Quadratus lumborum muscle stiffness in chronic non-specific low back pain: a diagnostic accuracy study

Background Evaluation of muscle tenderness is encouraged in the diagnosis of low back pain (LBP). However, manual palpation is poorly reliable and objective methods to quantify muscle stiffness are needed. Objectives To investigate quadratus lumborum muscle stiffness differences between individuals with chronic non-specific low back pain (LBP) and pain-free controls, analyze side-to-side stiffness differences, and assess the diagnostic accuracy of shear wave elastography. Methods A cross-sectional study was conducted recruiting 40 patients with chronic non-specific LBP and 40 asymptomatic controls. Variables assessed included muscle stiffness using shear wave elastography metrics (shear wave speed (SWS) and Young's modulus (YM)), pain chronicity, pain intensity, pain-related disability (using the Oswestry Disability Index) and symptoms associated with central sensitization (using the Central Sensitization Inventory). Diagnostic accuracy was evaluated through sensitivity, specificity, and receiver operating characteristic analysis. Results No significant side-to-side stiffness differences were observed within either group (both, p > 0.05). Quadratus lumborum muscle stiffness was significantly lower in LBP patients compared to controls (SWS p = 0.010; YM p = 0.008). The receiver operating characteristic analysis for both metrics showed poor discriminatory ability (< 0.7). A modest balance between sensitivity (SWS 70%; YM 75%) and specificity (SWS 97.5%; YM 50%) was found. Conclusions Although greater stiffness was expected due to the associated prevalence of trigger points in patients with LBP, quadratus lumborum muscle stiffness was significantly lower in patients suffering chronic non-specific LBP compared to controls. Despite significant differences, the diagnostic accuracy of shear wave elastography was poor, requiring further research to improve the diagnostic utility of shear wave elastography.