1 Department of Health, Kinesiology & Applied Physiology, Concordia University, Montreal, Canada.
2 School of Physical and Occupational Therapy, McGill University, Montreal, Canada.
3 NeuroPoly Lab, Institute of Biomedical Engineering, Polytechnique Montreal, Montreal, Canada.
4 Department of Electrical and Computer Engineering, Concordia University, Montreal, Canada.
5 Department of Psychology, McGill University, Montreal, Canada.
6 Department of Health, Kinesiology & Applied Physiology, Concordia University, Montreal, Canada. maryse.fortin@concordia.ca.
7 Centre de Recherche Interdisciplinaire en Readaptation (CRIR), Montreal, Canada. maryse.fortin@concordia.ca.

Purposes: While lumbar multifidus (MF) muscle alterations are linked to low back pain (LBP), the structure-function relationship is not fully understood. This study aims to evaluate the relationship between fatty degeneration of the lumbar MF muscle and its function in individuals with and without LBP.

Methods: The study included 25 participants with chronic nonspecific LBP and 25 age- and sex-matched healthy controls. Participants underwent MRI assessment for MF fat infiltration, utilizing IDEAL fat-water images. Ultrasound measures evaluated MF function, including shear-wave elastography (SWE) for stiffness/elasticity and thickness ratio from rest to submaximal contraction. All measurements were acquired at L4/L5 and L5/S1 spinal levels, bilaterally. Bivariate and multivariable linear regression models were used to assess the relationship between morphology and function, while age, sex, body max index (BMI), physical activity levels, and LBP status were considered as covariates.

Results: Fifty participants (26 females) were included (mean age: 39.22 ± 11.67). Greater % MF fat at L4/L5 was significantly associated with greater MF SWE ratio (p = 0.002). No significant bivariate or multivariable relationships were found between MF fat infiltration and MF thickness ratio. Participants with LBP exhibited lower contraction ratios (p = 0.017) and higher SWE during contraction (p = 0.03) at L4/L5 compared to controls.

Conclusion: This study highlights a positive association between MF fat infiltration and SWE-based stiffness measures at L4/L5, suggesting altered muscle composition may impacts MF function. However, no relationship was found between MF fat infiltration and contraction. Participants with LBP demonstrated distinct deficits in muscle activation, supporting the need for targeted rehabilitation strategies addressing these functional impairments.