The hidden architecture of back pain: ultrasound-based lumbar multifidus pennation angle analysis - a cross-sectional pilot study.

BACKGROUND: Chronic low back pain (CLBP) is associated with structural and functional alterations in the lumbar-multifidus (LM) muscle, a key-stabilizer of the spine. In this pilot study, Pennation-angle, a well-established muscle architecture marker not previously assessed in CLBP, was evaluated using ultrasound to compare LM structure between CLBP-patients and healthy-controls and examine its relationship with muscle thickness, pain-intensity and activity-levels. METHODS: This cross-sectional study included 40 adults aged 18–35 years (20 CLBP-patients, 20 healthy-controls) at AIIMS Jodhpur. Bilateral ultrasound-imaging at the L4–L5 level was performed, and the average of right and left measurements was used to quantify superficial and deep LM pennation-angle and muscle-thickness. Pain intensity (NRS-scale) and physical activity levels were recorded. Group comparisons were performed using t-tests or Mann–Whitney U tests; correlations by Pearson’s or Spearman’s coefficients. RESULTS: CLBP participants showed significantly reduced superficial-pennation angle (7.03 ± 0.98° vs. 8.85 ± 1.25°, p < 0.001; Cohen’s d = − 1.62), superficial muscle-thickness (1.07 ± 0.187 cm vs. 1.29 ± 0.258 cm, p = 0.004; d = − 0.96), and deep muscle-thickness (1.18 ± 0.190 cm vs. 1.32 ± 0.190 cm, p = 0.026; d = − 0.73), while deep-pennation angle showed no group difference (p = 0.235; d = − 0.38). Superficial-pennation angle correlated positively with superficial muscle-thickness (r = 0.678, p < 0.001). NRS showed weak negative trends with all LM parameters. Physical activity distributions differed but were statistically non-significant (χ²=2.88, p = 0.237), although effect size indicated a small-to-moderate trend toward higher sedentary behaviour in CLBP (Cramer’s V = 0.268). CONCLUSION: The observed reduction in LM pennation-angle and muscle-thickness reflects potential architectural compromise and disuse-related atrophy in CLBP. The strong angle–thickness correlation supports the interdependence of muscle size and fiber orientation. Although pain-intensity and activity-levels were not statistically associated to LM morphology, their negative and sedentary trends may still reflect behavioural and pain-related influences on subtle muscle decline. CLBP patients demonstrate distinct LM architectural alterations, emphasizing the value of ultrasound-based assessment and supporting targeted rehabilitation strategies focused on restoring LM function and morphology. INSTITUTIONAL ETHICS COMMITTEE REGISTRATION NUMBER: AIIMS/IEC/2025/5525.