The equations of equilibrium are set up for the thoracic and lumbar spine and involve body weight, external load, muscle forces and intervertebral reactions. These equations are solved using a linear programming technique that minimizes the total force in the system. The solution gives numeric values for the muscle forces and intervertebral reactions. The results for a subject in progressive forward flexion and in lateral bend are presented. Also examined are the effects of 3 different orthotic devices on the muscle forces of a patient with a scoliotic spine.