Cost-benefit of muscle cocontraction in protecting against spinal instability.

STUDY DESIGN Lifting dynamics and electromyographic 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 stability failure. OBJECTIVES To evaluate whether increased biomechanical stability associated with antagonistic cocontraction was capable of stabilizing the related increase in spinal load. SUMMARY OF BACKGROUND DATA Antagonistic cocontraction contributes to improved spinal stability and increased spinal compression. For cocontraction to be considered beneficial, stability must increase more than spinal load. Otherwise, it may be possible for cocontraction to generate spinal loads that cannot be stabilized. METHODS A biomechanical model was developed to compute spinal load and stability from measured electromyography and motion dynamics. As 10 healthy men performed sagittal lifting tasks, trunk motion, reaction loads, and electromyographic activities of eight trunk muscles were recorded. Spinal load and stability were evaluated as a function of cocontraction and trunk flexion angle. Stability was quantified in terms of the maximum spinal load the system 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 stability increased with trunk flexion. CONCLUSIONS Despite increases in spinal load that had to be stabilized, the margin between stability and spinal compression increased significantly with cocontraction. Antagonistic cocontraction was found to be most beneficial 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.