DOES A MUSCLE STRENGTH INDEX PROVIDE COMPLEMENTARY INFORMATION TO TRADITIONAL DISEASE-ACTIVITY VARIABLES IN PATIENTS WITH RHEUMATOID-ARTHRITIS

Objective. To develop a muscle strength index (MSI) and determine whet her it provides complementary information to traditional disease activ ity 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 a ssessed on the data from 3 observers on 2 strength measurements in eac h of 10 patients. The association of the MSI with variables of disease activity and severity was assessed in univariate analysis. The contri bution of the MSI in the explanation of physician's global disease act ivity after accounting far the effect of traditional measures of disea se 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, t he MST had a high internal consistency (Cronbach's alpha 0.95) and int ra and interobserver reliability (Pearson correlation coefficient 0.94 each). The MSI correlated moderately with traditional measures of dis ease activity and strongly with physical functional disability and rad iological damage. In contrast to grip strength, the MSI explained addi tional variation of phy sician'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.