Investigating the association between paraspinal soft tissue stiffness and spinal geometry during controlled forward flexion: A cross-sectional case-controlled investigation.

Abstract

This prospective case-controlled study examined whether apparent mechanical stiffness of the multifidus (MF), erector spinae (ESM) and thoracolumbar fascia (TLF) at rest is associated with spinal curvature and range of motion (RoM) during controlled flexion. Fifteen participants with chronic non-specific LBP and 18 asymptomatic participants underwent non-invasive prone stiffness assessment of the MF, ESM and TLF tissue using a validated IndentoPro device. Surface EMG recorded trunk and hip muscles, bilaterally, during a guided 60° flexion task and normalized to maximal voluntary contractions. A motion bucky apparatus constrained the flexion path. X-ray fluoroscopy captured L2-S1 intervertebral rotations (IVR) and a 3D motion-capture system quantified lumbar lordotic (LLA) and thoracic kyphotic angles (TKA). Group differences were assessed using independent t-tests and Mann-Whitney U test with effect sizes (Cohen's d, rank-biserial r), while Spearman correlations (p) stiffness-kinematics associations. LBP patients exhibited higher TLF stiffness (d = 1.1, p ≤ 0.05), reduced lumbar RoM (d = 0.9, p ≤ 0.05) and greater loss of LLA (d = 0.8, p ≤ 0.05), compared with controls. ESM stiffness correlated positively with TKA (p = 0.4, p ≤ 0.05), whereas TLF stiffness correlated negatively with L4-L5 and L5-S1 IVR (p = -0.4, p ≤ 0.05) and positively with ∆TKA/∆LLA (p = 0.4, p ≤ 0.05). Surface EMG showed low-to-moderate normalized activation with no significant between-group differences in trunk muscle recruitment. Higher apparent paraspinal tissue stiffness at rest was associated with changes in the spinal geometric profile.