Muscle fatigue in early fibromyalgia.

OBJECTIVE

To compare the processes connected with muscle fatigue during prolonged exercise in patients with fibromyalgia (FM) with studies of healthy individuals.

METHODS

Nine female patients with FM, all employed in full or part time, performed repetitive isometric contractions of the quadriceps muscles at 30% of maximal voluntary contraction (MVC) force in 6 s with 4 s rest between until exhaustion.

RESULTS

The MVC and electrically stimulated tetanic forces fell continually and almost in parallel, indicating no central fatigue. The relaxation rate of the muscle became gradually faster as previously seen in healthy subjects. Plasma catecholamine concentrations remained unchanged during the exercise period, whereas a modest rise in blood lactate and plasma K+ was seen. The electromyographic (EMG) amplitude recorded during contractions increased gradually, while no changes were seen in EMG recorded in resting periods between contractions. Heart rate and VO2 rose gradually and a small exercise induced pain was recorded.

CONCLUSION

Our study showed normal physiological muscle fatigue responses, except for the lack of increase in plasma catecholamine concentrations during exercise.