Associations Between Hip Mobility and Pain in Chronic Low Back Pain Using IMU and Markerless Motion Capture

Introduction: Chronic non-specific low back pain (CNLBP) is associated with altered lumbopelvic mechanics and impaired hip mobility. This study examined whether changes in pain-provoking hip flexion are associated with changes in low back pain and assessed agreement between inertial measurement units (IMUs) and a markerless motion capture system. Methods: Thirty-six patients with CNLBP completed a longitudinal repeated-measures rehabilitation protocol consisting of approximately 13 physiotherapy sessions over a period of up to 6 weeks. Active hip flexion was assessed in the symptomatic limb (the limb provoking lumbar pain). Hip flexion was recorded during the same movement trial using IMUs and a markerless system. Pain and disability were assessed using the Visual Analogue Scale and Oswestry Disability Index. Results: Improvements in hip flexion were moderately associated with pain reduction (markerless: r = −0.52; IMU: r = −0.57), with negligible associations with disability. Markerless and IMU measurements showed a strong correlation (r = 0.87), while Bland–Altman analysis showed consistent underestimation by the markerless system (bias = −3.67°). Conclusions: Symptom-specific hip mobility is associated with pain reduction in CNLBP, highlighting the role of lumbopelvic biomechanics. IMUs demonstrated higher consistency, while markerless systems offered a more accessible alternative for clinically meaningful movement assessment.