Xp were significantly increased when the ischio-femoral contact was reduced from 100 to 30 BP. It is concluded that isometric ramp efforts are accompanied by dynamics postural adjustments and that the greater these adjustments, the larger the maximal isometric force. These observations are interpreted as supporting the view that any variation in the external force perturbs the balance of the subject, and that the maximal value of this force depends on the intensity of the dynamic postural counter-perturbation.