INFLUENCE OF VELOCITY ON ANTICIPATORY POSTURAL ADJUSTMENTS IN AN ACROBATIC MOVEMENT

Abstract

Introduction: When intentional movement is performed, this is generally preceded by anticipatory postural adjustments (APA). These APA are dynamic phenomena. In upright and crouching posture, i.e., bipedal and quadruple stance, it has been shown that the duration and amplitude of the APA increase with the velocity of the forthcoming movement. The function of this anticipatory dynamic is, in the initiation of gait (Brenière et al., 1987) or in the sprint start (Natta et al., 1990), to create conditions to obtain a suitable speed of the center of mass (CM) and in the case of upper limb movements (Bouisset and Zattara, 1981) to minimize in advance the perturbation of balance created by the forthcoming movement. The aim of this study was to examine the influence of velocity on the APA in an acrobatic movement in which final balance is particularly unstable. Methods and procedures: From an initial quadruple stance, the feet placed one in front of the other along the anterior-posterior axis, and the hands placed on the same lateral axis, the subjects were requested to do handstands. Seven expert subjects performed the movement with either a natural (N) or a fast (F) velocity. The biomechanical data were subsequently analyzed. Variations of anteriorposterior (Rx) and vertical (DRz = Rz-mg) ground reaction forces and the anteriorposterior displacements of the center of foot pressure (CPx) were obtained from a force plate. These forces were used to calculate the acceleration (R/m) and velocity (by integration of the acceleration) of the center of mass (CM). The data were digitized at 750 Hz. Results and discussion: The first main result showed that the amplitudes of the anterior-posterior and vertical reactions of the anticipatory dynamic were different between the two conditions, they were greater in F than in N, and the duration was also different on the three axes (for example, on the anterior-posterior axis N = 333 ms ± 53, F = 257 ms ± 54, mean of subjects), but they were smaller in F than in N. These last results were in contrast to those of other studies. The second main result concerns the velocity of the CM. The peak velocity occurred at the time of the bi-manual stance, and the amplitude was similar following the anteriorposterior axis (N = 0.74 ms-1 ± 0.12, F = 0.77 ms-1 ± 0.14, mean of subjects) but different following the vertical axis (N = 0.89 ms-1 ± 0.16, F = 1.06 ms-1 ± 0.17, mean of subjects). Results suggest: 1) a functional dissociation between upper and lower parts of the body during the movement phase, 2) the perturbation of balance is all the more important because the speed is great. The dynamic strategy used was to increase the velocity of the body on the vertical axis on which balance is very stable.