A Biomechanical Assessment and Model of Axial Twisting in the Thoracolumbar Spine

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Abstract
Measured trunk kinematics, applied moments, and trunk muscle activities were employed in a biomechanical model to determine load experiences by the spine during dynamic torsional exertions. The purpose of this investigation was to examine the influence of dynamic twisting parameters on spinal load. Axial twisting of the torso has been identified as a significant risk factor for occupationally related low back disorders. However, previous studies have had difficulty describing how twisting is accomplished biomechanically, or how the spine is loaded during twisting motions. Electromyograph activity of 10 trunk muscles was monitored while 12 subjects performed twisting exertions under various conditions of force, velocity, position, and direction. An electromyograph-assisted biomechanical model was developed to interpret the effects of those twisting parameters on spine loading. Significant flexion-extension and lateral moments were generated during the twisting exertions. Muscle co-activity associated with twisting exertions was significantly greater than that associated with lifting exertions. Employing electromyograph data to represent muscle co-activity, the model accurately predicted trunk moments and hence was assumed to reasonably reflect spine loading. Under the conditions tested, the results indicated that relative spinal compression during dynamic twisting exertions was twice that of static exertions. Spine loading also varied as a function of whether the trunk was twisted to the left or right and according to the direction of applied torsion--i.e., clockwise or counterclockwise. The results may help explain, biomechanically, why epidemiologic findings have repeatedly identified twisting as a risk factor for low back disorder.

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