Twenty women with primary fibromyalgia syndrome and 20 age matched healthy women were investigated. The subjects performed maximum voluntary isokinetic contractions of the right quadriceps in an isokinetic dynamometer. Maximum voluntary isometric contractions of the right quadriceps were performed with superimposed transcutaneous electrical stimulation. The examination protocol was repeated after 1 h of resting. Isokinetic and isometric muscle strength was found to be, respectively, 45% (p = 0.0001) and 44% (p = 0.0001) lower in the patient group compared to the healthy subjects. The frequency of superimposed twitches was 65% in the patient group and 15% in the control group (p = 0.003). Patients with primary fibromyalgia have a lower maximum voluntary muscle strength than expected. The increased presence of superimposed electrically elicited twitches during maximum voluntary contraction indicates submaximal force application in primary fibromyalgia syndrome.