The Illusion of Lumbar Multifidus Muscle Size: Gross vs. Contractile Differences at L4 in Young Adult Women with Non-Specific Chronic Low Back Pain

Abstract

Background: Non-Specific Chronic Low Back Pain (NSCLBP) is associated with alterations in lumbar muscle morphology, fat infiltration, and reduced contractile tissue, which may affect function and clinical evaluation in younger populations when early structural changes may be underrecognized. Objective: To compare gross muscle (GM) and contractile muscle (CM) measurement-related differences in muscle composition at the L4 level in the upper and lower regions in young adults with NSCLBP. Methods: A cross-sectional observational study was conducted involving seven young adults with NSCLBP. Lumbar multifidus (LM) morphology was assessed using Dixon magnetic resonance imaging (MRI) at the upper and lower regions of the L4 vertebral level. Measurements included total cross-sectional area (CSA), fat CSA, muscle CSA, and muscle fat infiltration percentage (MFI%). Paired-samples t-tests were used to compare GM and CM measurements. Results: Significant differences were observed between GM and CM measurements across all variables (p<0.003). GM values were consistently higher than CM for total CSA, fat CSA, muscle CSA, and MFI%. At the upper L4 level, total CSA differences ranged from 6.05 to 6.28, whereas larger differences were observed at the lower L4 level (9.06–10.66). MFI% and fat CSA were also significantly greater in GM measurements, indicating substantial contributions from non-contractile tissue. Differences were consistently more pronounced at the lower L4 region. Conclusions: GM measurements significantly overestimate LM morphology due to the inclusion of intramuscular fat. Distinguishing contractile from non-contractile tissue provides a more accurate assessment of muscle quality and may improve NSCLBP evaluation and management.