Effect of physical training on the proportion of slow-twitch type I muscle fibers, a novel nonimmune-mediated mechanism for muscle impairment in polymyositis or dermatomyositis.
OBJECTIVE
To compare muscle fiber type composition and muscle fiber area in patients with chronic polymyositis or dermatomyositis and healthy controls, and to determine whether physical training for 12 weeks could alter these muscle characteristics.


METHODS
Muscle fiber type composition and muscle fiber area were investigated by biochemical and immunohistochemistry techniques in repeated muscle biopsy samples obtained from 9 patients with chronic myositis before and after a 12-week exercise program and in healthy controls. Muscle performance was evaluated by the Functional Index (FI) in myositis and by the Short Form 36 (SF-36) quality of life instrument.


RESULTS
Before exercise, the proportion of type I fibers was lower (mean +/- SD 32% +/- 10%) and the proportion of type IIC fibers was higher (3% +/- 3%) in patients compared with healthy controls. After exercise, percentage of type I fiber increased to 42% +/- 13% (P < 0.05), and type IIC decreased to 1% +/- 1%. An exercise-induced 20% increase of the mean fiber area was also observed. The functional capacity measured by the FI in myositis and the physical functioning subscale of the SF-36 increased significantly. Improved physical functioning was positively correlated with the proportion of type I fibers (r = 0.88, P < 0.01) and type II muscle fiber area (r = 0.70, P < 0.05).


CONCLUSIONS
Low muscle endurance in chronic polymyositis or dermatomyositis may be related to a low proportion of oxidative, slow-twitch type I fibers. Change in fiber type composition and increased muscle fiber area may contribute to improved muscle endurance and decreased muscle fatigue after a moderate physical training program.
Dastmalchi
Maryam
M
Karolinska University Hospital, Solna, Karolinska Instituet, Stockholm, Sweden. maryam.dastmalchi@karolinska.se
Alexanderson
Helene
H
Loell
Ingela
I
Ståhlberg
Marcus
M
Borg
Kristian
K
Lundberg
Ingrid E
IE
Esbjörnsson
Mona
M
D000328:Adult; D002908:Chronic Disease; D003882:Dermatomyositis / Q000378:metabolism* / Q000534:rehabilitation; D015444:Exercise / Q000502:physiology*; D005081:Exercise Therapy; D005260:Female; D006305:Health Status Indicators; D006801:Humans; D007150:Immunohistochemistry; D008297:Male; D008875:Middle Aged; D009119:Muscle Contraction / Q000502:physiology; D018763:Muscle Fatigue / Q000502:physiology; D018657:Muscle Fibers, Slow-Twitch / Q000378:metabolism*; D017285:Polymyositis / Q000378:metabolism* / Q000534:rehabilitation