The Form and Function of the Loaded Human Spine

Klausen, K. The form and function of the loaded human spine. Acta physiol. scand. 1965. 65. 176–190. — The reaction of the spine and its muscles to an increase of the pull of gravity was investigated in different ways: The change of the curves of the spine was measured with an inclinometer. The placement of the line of gravity in relation to L4 and the ankle joints was determined, and the activity of the trunk muscles and some of the leg muscles was investigated electromyographically. From a discussion of the mechanical effect of gravity on the individual joints, and on the spine as a whole it is concluded that the short, deep muscles of the back must play an important part in stabilizing the individual joints, and that the long back muscles or the abdominal muscles are responsible for the stabilization of the spine as a whole. It was found that an increased pull of gravity always was counteracted by increased activity in one set of muscles only, i. e. either in the back muscles or in the abdominal wall muscles. An increased pull of gravity gave rise to a flattening of the lumbar lordosis probably due to a deformation of the discs, while the thoracic kyphosis was almost unaltered. A load placed high on the back caused the trunk to lean, slightly forward, and tbe increased pull of gnavity was counteracted by an increased activity of the lower back muscles. A load placed low on the back caused the activity of the back muscles to decrease, and in most cases the pull backwards of the load was counteracted by the psoas muscle.