title: Increased proportion of megafibers in chronically painful muscles.
abstract: 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, we 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 (MYA) and 20 healthy matched controls (CON) 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 MYA had a significantly higher proportion of type I megafibers than CON, in spite of no significant difference in overall type I fiber size. In MYA and CON 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 CON and MYA, 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.
authors.affiliation: National Research Centre for the Working Environment, Work Physiology, Lersø Parkalle 105, DK 2100 Copenhagen Ø, Denmark Institute of Sports Medicine Copenhagen, Bispebjerg Hospital, Building 8, 1st Floor, Bispebjerg Bakke 23, DK-2400 Copenhagen NV, Denmark Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark.
authors.forename: Lars L
authors.initials: LL
authors.lastname: Andersen
authors.forename: Charlotte
authors.initials: C
authors.lastname: Suetta
authors.forename: Jesper L
authors.initials: JL
authors.lastname: Andersen
authors.forename: Michael
authors.initials: M
authors.lastname: Kjær
authors.forename: Gisela
authors.initials: G
authors.lastname: Sjøgaard
mesh_terms: D000328:Adult; D002196:Capillaries / Q000648:ultrastructure; D016022:Case-Control Studies; D002908:Chronic Disease; D003201:Computers; D005260:Female; D006801:Humans; D006984:Hypertrophy; D008875:Middle Aged; D018485:Muscle Fibers, Skeletal / Q000648:ultrastructure*; D009135:Muscular Diseases / Q000473:pathology*; D019547:Neck Pain / Q000473:pathology; D009784:Occupational Diseases / Q000473:pathology*; D010146:Pain / Q000473:pathology*; D012782:Shoulder; D011795:Surveys and Questionnaires