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Introduction: We hypothesized that microvascular disturbances in muscle tissue play a role in the reduced exercise capacity in juvenile dermatomyositis (JDM). Methods: Children with JDM, children with juvenile idiopathic arthritis (clinical controls), and healthy children performed a maximal incremental cycloergometric test from which normalized concentration changes in oxygenated hemoglobin (Δ[O2Hb]) and total hemoglobin (Δ[tHb]) as well as the half-recovery times of both signals were determined from the vastus medialis and vastus lateralis muscles using near-infrared spectroscopy. Results: Children with JDM had lower Δ[tHb] values in the vastus medialis at work rates of 25%, 50%, 75%, and 100% of maximal compared with healthy children; the increase in Δ[tHb] with increasing intensity seen in healthy children was absent in children with JDM. Other outcome measures did not differ by group. Conclusions: The results suggest that children with JDM may experience difficulties in increasing muscle blood volume with more strenuous exercise. Muscle Nerve, 2013 [ABSTRACT FROM AUTHOR]