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Arthritis Rheumatol. 2019 Nov 20. doi: 10.1002/art.41174. [Epub ahead of print]

Functional and Structural Adaptations of Skeletal Muscle in Long-Term Juvenile Dermatomyositis; a Controlled Cross-Sectional Study.

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Department of Rheumatology, Oslo University Hospital, Rikshospitalet, Oslo, Norway.
Department of Physical Performance, Norwegian School of Sport Sciences, Oslo, Norway.
Institute for Experimental Medical Research and K.G. Jebsen Center for Cardiac Research, Oslo University Hospital and University of Oslo, Oslo, Norway.
Department of Radiology, Oslo University Hospital, Rikshospitalet, Oslo, Norway.
Institute of Clinical Medicine, Faculty of Medicine, University of Oslo, Oslo, Norway.
Department of Cardiology, Oslo University Hospital, Ullevål, Oslo, Norway.
Norwegian National Advisory Unit on Rheumatic Diseases in Children and Adolescents, Oslo University Hospital, Rikshospitalet.
Bjørknes University College, Oslo, Norway.



To compare muscle strength/endurance of knee extensors between patients with long-term juvenile dermatomyositis (JDM) and controls, and between patients with active and inactive disease. In patients; to explore associations between strength/endurance and i)clinical parameters ii)physical activity, and iii)humoral/structural adaptation in skeletal muscle.


In a cross-sectional study (n=44 patients and 44 age/sex-matched controls), we tested isometric muscle strength (peak torque) and dynamic muscular endurance (total work) of knee extensors, physical activity (accelerometers), and myokines (ELISA). Patients were examined by validated tools (clinical muscle tests, disease activity/damage, and inactive disease) and magnetic resonance imaging of thigh muscles including cross-sectional area (CSA). Needle biopsies of m. vastus lateralis (n=12, age ≥18y) were assessed by histochemistry.


After mean 21.8±11.8y disease duration, peak torque was 29(95%CI 13-46) Nm lower in patients vs. controls (p=0.001), and total work was 738(IQR 565-1155)J in patients and 1249(IQR 815-1665)J in controls (p<0.001). Both were lower in active vs. inactive disease (p<0.019); only work remained lower controlling for quadriceps CSA. Torque and work correlated with clinical muscle tests in patients with active disease (r=0.57-0.84). Muscle biopsy results indicated different fiber type composition but similar capillary density between active and inactive disease.


In long-term JDM, muscle strength/endurance of knee extensors were lower compared to controls, and in active vs inactive disease. Our results indicate a need for more sensitive muscle tests in this clinical setting. We hypothesize that impaired muscular endurance in active disease may be influenced by structural/functional adaptations of muscle tissue independent of muscle size.


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