Electromyographic measurement of fatigue in rheumatoid arthritis and neuromuscular disease.

Stiffness, particularly "morning stiffness" is a characteristic symptom of active Rheumatoid Disease. An articular component of stiffness has been measured directly in a number of studies (Scott, 1960; Wright and Johns, 1961). Stiffness in muscle is less readily measured and has been given relatively little attention. Stiffness in both muscle and joint increase the load on actively contracting muscle, and can therefore be expected to hasten the onset of muscular fatigue. In an isometric contraction, however, the effect of joint stiffness would presumably be minimal, and the development of fatigue would reflect mainly the muscular component of stiffness. If the rate of development of fatigue could be measured objectively, this should provide information regarding the state of the muscles in rheumatoid disease, as well as in other disorders associated with stiffness or weakness. This report presents the results of a study of the changes in electrical activity of muscle occurring with fatigue in healthy individuals, patients with rheumatoid disease, and patients with neuromuscular disorders. The changes in the pattern of the myogram occurring with fatigue can be studied in records obtained with surface electrodes. It has been reported that the potentials increase in amplitude and diminish in frequency as fatigue develops (Cobb and Forbes, 1923). Quantitative information about these changes may be derived by graphic or electronic integration of the myogram. It has been shown that the mean voltage of the electromyogram is proportional to the tension of an isometric contraction (Lippold, 1952). The electrical activity