OBJECTIVE To measure maximal voluntary strength and central activation without fatigue, and to assess both peripheral and central components of muscle fatigue of the elbow flexor muscles, during exercise, in a group of patients with fibromyalgia (FM) (n = 11). Results are compared with data from control subjects (n = 36). METHODS Maximal voluntary activation and strength of elbow flexors were quantified using twitch interpolation during attempted maximal isometric contractions both in unfatigued muscles and during fatigue produced by 45 min of submaximal exercise. RESULTS Maximal voluntary strength of the elbow flexors before and during exercise was within the normal range. Central fatigue did not develop to a greater extent in the patient group. No patient had a decline in twitch amplitude during exercise below the 95% confidence limit for the decline in control subjects. However, the increment in perceived effort (Borg Scale) was abnormally large in 5 patients during the fatiguing exercise. CONCLUSIONS Neither poor motivation, reflex pain inhibition, nor muscle contractile failure are important in the pathogenesis of fatigue in patients with FM. However, the subjective response to exercise is commonly excessive.