Electrical stimulation pain thresholds and EMG activity were studied, using the vastus medialis muscle of healthy control subjects and of patients with osteoarthritis of the knee. Various categories of sensation elicited by progressive increases of the level of electrical stimulation (including one defined as pain threshold) were defined for control subjects. For patients, muscular pain thresholds differed significantly for the two sides of the body; they were usually lower in the more affected side. Involuntary activity of certain motor units and delayed relaxation following voluntary contraction were consistently observed in patients. The involuntary activity was affected by limb position and by mechanical stimulation of tender areas of the muscle or joint. Differences in pain threshold between the two sides were significantly reduced and the EMG pattern became normal following treatment with injections of local anaesthetic into tender periarticular areas and systemic administration of lysine acetylsalicylate. In standing patients, abnormal EMG activity (which was characteristically sensitive to body load and its variations) was found. Injection of a local anaesthetic into the joint cavity was able to induce a rapid subjective improvement and a consistent reduction of EMG activity.