Characteristics of the upper trapezius muscle in patients with chronic shoulder impingement syndrome: a comparative study

The upper trapezius (UT) muscle is critical for proper scapular kinematics, yet the role of intrinsic UT muscle characteristics remains unclear. Our study aimed to compare UT muscle quality (echo intensity, EI), morphology (thickness change), and tone and stiffness between patients with CSIS and controls. This observational study recruited 30 participants with unilateral CSIS (27.5 ± 5.5 years old; pain duration 23.0 ± 19.7 months) and 30 controls (24.4 ± 3.0 years old) from the community. B-mode ultrasound was utilized to measure UT EI and muscle thickness at rest and during maximal contraction. Myotonometry assessed oscillation frequency (tone) and dynamic stiffness at rest and during 60° shoulder flexion. Data were analyzed using analysis of covariance (age). The CSIS group exhibited greater UT muscle EI than controls on both affected and unaffected sides (p = 0.007, 95% Confidence Interval [CI]: 2.135-12.894). The CSIS group exhibited greater muscle tone and stiffness during 60° shoulder flexion, including elevated oscillation frequency (p = 0.005, 95% CI: 0.351-1.873) and dynamic stiffness (p = 0.006, 95% CI: 10.678-61.782) than the controls. Our results suggest that patients with CSIS had bilateral deficits in muscle quality and muscle tone during active tasks. These findings provide objective evidence of altered intrinsic muscle characteristics and mechanical properties in the UT of patients with CSIS.Trial registration: The protocol of this study was approved by the Institutional Review Board of National Yang Ming Chiao Tung University and prospectively registered with ClinicalTrials.gov (NCT06472349).