Maintenance of upright standing posture during trunk rotation elicited by rapid and asymmetrical movements of the arms

Abstract: Nine healthy subjects standing upright, initiated small, medium and large (S, M and L conditions, respectively) forward movements of their right (Rt) arm together with backward movements of their left (Lt) arm. They also performed medium-size movements while holding a 3kg dumbbell in each hand (D condition). Movements started with the arm hanging alongside the body and ended when the shoulder reached a desired orientation. The arm and trunk movements were videotaped and recorded by accelerometers taped to the wrists, shoulders, and hips bilaterally. The torque around the vertical axis was measured using a force-plate on which the subjects stood. EMGs were recorded with surface electrodes bilaterally from the shoulder, trunk, and thigh muscles. Trajectories of the center of foot pressure were measured in additional experiments. In association with arm movements, there was a small counterclockwise (ccw: the Rt shoulder forward and the Lt backward) rotation of the trunk, followed by large alternate rotations of the trunk, first clockwise (cw) and subsequently ccw. The intervals from the hand acceleration to the shoulder and hip accelerations were, respectively, 0±15ms (mean and S.D. for all subjects) and −17±15ms. The force-plate showed initial cw and later ccw torques 63±41ms after hand acceleration. The EMGs of the Rt hamstrings (Ham) and Lt rectus femoris (RFem) were followed by those of the Lt Ham and Rt RFem which, respectively preceded the alternate trunk rotations. The integrated EMGs and torques increased with increasing amplitude of arm movement and load. The integrated torques increased in the order of S, M, L, and D conditions. The integrated EMGs of the thigh muscles correlated with the integrated torques (medians: r =0.880, 0.696, 0.785, and 0.688, respectively, in the Rt Ham, Lt Ham, Rt RFem, and Lt RFem). The trajectories of the center of foot pressure showed variations, initially toward the Rt side and then the Lt side which, respectively coincided with the initial and later phases of the trunk rotations and the muscle activation. The trunk muscles were generally coactivated between the Rt and Lt muscles, and the integrated EMGs increased with increasing the integrated torques. Our results showed that alternate rotations of the upper trunk, produced by rapid arm movements, were transmitted to the hip in part due to cocontraction of trunk muscles, and each pair of hip joint muscles contributed to the maintenance of the standing posture by stabilizing the hip joints against alternating trunk rotations. [Copyright &y& Elsevier]