Background Inflammatory myopathies (IMs) are systemic autoimmune diseases, characterized by reduced muscle strength and endurance. Although IMs are relatively rare, patients' disabilities in daily life are substantial. Thus, there is a need for appropriate and reliable assessments to quantify muscle weakness. Although the manual muscle test (MMT) and the quantitative muscle test (QMT), which employs a dynamometer, are commonly used tests to evaluate muscle strength, their psychometric properties (e.g. reliability) have not yet been sufficiently studied1.
Objectives To determine the reliability of the MMT8 and the QMT in patients with IM.
Methods Twenty-four adult, German speaking patients with established diagnosis of IM were included. The majority of patients was female (83%), 57±12 years old with a mean disease duration of 28±29 months. The maximum isometric strength of eight muscle groups, which are most often affected, was evaluated with the MMT8 using the 10 point Kendall scale and the QMT indicating strength in Newton (N). For intra-rater reliability, one tester performed the MMT8 and the QMT twice and for inter-rater reliability, they were tested by a second independent tester. The Intraclass Correlation Coefficient (ICC) and the Standard Error of Measurement (SEM) were evaluated.
Results The intra-rater reliability of the MMT8 yielded moderate to good ICCs (0.63–0.89) for all muscle groups and an excellent ICC (0.92) for the total score. Inter-rater reliability yielded moderate to good ICCs (0.54–0.84) for shoulder abductors, wrist and hip extensors, hip abductors, neck flexors and the total score, and low ICCs (0.08–0.34) for the remaining muscle groups (elbow flexors, knee and foot extensors). The SEM was smaller for the intra-rater (SEM 0.5/10 point of Kendall scale for muscle groups and 2/10 points for the total score) than for inter-rater reliability (SEM 0.5–1/10 point for muscle groups and 3/10 points for the total score). QMT presented moderate to excellent intra- and inter-rater reliability (ICCs 0.52–0.92) for all muscle groups. The intra-rater comparison for shoulder abductors was 10N, inter-rater comparison 13N and the intra-rater and inter-rater comparisons for the knee extensors were 28N and 35N, respectively. In contrast to the QMT, the MMT8 showed high ceiling effects between 18% and 77% for all but one muscle group (hip extensors).
Conclusions In this study, the QMT showed a better reliability than the MMT8 because of missing ceiling effects and due to the good to excellent intra- and inter-rater reliability. Notably, the QMT seems to be more accurate to measure single muscle groups especially in patients with mild weakness as for example in early disease. If no hand-held dynamometer is available, the total score of MMT8 represents an appropriate alternative.
Rider LG, Werth VP, Huber AM, Alexanderson H, Rao AP, Ruperto N, Herbelin L, Barohn R, Isenberg D, Miller FW: Measures of adult and juvenile dermatomyositis, polymyositis, and inclusion body myositisArthritis Care Res (Hoboken) 2011
Disclosure of Interest None declared