MRI-Based Assessment of Trunk and Hip Muscle Morphology and Strength in Chronic Low Back Pain

BACKGROUND Low back pain (LBP) is a leading cause of disability worldwide, yet the relationship between muscle morphology, strength imbalances, and chronic LBP remains incompletely understood. This study investigated cross-sectional area (CSA) and strength differences in trunk and hip muscles between chronic LBP patients and healthy controls. MATERIAL AND METHODS Fifty patients with chronic LBP (age 53±13.5) and 30 (age 42.3±8.3) healthy controls underwent lumbosacral magnetic resonance imaging to measure CSA of paravertebral (psoas major, quadratus lumborum, erector spinae, multifidus), abdominal (rectus abdominis), and hip muscles (iliacus, gluteus maximus). Isokinetic dynamometry assessed trunk and hip flexor/extensor strength. Statistical analyses included t tests confirmed with Cohen's d and Pearson correlations. RESULTS Patients with LBP showed smaller psoas major CSA at L2/L5 and rectus abdominis CSA at S2/S3 than controls (all P<0.05), with no differences in quadratus lumborum, erector spinae, multifidus, iliacus, or gluteus maximus. Trunk flexor and extensor strength was lower in LBP patients, with a reduced trunk flexor/trunk extensor ratio (0.77±0.20 vs 0.96±0.16, P<0.001); hip flexor/extensor ratios showed a trend toward imbalance (left hip flexor/hip extensor: 0.60±0.15 vs 0.67±0.12, P=0.047). CSA-strength correlations were stronger in patients with LBP, particularly for the psoas major (r=0.42-0.58, P<0.05). CONCLUSIONS Chronic LBP is associated with selective atrophy of the psoas major and rectus abdominis, alongside significant strength deficits in trunk and hip flexors. CSA-strength correlations in LBP patients suggest morphological changes exacerbate functional imbalances, contributing to LBP pathophysiology. These findings highlight the importance of targeted rehabilitation addressing trunk and hip musculature to restore strength symmetry and mitigate disability.