The effects of abdominal muscle coactivation on lumbar spine stability

Study Design. A biomechanical model of the lumbar spine was used to calculate the effects of abdominal muscle coactivation on spinal stability. Objectives. To estimate the effects of abdominal muscle coativation on lumbar spine stability, muscle fatigue rate, and lumbar spine compression forces. Summary of Background Data. The activation of human trunk muscles has been found to involve coactivation of antagonistic muscles, which has not been adequately predicted by biomechanical models. Antagonistic activation of abdominal muscles might produce flexion moments resulting from abdominal pressurization. Qualitatively, antagonistic activity also has been attributed to the need to stability the spine. Methods. Spinal loads and spinal stability were calculated for maximum and submaximum (40%, 60% and 80%) efforts in extension and lateral bending using a previously published, anatomically realistic biomechanical model of the lumbar spine and its musculature. Three different antagonistic abdominal muscle coactivation patterns were imposed, and results were compared with those found in a model with no imposed coactivation. Results. Results were quantified in terms of the sum of cubed muscle stresses (Sigma sigma m(3), which is related to the muscle fatigue rate), the maximum compressive loading on the lumbar spine, and the critical value of the muscle stiffness parameter (q) required for the spine to be stable. Forcing antagonistic coactivation increased stability, but at the cost of an increase in Sigma sigma m(3) and a small increase in maximum spinal compression. Conclusions. These analyses provide estimates of the effects of antagonistic abdominal muscle coactivation, indicating that its probable role is to stabilize the spine.