Fat matters: paraspinal muscle quality is associated with electromyographic fatigability and endurance in lumbar spinal stenosis.

Background: Paraspinal muscle degeneration with fatty infiltration and atrophy is well-documented in symptomatic lumbar spinal stenosis (sLSS). However, it remains unclear whether these structural alterations are associated with paraspinal muscle function. This study investigated associations between paraspinal muscle morphology, surface electromyography (sEMG)-derived fatigability and endurance capacity in sLSS. Methods: A total of 105 preoperative patients with sLSS underwent quantitative lumbar MRI (VIBE Dixon sequence) and a modified Biering-Sorensen test with concurrent sEMG. Fat fraction, total and lean cross-sectional area (CSA) were derived from machine-learning-assisted segmentation at L1, L3, and L5. Normalized median frequency (MDF) slope, MDF decline, and initial MDF were derived from sEMG. Spearman correlations and multilevel linear models adjusted for age, sex, BMI, and endurance, examined associations between morphology and fatigability. Results: Mean endurance was 101.2 ± 72.5 seconds. Fat fraction was consistently high in the multifidus (46%-49%) and increased caudally in the erector spinae (31% at L1 to 61% at L5), correlating negatively with endurance bilaterally (-0.34 ≤ρ≤ -0.50). In multilevel models, fat fraction was independently associated with more negative MDF slopes (-0.005, 95% CI [-0.011; -0.001]), greater MDF decline (0.619 [0.15; 1.12]), and greater initial MDF (9.883 [9.01; 10.65]). Total CSA was associated only with initial MDF. Conclusions: Paraspinal fat fraction was associated with the rate and magnitude of MDF decline, independent of muscle size, endurance, age, and sex. These findings suggest that morphological changes in paraspinal muscles have functional correlates that may contribute to activity limitations in sLSS. Muscle quality appears more relevant than muscle quantity for fatigability in this population.