The transmission of load through the lumbar spine was analyzed in a model of the five lumbar vertebrae, the sacrum/pelvis and the thorax, and 66 symmetric pairs of multijoint muscles. The model was used to test the hypotheses that (1) the need to maintain equilibrium simultaneously at all vertebral levels precludes simultaneous maximum activation of synergistic muscles and (2) that the maximum loads which could be carried by the spine and the degree of muscle activation increases with increasing motion segment stiffness. Maximum moments applied to T12 were calculated for moments in three principal directions, subject to equilibrium at all six joints and to constraints on the maximum muscle stress 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 agreement with published results from maximum effort experiments. The differences in maximal effort were greater than the moments transmitted through the joints. While muscle activation levels were greater, many synergistic muscles were still submaximally activated. Antagonistic muscles were recruited to maintain multijoint equilibrium. We concluded that (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 representation of the lumbar spine.