Human muscle fibre abnormalities related to occupational load.

Ten biopsy studies addressing structural and histochemical muscle fibre abnormalities related to occupational work and myalgia have been reviewed. Eight of these were focused on the upper trapezius muscle while the two others were made on the first dorsal interosseous muscle and the extensor carpi radialis brevis muscle. Standard histological methods have mainly been used with some variations. All except two studies were on women. To achieve desirable contrasts in effect, subject groups usually were individuals with myalgia exposed to static and/or highly repetitive work and healthy non-exposed subjects. In three studies work-exposed subjects without myalgia were included, in two others two groups with myalgia were formed based on the degree of disorder. Major findings in the subjects with myalgia or exposed to work load were increased fibre cross-sectional areas, various indications of mitochondrial disturbances in type I fibres and reduced capillarisation per normal fibre cross-sectional area, the last only in women. The relationship between these findings and pain perception are unclear and frequently similar findings are made also in work-exposed pain-free subjects. Some signs of mitochondrial disturbances in the trapezius muscle were also found in reference groups. However, there are some indications of a relationship between degree of abnormality and degree of pain. There are also indications of differences between the type of finding in the trapezius muscle and in the hand/forearm muscles.