Cost-benefit of muscle cocontraction in protecting against spinal instability

Study Design. Lifting dynamics and eletromyographic activity were evaluated using a biomechanical model of spinal equilibrium and stability to assess cost-benefit effects of antagonistic muscle cocontraction on the risk of st ability failure. Objectives. To evaluate whether increased biomechanical stability associate d with antagonistic cocontraction was capable of stabilizing the related in crease in spinal load. Summary of Background Data. Antagonistic cocontraction contributes to impro ved spinal stability and increased spinal compression. For cocontraction to be considered beneficial, stability must increase more than spinal load. O therwise, it may be possible for cocontraction to generate spinal loads tha t cannot be stabilized. Methods. A biomechanical model was developed to compute spinal load and sta bility from measured electromyography and motion dynamics. As 10 healthy me n performed sagittal lifting tasks, trunk motion, reaction loads, and elect romyographic activities of eight trunk muscles were recorded. Spinal load a nd stability were evaluated as a function of cocontraction and trunk flexio n angle. Stability was quantified in terms of the maximum spinal load the s ystem could stabilize. Results. Cocontraction was associated with a 12% to 18% increase in spinal compression and a 34% to 64% increase in stability. Spinal load and stabili ty increased with trunk flexion. Conclusions. Despite increases in spinal load that had to be stabilized, th e margin between stability and spinal compression increased significantly w ith cocontraction. Antagonistic cocontraction was found to be most benefici al at low trunk moments typically observed in upright postures. Similarly, empirically measured antagonistic cocontraction was recruited less in high- moment conditions and more in low-moment conditions.