The mechanism of the lumbar spine.

The moments generated in the lumbar spine by a weight lifter are balanced by moments generated internally by muscles and by the intervertebral ligamentous structures including the disc. Unique real number solutions have been obtained by a optimization technique minimizing shear and penalizing excessive muscle power. The model faithfully reproduces the electromyographic responses and all known experimentallydetermined values for muscle power and compression. The outputs can be used to calculate a realistic motion of the spine under various loads. The model suggests that in handling heavy loads, the stress at each intervertebral joint is identical and with maximal voluntary effort, the weight lifter does not exceed 67% of the ultimate strength of his tissues.