1. Thirty-five female patients, mean age 63 years, suffering from rheumatoid arthritis participated the study. Twenty patients had been on long-term low-dose corticosteroid treatment. Fifteen patients had never received corticosteroids. A control group of 15 age- and sex-matched healthy subjects was also studied.

2. Examination of muscle biopsies from the (right) vastus lateralis and measurements of isokinetic and isometric knee-extension muscle strength were performed in all subjects.

3. Rheumatoid arthritis patients treated with corticosteroid showed a low percentage of type I fibres, mean 35.7 (range 17–66) % compared with patients who did not receive corticosteroid (P < 0.005). The latter group did not differ from the controls.

4. The muscle fibre areas also were affected in the corticosteroid treated rheumatoid patients. Type I and type II mean fibre areas were reduced by 32% and 50%, respectively, when compared with non-prednisone treated patients. The latter group did not differ from the controls in this respect.

5. A correlation was found between the isokinetic muscle strength of the knee extensors and the mean areas of type I and type II in patients treated with prednisone (r = 0.48, P < 0.05 and r = 0.58, P < 0.02 respectively). No such correlation was found when using isometric measurements of the knee extensors. A positive correlation was found in both groups of rheumatoid arthritis patients between the areas of the type I and type II fibres (r = 0.66–0.68, P < 0.05–0.02).

6. It is concluded that the observed abnormal muscle morphology and low knee-extension capacity found in the corticosteroid treated patients is due to the treatment rather than to other factors, as there were no other differences between the two groups of patients.

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