Introduction: Chronic low back pain (LBP) is one of the most prevalent causes of disability worldwide as it negatively affects an individual’s quality of life. Chronic LBP is associated with weakened paraspinal muscles, specifically, those involved with extension; erector spinae (ES) and multifidus (MF). The relationships between muscle size (e.g., cross-sectional area [CSA]), composition (e.g., fatty infiltration [FI]) and strength, however, is poorly understood and warrants further investigation. This study investigated the relationship between paraspinal muscles CSA, FI, and lumbar extensor muscle strength at each spinal level in subjects with chronic LBP.
Methods: A total of 25 participants (17 females, 8 males) with chronic LBP (average age of 43.92 ± 11.82 years old) were recruited. Isometric contractions were performed to measure the maximum strength of extensor muscles in 7 different angles using MedX lumbar extension machine. CSA and signal interference (SI) of the ES, MF, psoas (PS) and quadratus lumborum (QL) were measured at mid-disc bilaterally, for each lumbar level on IDEAL fat and water axial MR images. The SI of water and fat images were used to assess the percent-fat signal fraction (%FSF), representing FI. Spearman’s correlation was used to assess the relationship between CSA, %FSF, and overall mean strength at each lumbar level.
Results: Greater ES, MF, PS and QL muscle CSA were all positively correlated to higher mean extension strength at the upper 4 spinal levels (L1-L2 to L1-L4) (e.g., r ranged between 0.457-0.668), with PS showed the strongest correlations. No correlations were found between CSA and strength at L5-S1 for any of the paraspinal muscles. FSF% was not correlated with mean extension strength at any level.
Discussion: This study analysed the relationship between FI and CSA and lumbar extension strength for every paraspinal muscle individually, at each lumbar level (L1 to S1). Paraspinal muscle size was strongly correlated with mean extension strength in individuals with chronic LBP, with the PS muscle showing the strongest correlation at the L1-L4 levels. Further research is needed to clarify the relationship between paraspinal muscle strength and composition, expand our findings and investigate whether sex and BMI influence this association in subjects with LBP.