Objectives. To compare muscle performance in women with fibromyalgia and in healthy women. Patients and methods. Sixteen women with fibromyalgia syndrome (FMS) and 85 healthy women who were physically inactive or engaged in recreational sports underwent measurements of four parameters: maximal concentric isokinetic muscle strength of the knee extensors and flexors in the dominant limb, isometric grip strength on a Colin dynamometer, muscle fatigue resistance during 30 maximal concentric isokinetic contractions of the dominant knee flexors and extensors at 180° angular velocity, and static endurance during posture maintenance. Results. All muscle variables were decreased in the FMS patients as compared to the controls. The decreases were more marked during aerobic than during anaerobic exercise. Mean decreases were 39% (P<0.001) for muscle strength, 40% (P<0.0001) for fatigue resistance, and 81% (P<0.0001) for static endurance. Pain during exercise as evaluated using a visual analog scale was more marked in the FMS patients. Conclusion. This study of the three pathways supplying energy to muscle confirms that muscle function is globally impaired in FMS patients. The results suggest that the impairment predominates on aerobic processes.