Role of the Trunk in Stability of the Spine

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Abstract
There is a great discrepancy between the force that can theoretically be applied to the spine if the role of intracavitary pressures is ignored and the force that can be tolerated experimentally by the isolated ligamentous human spine. In estimating that a force of approximately 2,071 pounds is imposed on the lower lumbar part of the spine when a weight of 200 pounds is lifted in the flexed position, the vertebral column is considered only as an isolated unit being acted on by the forces of the weight lifted, the body weight, and the contraction of the deep muscles of the back. Such a picture, however, is quite simplified, since no account is taken of the important role of the trunk—thorax and abdomen—in support of the spine. ("Support" is used here in its broadest sense, meaning maintenance of stability of the column against deformity and against structural damage of its components.) The answer to the question of how the vertebral column in a living subject is able to withstand a far greater force than the isolated column must be found by consideration of the extrinsic supporting structures of the trunk. In this study, data on the mechanism of support of the spine by the trunk have substantiated the hypothesis made at the beginning of the study: The spinal column is attached to the sides of and within two chambers, the abdominal and thoracic cavities; the action of the trunk muscles converts these chambers into nearly rigid-walled cylinders containing (1) air and (2) liquid and semisolid material. Both these cylinders are capable of transmitting part of the forces generated in loading the trunk and thereby of relieving the load on the spine itself. When large forces are applied to the spine—for example, lifting weights of 100 to 200 pounds—there is generalized contraction of the trunk muscles, including the intercostals, the muscles of the abdominal wall, and the diaphragm. The muscles about the shoulder girdle and those of the back are, of course, active during lifting, just as the muscles of the thighs help maintain body balance and the erect position. The action of the intercostals and of the muscles of the shoulder girdle renders the thoracic cage a quite rigid structure firmly bound to the thoracic part of the spine. When inspiration and the action of the intercostal muscles, which stabilize the rib cage, increase intrathoracic pressure, the thoracic cage and spine become a solid, sturdy unit capable of transmitting large forces. By the contraction of the diaphragm, attached at the lower margin of the thorax and overlying the abdominal viscera, and of the muscles of the abdominal wall, especially the transversus abdominis, the abdominal contents are compressed into a semirigid cylinder. The force of weights lifted by the arms is thus transmitted to the spinal column by the shoulder-girdle muscles, principally the trapezius, and then to the abdominal cylinder and to the pelvis, partly through the spinal column but also through the rigid rib cage. When larger forces are involved, there is need for increased rigidity of the rib cage and compression of the abdominal contents. This accounts for the increased activity of the trunk muscles and the increase in intracavitary pressures when greater forces are applied. Thus, an increased intra-abdominal pressure is due to the contraction of the abdominal muscles and to the compression of the abdomen by the force which is transmitted through the trunk. This view is well substantiated by the fact that when an air-pressure corset is worn, although the resting abdominal pressure is considerably elevated (by approximately twenty millimeters of mercury), the pressures recorded during loading of the spine are similar to those recorded without the corset. However, the activity of the abdominal muscles is markedly decreased when the inflatable corset is worn. It appears, therefore, that the contracted muscles of the abdominal wall or the rigid external-pressure apparatus act to contain the abdominal contents in a compressed state capable of transmitting force. When the compression or restraint is accomplished by an external apparatus, there is little need for contraction of the abdominal muscles. However, a small amount of activity is noted in the abdominal muscles even when the corset is worn, especially when greater forces or heavier loads are applied. The amount of compression of the viscera necessary to transmit these great forces can be tolerated briefly but not for prolonged periods. Since, in this study, the apparatus was inflated to the limit of comfort for extended periods, the abdomen was not compressed enough to obviate entirely the need for muscle activity with the larger forces. It should be emphasized that the mechanism discussed here is a reflex mechanism. When a load is placed on the spine, the trunk muscles are involuntarily called into action to fix the rib cage and to restrain or compress the abdominal contents. The intracavitary pressures are thereby increased, aiding in support of the spine. It may be concluded, from this calculation of the contribution of the trunk compartments to the support of the spine, that the actual force on the spine is much less than that considered to be present when support by the trunk, or the effect of the intracavitary pressures, is omitted. The calculated force on the lumbosacral disc is about 30 per cent less, and that on the lower thoracic portion of the spine is about 50 per cent less, than would be present without support by the trunk. These particular results are applicable under the conditions described in this report—near-maximum loading of the trunk (lifting a 200-pound weight) in the flexed position. However, with appropriate consideration of the surrounding structures, the position of the spine, and the forces acting at a specific level, the general principles illustrated by this investigation can be used to determine the force at any level of the spine...

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