The aim was to study the methodological aspects of the muscle twitch interpolation technique in estimating the maximal force of contraction in the quadriceps muscle utilizing commercial muscle testing equipment. Six healthy subjects participated in seven sets of experiments testing the effects on twitch size of potentiation, time lag after potentiation, magnitude of voluntary force, stimulus amplitude, stimulus duration, angle of the knee, and angle of the hip. In addition, the consequences of submaximal potentiation on the estimation of maximal force from twitch sizes were studied in five healthy subjects. We found an increase in twitch size with increasing levels of potentiation and twitch size decreased exponentially following potentiation. We found a curvilinear relationship between twitch size and voluntary force, and these properties were more obvious when the stimulation intensity of the preload was reduced. The relationship between twitch size and force was only linear, for force levels greater than 25% of maximum. It was concluded that to achieve an accurate estimate of true maximal force of muscle contraction, it would be necessary for the subject to be able to perform at least 75% of the true maximal force.

The objective of the study was to quantify the voluntary dynamic muscular endurance (DME) in patients with primary fibromyalgia (PF) since easy fatigability of muscles is one of the major symptoms in this syndrome. Consecutive outpatients referred to a rheumatology clinic specializing in PF were investigated. Thirty-six patients with PF were compared with 18 patients with chronic myofascial pain (CMP) syndrome. Subjects were matched according to gender, age, height, weight, peak torque, and contractional work. The DME was defined as the number of repeated knee extensions necessary for the contractional work in two successive knee extensions to be equal to or below 70% of the initial value measured with an isokinetic dynamometer. A significantly lower DME was found in the PF group than in the CMP group: 11 (7-13) and 18 (11-25), respectively (median and quartiles), p less than .005. Patients with PF have a low voluntary muscular endurance compared to CMP patients.