Polymyositis and dermatomyositis patients have lower aerobic capacity and endurance exercise performance than healthy controls. A. VO2 max vs. the corresponding power output obtained in the progressive bicycle test. B.Concentration of lactate in vastus lateralis muscle dialysate after cycling vs. cycling time in the endurance test performed at the same relative work load in all individuals. Individual data from 23 patients (open circles) and 12 healthy controls (open triangles). Mean data (± SD) from patients (filled circle) and healthy controls (filled triangle) are also shown.