INTRODUCTION: Studies have linked chronic back pain with abnormal muscle use and structure. However, cause‐effect relationships among back pain, injury, and muscle dysfunction remain elusive. Defining such relationships is essential to understand and treat back pain. This study was designed to assess changes in passive mechanical properties, protein structure, and histology of multifidus in response to disc degeneration produced by experimental needle puncture.
METHODS: 12 rabbits were studied; 4 in each of 3 groups: control, 4‐weeks disc degeneration, and 12‐weeks disc degeneration. Single multifidus fibers and fiber bundles were isolated and tested for slack sarcomere length and elastic modulus. Titin isoform mass, myosin heavy chain (MHC) distribution, and muscle histology were also examined.
RESULTS: Compared to control, muscle fibers were 34% stiffer and fiber bundles 107% stiffer in the 12‐week degeneration group. No changes were detected at 4‐weeks degeneration. No statistically significant change was found for MHC distribution in the degeneration groups compared to control, while titin isoforms were larger (p < 0.05) than control at 12‐weeks degeneration. Histology revealed select regions of multifidus, at 12‐weeks degeneration, with increased space between fiber bundles which was partly occupied by adipose.
CONCLUSION: Multifidus becomes stiffer, in individual fibers and fiber bundles, in response to experimentally‐induced intervertebral disc degeneration. This cannot be explained by change in fiber‐type, nor by increased size of the protein titin (which would reduce stiffness). We hypothesize that fiber bundles become stiffer by proliferation and re‐organization of collagen within the muscle but the basis for fiber stiffening is unclear. These results represent the first direct evidence of changes in mechanical properties of back muscles in response to disc degeneration and indicate a cause‐effect between low back injury and back muscle dysfunction.