Morphological Differences in the Upper Trapezius Muscle Between Female Office Workers With and Without Trapezius Myalgia: Facts or Fiction?

OBJECTIVE: Work-related trapezius myalgia is a common musculoskeletal disorder in office workers. Prolonged low-level muscle activity during office work may lead to morphological changes in the muscle tissue, causing pain and fatigue. The aim of the present study was to investigate differences in muscle morphology between office workers with and without trapezius myalgia. DESIGN: Muscle biopsy samples were obtained from the upper trapezius of female office workers with trapezius myalgia (n = 17) and healthy controls (n = 15). Myosin heavy chain immunohistochemistry and Gomori trichrome stainings were performed to identify differences in muscle fiber type proportion, Feret's diameter, and internal nuclear proportion. RESULTS: The myalgia group showed significantly more type IIA and IIA/IIX fibers and less type I and IIX fibers, compared with the control group (P < 0.001 to P = 0.005). No significant differences were found for Feret's diameter and internal nuclear proportion (P > 0.05). However, a significantly higher Feret's diameter was found for type I fibers, compared with type II fibers in both groups (P < 0.001 to P = 0.002). Several subjects of both groups displayed an internal nuclear proportion of more than 3%. CONCLUSIONS: Female office workers with trapezius myalgia show a different fiber type distribution compared with their healthy colleagues but display no differences in fiber size and internal nuclear proportion.