Effects of Abdominal Muscle Coactivation on the Externally Preloaded Trunk: Variations in Motor Control and Its Effect on Spine Stability

In Brief Study Design. A repeated measures biomechanical analysis of the effects of abdominal bracing in preparation for a quick release of the loaded trunk. Objectives. To quantify the ability of individuals to abdominally brace the externally loaded trunk, and assess their success in achieving and enhancing appropriate spine stability. Summary of Background Data. Spine stability requires trunk muscle coactivation, which demands motor control skill that differs across people and situations. The quick release protocol may offer insight into the motor control scheme and subsequent effect on spine stability. Methods. There were 10 individuals who sat, torso upright, in an apparatus designed to foster a neutral spine position. They were instructed to support a posteriorly directed load to the trunk in either their naturally chosen manner, or by activating the abdominal muscles to 10%, 20%, or 30% of maximum ability. The externally applied load was then quickly released, thereby unloading the participant. Muscle pre-activation patterns, spine stability, and kinematic measures of trunk stiffness were quantified. Results. Participants were able to stabilize their spine effectively by supporting the load in a naturally selected manner. Conscious, voluntary overdriving of this natural pattern often resulted in unbalanced muscular activation schemes and corresponding decreases in stability levels. Conclusions. Individuals in an externally loaded state appear to select a natural muscular activation pattern appropriate to maintain spine stability sufficiently. Conscious adjustments in individual muscles around this natural level may actually decrease the stability margin of safety. The ability of externally loaded individuals to brace successfully their abdominal musculature to improve spine stability was assessed. Participants tended to find a natural bracing state that sufficiently ensured the stability of the spine. Overdriving this natural bracing pattern often led to unbalanced muscle coactivation patterns and decreases in spine stability.