Changes in the mechanical properties of the trunk in low back pain may be associated with recurrence.

Exercise is one of the few effective treatments for LBP. Although exercise is often based on the premise of reduced spinal stiffness, trunk muscle adaptation may increase stiffness. This study developed and validated a method to assess trunk stiffness and damping, and tested these parameters in 14 people with recurring LBP and 17 pain-free individuals. Effective trunk stiffness, mass and damping were estimated with the trunk modeled as a linear second-order system following trunk perturbation. Equal weights (12-15% body weight) were attached to the front and back of the trunk via pulleys such that the trunk could move freely and no muscle activity was required to hold the weights. The trunk was perturbed by the unexpected release of one of the weights. Trunk kinematics and cable force were used to estimate system properties. Reliability was assessed in 10 subjects. Trunk stiffness was greater in recurrent LBP patients (forward perturbation only), but damping was lower (both directions) than healthy controls. Estimates were reliable and validated by accurately estimated mass. Contrary to clinical belief, trunk stiffness was increased, not reduced, in recurrent LBP, most likely due to augmented trunk muscle activity and changes in reflex control of trunk muscles. Although increased stiffness may aid in the protection of spinal structures, this may have long-term consequences for spinal health and LBP recurrence due to compromised trunk dynamics (decreased damping).