Contribution of ribcage movement to thoracolumbar posteroanterior stiffness

Objective: To investigate (1) whether thoracolumbar posteroanterior (PA) stiffness differs between 2 conditions of ribcage movement: unconstrained and constrained, and (2) whether the effect of ribcage constraint on PA stiffness varies according to where the PA force is applied. Design: Two-factor within-subjects design. Setting: Spinal Mechanics Laboratory, University of Sydney. Intervention: A convenience sample of 41 subjects, asymptomatic for back pain, participated. PA stiffness at T12-L4 was measured in the unconstrained and constrained ribcage conditions with a mechanical device. For the constrained condition, we used a clamping device to apply a force to the subject's lower thorax to reduce movement. Main Outcome Measures: PA stiffness at T12-L4 under both ribcage conditions. Results: PA stiffness at T12-L4 significantly increased when the ribcage was constrained (P < .05). However, the effect of ribcage movement did not depend on the location of the PA force. Conclusions: These findings suggest that the properties of the ribcage influence measures of PA stiffness in the thoracolumbar (T12-L4) spine uniformly. Variations in PA stiffness in segments T12-L4 may reflect the properties of the intervertebral joints.