Oral Presentation 51st International Society for the Study of the Lumbar Spine Annual Meeting 2025

INTRODUCTION

Spinal degeneration and deformity alter the natural structure of the spine, which can necessitate accommodating alterations in muscle function. In addition, paraspinal muscle dysfunction may act as a causative factor for spine degeneration and deformity. In a study of adult spinal deformity patients undergoing corrective surgery, 17% of paraspinal muscle biopsies did not exhibit any contractile function, and characteristics of muscle degeneration were widespread across the patient group [1]. To maintain some function, it’s possible that certain fibres within these degenerated muscles develop compensatory mechanisms to enhance their own contractile ability; however, within-patient observations into muscle fibre contractile function have not been conducted.  Therefore, this study was designed to explore within-patient paraspinal muscle single fibre contractile variability in patients with adult spine deformity. It was hypothesized that muscles with higher overall functional capability would demonstrate lower between-fibre (within-muscle) variability.

METHODS

48 paraspinal muscle biopsies (bilateral multifidus and longissimus) were collected from 12 adult spinal deformity patients at the L4-5 level. The biopsies were chemically permeabilized and single muscle fibres were isolated. These isolated fibres were placed into a calcium bath to induce maximal contraction where specific force (maximum normalized force generating capability), active modulus (normalized stiffness), and rate of force redevelopment (KTR) were measured. An average of 20 fibres were tested per biopsy; fibres were separated as either Type 1 (slow) or Type 2 (fast). Mean values and the relative (coefficient of variation) and absolute (standard deviation) variability were calculated for the pooled Type 1 and Type 2 fibres from each biopsy, and correlation analyses were performed between the means and variabilities.

RESULTS

A significant negative correlation was observed between the mean and coefficient of variation of specific force (Figure 1; Type1: r=-0.58, p<0.01; Type2: r=-0.57, p=<0.01), as well as between the mean and coefficient of variation of active modulus (Type1: r=-0.72, p<0.01; Type2: r=-0.71, p<0.01), suggesting increased relative variability in weaker muscles.  Conversely, stronger muscles showed higher, but not statistically significant, absolute variability (Figure 1; specific force: Type1: r=0.18, p=0.36; Type2: r=0.21, p=0.32; active modulus: Type1: r=0.07, p=0.69; Type2: r=0.09, p=0.69). For KTR, positive correlations between the mean and both relative variability in Type 1 fibres (r=0.16, p=0.4) and absolute variability in Type 1 and 2 fibres were observed (Type1: r=0.76, p<0.01; Type2: r=0.50, p=0.01). Conversely, Type 2 muscle fibres demonstrated a significant negative correlation between mean KTR and relative variability (Type2: r=-0.56, p<0.01).

DISCUSSION

These results highlight the complex interplay between muscle fibre function and variability in relation to spinal degeneration. As a muscle’s average force generating capacity decreases, there is a statistically significant greater relative variability, suggesting a relative compensation in function scaled to the overall reduced force generating capability.  For KTR, which represents the rate at which muscle fibres can generate force, the predominant positive relationship between the mean and variability indicates a lack of compensation for this contractile property. These findings emphasize the need for further research to distinguish naturally occurring variability in muscle fibre functional capabilities from those associated with spinal degeneration.