The Influence of Trunk Muscle Coactivity on Dynamic Spinal Loads

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Abstract
Study Design Measured trunk muscle activity was employed in a biomechanical model to determine the influence of including or neglecting muscle coactivity on predicted spinal loads. Objectives The purpose of this investigation was to examine the influence of muscle coactivity on spinal load. Summary of Background Data Electromyographic patterns in the trunk musculature have demonstrated significant levels of cocontraction during lifting exertions. Biomechanical analyses of musculoskeletal loading are often mathematically constrained from including muscle coactivity. Models that attempt to include coactive behavior are complex and difficult to implement. Methods Electromyographic data were collected from five trunk muscle pairs while subjects performed dynamic lifting excertions. A validated, electromyographically assisted biomechanical model was used to compute relative muscle force, lifting moment, and spinal load. Results were generated and compared from analyses that included from one to five simultaneously active muscle pairs. Results Trunk extensor muscles generate lifting moments as much as 47% greater than the applied lifting moment to offset flexor antagonism. Analyses that neglect muscle coactivity during dynamic lifting exertions may underestimate spinal compression by as much as 45% and shear forces by as much as 70%. Conclusions The level of coactive spinal loading is significantly influenced by the weight of the lifted load as well as trunk extension velocity. Muscle coactivity significantly influences the modeled load in the lumbar spine during lifting exertions as should be considered if an accurate measure of spinal loading of desired.

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