Muscle changes in work-related chronic myalgia.

Muscle biopsies from the descending portion of the trapezius muscle were studied in 9 healthy subjects and in 10 patients with localized chronic myalgia related to static load during repetitive assembly work for 16 (10-31) years, with 10 (4-26) months of sick leave at the time of biopsy. Both categories showed isolated atrophic muscle fibers and occasional abnormal fibers with internally situated nuclei, some variation in fiber diameter, and fiber splitting. Fibers with a "moth-eaten" appearance due to a multifocal loss of oxidative enzyme activity were frequent both in the healthy and in the myalgic individuals. In contrast, isolated pathologic "ragged red" fibers were only found in the cases with myalgia (8 of 10), strongly suggesting mitochondrial damage. The phenomenon was confined to the Type 1 fibers. The frequency of Type 1 fibers was increased. Levels of adenosine triphosphate and adenosine diphosphate were reduced in myalgia patients, whereas lactate, pyruvate, and glycogen levels were normal, as well as phosphoryl creatine and total creatine.