Muscle Activation Strategies and Symmetry of Spinal Loading in the Lumbar Spine With Scoliosis

Study Design. Biomechanical analysis of muscle and spinal forces in a lumbar spine with scoliosis. Objectives. To calculate spinal loading asymmetry and its dependence on muscle activation strategy. Summary of Background Data. It is commonly assumed that a spine with scoliosis experiences greater loading on the concave side and that this asymmetric loading causes asymmetric growth and progression of deformity. However, neither the magnitude of the asymmetric loading imposed on the spine as a function of the scoliosis curve nor the resulting mechanically altered vertebral growth and disc remodeling have been quantified. Methods. Spinal loading was estimated in a lumbar spine model with increasing degrees of scoliosis. External loading was each of three pure moments or forces acting at T12, with magnitudes of either 50% or 75% of maximum effort. For each external loading, the muscle activation patterns were determined with each of three different muscle activation strategies in an optimization model: 1) minimize the sum of cubed muscle stresses; 2) minimize spinal asymmetric load (i.e., “follower load”); and 3) reverse the spinal load asymmetry (increased compression on convex side) at the level of the apex. Results. The first strategy produced loading that tended to increase the curve magnitude, with the resultant force acting at up to 15 mm lateral to the intervertebral disc center. Both Strategies 2 and 3 had increased muscle stress averaging between 42% and 75%. Conclusions. We speculate that individuals with scoliosis can adopt different muscle activation strategies and that these strategies may determine whether or not the spinal loading causes scoliosis progression during growth. Muscle activation patterns generating spinal loading that does not promote curve progression during growth have greater physiologic cost.