Increased proportion of megafibers in chronically painful muscles
From: Pain. 2008 Oct 31;139(3):588-93
Trapezius myalgia – chronic pain from the upper trapezius muscle – is frequent in female employees in monotonous stressful jobs, potentially due to chronic overload of type I muscle fibers. In this study, the authors investigated the intra-individual distribution of trapezius muscle fiber size, and hypothesized that females with myalgia compared with matched healthy controls have a higher percentage of grossly hypertrophied type I fibers with poor capillarization.
Forty-two female office workers with trapezius myalgia and 20 healthy matched controls participated in the study. Standard histochemical methods were used to determine fiber size, fiber type, and capillarization. Type I megafiber were defined as at least twice the size of the median type I fiber size of each individual. The main finding was that trapezius myalgia had a significantly higher proportion of type I megafibers than healthy matched controls, in spite of no significant difference in overall type I fiber size. In trapezius myalgia and healthy matched controls type I megafibers were located in 46% and 11% of the females, respectively. Capillarization of the overall type I fiber pool was not different between healthy matched controls and trapezius myalgia, but was significantly lower in type I megafibers of both groups. The percentage of megafibers was positively related to age and weekly working hours, indicating an effect of long-term exposure. In conclusion, this study shows that trapezius myalgia is associated with a significantly higher percentage of grossly hypertrophied type I muscle fibers with poor capillarization – type I megafibers.
