Effects of muscle activity on lumbar spinal stiffness in asymptomatic adults: An investigation using a novel rolling device

Background: Device-based measurement of lumbar spinal stiffness has the potential to identify patients with low back pain who are more likely to improve with spinal manipulative therapy. This study evaluates how voluntary contraction of spine muscles may impact stiffness measures.

Objective: To determine how the contraction of different spinal muscles may influence spinal stiffness at all lumbar levels.

Design: Experimental study.

Method: A mechanical device was used to measure spinal stiffness (N/mm) from L1 to L5 in 12 asymptomatic participants, while muscle activity from four pairs of thoracolumbar muscles was recorded. A baseline measurement was collected with the participants holding their breath at normal exhalation. Participants stiffness was then measured while performing (1) an isometric hip extension, (2) an isometric shoulder flexion, and (3) a deep held inhalation. Mixed-model ANOVAs were used to evaluate the effects of the perturbations on spinal stiffness at each lumbar level. Friedman's test was then computed to evaluate the differences in muscle activity between the perturbations.

Results: Globally, the designed perturbations generated activity in different muscles with different magnitudes (P-values≤0.05). Increased spinal stiffness was observed at each spinal level during the hip extension, and at L5 during the held inhalation (P-values<0.05). A differential effect of the spinal levels on the spinal stiffness was observed during the hip extension and held inhalation (P-values<0.05).

Conclusion: This study provides evidence that the magnitude of muscle activity influences spinal stiffness, but not equally between lumbar levels.