Sex and Lumbar Lordosis as Determinants of Segmental Lumbar Muscle Fat Infiltration: A Cross‑Sectional MRI Study

Purpose To investigate segmental lumbar paraspinal muscle fat infiltration using the Quantallier classification and explore the influence of sex, age, BMI, and sagittal spinal alignment Methods A total of 120 patients followed between March 2024 and December 2025 were included in the study. The patients underwent lumbar MRI with fat infiltration graded 0–4 (Quantallier) for multifidus, erector spinae, and psoas muscles at L1–S1 levels. Lumbar lordosis angle and sacral slope were measured. Mann–Whitney U tests compared sexes, and ordinal logistic regression identified predictors of L5–S1 fat infiltration. Results Males had higher BMI and lordosis (p < 0.01). L5–S1 erector spinae infiltration was higher in males (p < 0.001). Regression indicated male sex (OR = 13.0) and BMI (OR = 1.16) increased erector spinae infiltration odds; greater lordosis was protective (OR = 0.89). For multifidus, male sex (OR = 5.08) and lordosis (OR = 0.90) were significant. Nagelkerke R² ≈ 0.24. Conclusions Male sex strongly predicts erector spinae fat infiltration. Greater lumbar lordosis protects multifidus from fat accumulation. BMI contributes to lower lumbar muscle degeneration. Sex and lumbar lordosis are significant determinants of lower lumbar muscle fat infiltration, suggesting demographic and biomechanical influences on muscle degeneration.