Lateral weight shift to one side in the standing position is performed to facilitate the initiation and change of direction in patients with trunk muscle instability. In this study, the muscle activity patterns of the multifidus, iliocostalis, and longissimus during lateral weight shift in the standing position were analyzed. The subjects were 10 healthy males who consented to participation. The subjects initiated lateral weight shift in 2 s in the standing position, and maintained the lateral weight-shift posture for a maximum of 2 s. Rotation of the trunk and pelvis did not occur, and both the acromion processes were held horizontal. Lateral movement distance is the distance that a subject can move within the specified time, while the toes of the limb on the side opposite the weight shift direction remain touching the ground lightly. The items measured were the center of pressure (COP), the electromyographic activities of the multifidus, iliocostalis, and longissimus of both the sides, and a video image taken from the front. The muscle activity on the non-moving side multifidus tended to increase with the displacement of the COP of the moving side. In addition, muscle activities of the iliocostalis and longissimus of the non-moving side tended to increase. Furthermore, elevation of the pelvis and flexion of the trunk on the non-moving side were observed with the displacement of the COP to the moving side. The multifidus of the non-moving side appears to contribute to the movement, because of the physiological lordosis of the lumbar spine, and both the acromion processes are held horizontal for elevation of the pelvis on the non-moving side, which is the result of lumbar extension and flexion on the non-moving side. In addition, the iliocostalis and longissimus of the non-moving side also appear to contribute to the movement, because of the curvature of the thoracolumbar segments, and both the acromion processes are held horizontal for the elevation of the pelvis on the non-moving side, which is the result of thoracolumbar extension and flexion on the non-moving side. In physical therapy, it has been suggested that the lateral weight shift that occurs because of the elevation of pelvis on the non-moving side is important, and that thoracolumbar flexion on the non-movable side is generated by the posterior trunk muscles (multifidus, iliocostalis, and longissimus) on the non-moving side.
