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Editor’s note: Although patients with idiopathic inflammatory myopathy often display muscle pain, weakness, and wasting, there are few data to show whether the distribution and severity of muscle weakness varies across different disease subtypes. Manual muscle testing (MMT) is the most widely used method of clinical assessment for these patients, and is also a commonly used primary outcome measure in therapeutic trials. In this cross-sectional study, 172 patients with polymyositis, dermatomyositis, and juvenile dermatomyositis (JDM) were examined at the National Institutes of Health Clinical Center, Bethesda, MD, USA, from 1994 to 2005. MMT was performed by physiotherapists using a standardized methodology, and included 13 muscle groups. The potential maximum score was 240. The total MMT score was significantly lower in polymyositis patients compared with JDM patients. Polymyositis patients were also significantly weaker than both dermatomyositis and JDM patients in lower extremity and proximal MMT scores. In all three clinical groups, the five weakest muscle groups were hip flexors, extensors, and abductors; neck flexors; and shoulder abductors. Some degree of weakness in the three hip muscle groups was observed in
>86% of patients with polymyositis and dermatomyositis.