STRESS ON THE SPINE AND THE SPINE AND GRAVITY

The spine can never be at rest while life lasts, because the thoracic vertebrae, which are attached to the rib cage, move with every breath, straightening as you breathe in and flexing forward as you breathe out.

The spine plays a part in the movements of most other parts of the body. Moving the head in relation to the chest causes movement in the cervical and upper thoracic vertebrae; moving the arms also moves the thoracic spine; the lumbar spine is involved in movements of the upper part of the trunk in relation to the pelvis, and in moving the legs.

The spine and gravity-The vertical force exerted on the human body by gravity varies according to the position the body assumes. Gravity is least stressful in the lying-down position, and the stress is well distributed in moving on all fours (which is one reason why back pain victims sometimes have to revert to crawling); but the erect stance of homo sapiens is maintained in defiance of gravity, and the gravitational load exerted on the head, arms and upper trunk is taken mainly by the vertebral bodies, and the intervertebral discs. (Back problems are, however, not exclusive to those who happen to stand erect. There is a popular misconception that if only we walked on all fours, we would have no back problems. But some animals undoubtedly do have back troubles: horses, dogs especially dachshunds.)

The muscles, too, undergo gravitational stress, depending on the direction of movement. In bending, the downward movement of the trunk is helped by gravity, with the muscles and ligaments controlling and limiting the descent; in straightening up, the muscles must do all the work, against gravity, but once the upright position is achieved, they do not need to work very hard to maintain it.

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