Body orientation and regulation of the center of gravity during movement under water.
Professional divers were instructed to adopt a vertical posture under water with their feet fixed to the ground and to perform a fast forward or backward upper trunk bending movement in response to a tone. Kinematic and EMG analyses were performed. It was first noted that the divers adopted a forward inclined, erect posture, suggesting that the verticality was misevaluated, although the effects of gravity were still exerted on the otoliths. Second, the upper trunk movements were still accompanied by opposite movements of lower segments and, as a result, the center of gravity displacement was still minimized, although not so accurately as on the ground. The EMG pattern consisting of early activation of a set of trunk, thigh, and shank muscles continued to occur under water. These results suggest that "axial synergies" associated with upper trunk movements are learned motor habits that regulate the center of gravity position regardless of the equilibrium constraints.