M-Mode Ultrasound and Strength Assessment of the Lower Trapezius Muscle in Patients with Chronic Neck Pain: A Cross-Sectional Study

Background: Chronic neck pain (CNP) is considered a widespread musculoskeletal condition which affects the quality of life and is associated with scapular stabilizer dysfunction patterns. This study aimed to compare the morphological, functional, and strength-related characteristics of the lower trapezius muscle (such as muscle thickness at rest and during contraction, contraction-to-relaxation ratios, contraction and relaxation velocities, and force production) between individuals with CNP and asymptomatic controls. Methods: This cross-sectional study comprises 144 participants (78 with CNP and 76 controls) aged 18–30 years. Ultrasound imaging was used to assess muscle thickness, contraction and relaxation velocities, and contraction ratios. Muscle strength, including peak force and rate of force development, was evaluated using dynamometry. Participants with CNP were categorized by neck disability index (NDI) scores into mild, moderate, and severe groups. Results: Participants with CNP showed reduced contraction velocity (0.60 ± 0.24 cm/s vs. 0.81 ± 0.44 cm/s; p < 0.001; d = 0.61), lower relaxation velocity (0.48 ± 0.28 cm/s vs. 0.58 ± 0.32 cm/s; p = 0.038; d = 0.34), and decreased peak force (4.83 ± 3.27 kg vs. 6.00 ± 3.87 kg; p = 0.043; d = 0.33) compared to controls. Muscle thickness and contraction ratio differences were non-significant. Among CNP subgroups, the contraction ratio was lower in the severe disability group compared to the mild group (0.25 ± 0.19 vs. 0.53 ± 0.33; p = 0.015). Conclusions: The present study showed functional impairments of the lower trapezius muscle in individuals with CNP, particularly reduced contraction and relaxation velocities and lower peak force, with more pronounced deficits in those with severe disability. While no significant differences in muscle thickness were found, these findings suggest that functional alterations may precede structural changes.