Isometric muscle strength is an indicator of self-reported physical functional disability in patients with rheumatoid arthritis.

The objective of our study was to examine (1) whether isometric muscle strength contributes to the explanation of the physical functional disability of a rheumatoid arthritis (RA) patient population after accounting for other disease parameters and demographic variables and (2) whether change in isometric muscle strength is an indicator of change in physical functional disability. Sixty-five consecutive patients fulfilling the American Rheumatism Association 1987 revised criteria for RA were included in the study. Isometric muscle strength was measured with a validated Muscle Strength Index (MSI) calculated as the mean score of standardized isometric extension and flexion strength of the knee and elbow joints. Physical functional disability was measured with the physical dimension of the Health Assessment Questionnaire (HAQ). For 56 patients, we could obtain 1 yr follow-up data. Cross-sectionally, there was a significant correlation (r = -0.51, P < 0.01) between the MSI and the HAQ. Muscle strength remained a significant determinant of the HAQ in multivariate analysis accounting for disease and demographic variables. Longitudinally, change in MSI explained additional variance of change in HAQ after accounting for change in pain, the most important correlate of change in HAQ.