• Gregory James, Magali Millecamps, Laura S Stone, and Paul W Hodges.
• The University of Queensland, NHMRC Centre of Clinical Research Excellence in Spinal Pain, Injury and Health, School of Health and Rehabilitation Sciences, Brisbane, Australia.
• Spine. 2021 Dec 1; 46 (23): 161216201612-1620.

Study DesignCase-controlled animal study.ObjectiveThe aim of this study was to investigate whether multifidus muscle fiber type distribution changes in models of interverbal disc (IVD) degeneration and whether this is resolved by physical activity (PA).Summary Of Background DataThe loss of slow type I muscle fibers in the multifidus muscle in people with low back pain is contentious. Data from animal models of IVD degeneration suggest some discrepancies in human studies might be explained by the dependence of slow muscle fiber changes and their underlying mechanisms, on the time since injury and progression of IVD degeneration. It is not yet resolved what changes are apparent once the chronic phase is established. It is also not known whether muscle fiber changes can be resolved by whole body PA. This study aimed to examine slow fiber distribution in the multifidus muscle in models of IVD injury or spontaneous degeneration in animals with or without exposure to PA.MethodsTwo models of IVD degeneration were used. The first model used a genetically modified mouse (SPARC-null) that spontaneously develops IVD degeneration. The second model involved a surgically induced IVD lesion to induce degeneration. Mice in each study were allocated to housing with or without a running wheel for PA. At 12 months of age, the multifidus muscle was harvested. Slow muscle fiber distribution and the mRNA expression of genes associated with muscle fiber type transformation were examined.ResultsThe proportion and cross-sectional area of slow muscle fibers were reduced in both models of IVD degeneration compared to controls, without evidence of ongoing fiber transformation. Whole-body PA did not attenuate these alterations.ConclusionResults confirmed slow muscle fiber loss in the multifidus in the chronic phase of IVD degeneration induced spontaneously and by injury. Whole-body PA did not attenuate changes to muscle fiber distribution. More specific approaches to muscle activation might be required to achieve more complete reversal of muscle fiber changes, with potential implications for therapy in humans.Level of Evidence: N/A.Copyright © 2021 Wolters Kluwer Health, Inc. All rights reserved.

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