LUMBAR SPINE MAXIMUM EFFORTS AND MUSCLE RECRUITMENT PATTERNS PREDICTED BY A MODEL WITH MULTIJOINT MUSCLES AND JOINTS WITH STIFFNESS

The transmission of load through the lumbar spine was analyzed in a mo del of the five lumbar vertebrae, the sacrum/pelvis and the thorax, an d 66 symmetric pairs of multijoint muscles. The model was used to test the hypotheses that (1) the need to maintain equilibrium simultaneous ly at all vertebral levels precludes simultaneous maximum activation o f synergistic muscles and (2) that the maximum loads which could be ca rried by the spine and the degree of muscle activation increases with increasing motion segment stiffness. Maximum moments applied to T12 we re calculated for moments in three principal directions, subject to eq uilibrium at all six joints and to constraints on the maximum muscle s tress and intervertebral displacements. A model with realistic motion segment stiffness predicted maximum efforts between 1.4 and 3.3 times greater than a model with 'ball-and-socket' joints, and in better agre ement with published results from maximum effort experiments. The diff erences in maximal effort were greater than the moments transmitted th rough the joints. While muscle activation levels were greater, many sy nergistic muscles were still submaximally activated. Antagonistic musc les were recruited to maintain multijoint equilibrium. We concluded th at (1) muscle activations permitted in single anatomic level analyses are generally not compatible with equilibrium at other levels; (2) the effect of moment transmission in the joints gives a more realistic re presentation of the lumbar spine.