Structural changes in male trapezius muscle with work-related myalgia.

Muscular changes in male forest machine operators with work-related neck and shoulder myalgia were studied. Enzyme cyto- and immunohistochemical analysis was carried on muscle biopsies obtained from ten myalgic subjects (M), nine non-myalgic selected in the same work place (NM) and six healthy young men (C). The M group displayed a significant increase in type IIA fibres in comparison to the C group. This hypertrophy was accompanied by a parallel increase in the capillary bed. Both the M and NM groups exhibited an increase in fibres with a disorganised mitochondrial pattern. Interestingly, fibres lacking cytochrome c oxidase occurred in the M group (0.9%) but also in the NM group (0.5%), suggesting a mitochondrial defect. Central nuclei (5.2%) and developmental myosin (3%) were also more frequent in the M group. These changes are probably related to injury-regeneration cycles. These data support the association between the work conditions and muscle changes in work-related trapezius myalgia.