Paraspinal muscle fibres demonstrate a complex relationship between contractile variability and function in canines...

Paraspinal muscle dysfunction is associated with spinal degeneration, yet our understanding of the specifics of this muscle dysfunction is incomplete. Muscles consist of thousands of individual fibres, but little is known about the variability in functional capability amongst these fibres, particularly in muscles that display pathological features. This study investigated individual muscle fibre contractile function variability within paraspinal muscle biopsies from 19 chondrodystrophic canines undergoing surgery for intervertebral disc extrusion, and a control group of 10 healthy rats. Specific force, active modulus, and rate of force redevelopment (KTR) were tested in an average of 20 fibres from each biopsy. Correlations were computed between means and both standard deviations (absolute variability) and coefficient of variations (relative variability) for each contractile variable. Across contractile variables, canine muscle fibres demonstrated greater relative variability and lower means compared to rat controls, confirming pathological impairment. Notably, most relationships demonstrated negative correlations between contractile performance and relative variability, consistent with the hypothesis that more impaired muscles exhibit greater fibre-level heterogeneity. Interestingly, two canines demonstrated unusually high KTR in type 1 fibres, suggesting possible compensatory adaptation. These findings highlight altered within-muscle contractile function in spinal pathology and underscore the importance of fibre-level analysis for understanding muscle dysfunction.