Objective: To develop a muscle strength index (MSI) and determine whether it provides complementary information to traditional disease activity variables in patients with rheumatoid arthritis (RA).
Methods: The MSI was developed on the basis of practical and empirical aspects and statistical considerations. Intra and interobserver reliability was assessed on the data from 3 observers on 2 strength measurements in each of 10 patients. The association of the MSI with variables of disease activity and severity was assessed in univariate analysis. The contribution of the MSI in the explanation of physician's global disease activity after accounting for the effect of traditional measures of disease activity was assessed in multiple linear regression models.
Results: Eight strength measurements (extension and flexion of knee and elbow joints) obtained with a hand held pull gauge were aggregated into the MSI as the mean of the standardized scores. In 65 patients with RA, the MSI had a high internal consistency (Cronbach's alpha 0.95) and intra and interobserver reliability (Pearson correlation coefficient 0.94 each). The MSI correlated moderately with traditional measures of disease activity and strongly with physical functional disability and radiological damage. In contrast to grip strength, the MSI explained additional variation of physician's global assessment of disease activity if added to variables of pooled activity indices.
Conclusion: The MSI is a reliable and valid measure of disease activity and severity and may improve the content validity of pooled disease activity indices.