Electrical activity and arm muscle force in postoperative fatigue.

Muscle force of elbow flexors and quantitative electromyographic analyses were assessed before and 10 and 20 days after uncomplicated elective abdominal surgery in 20 patients. The findings were compared with subjective fatigue. Fatigue increased significantly from a mean preoperative level of 3.4 +/- 0.4 arbitrary units to 7.0 +/- 0.5 and 6.1 +/- 0.5 on postoperative days 10 and 20. Muscle force decreased by about 10% postoperatively. The duration of sustained contraction (endurance) at a force adjusted to 30% of maximum fell from 426 +/- 64 sec preoperatively to 301 +/- 22 sec 20 days after surgery (all differences significant). Postoperative increase in fatigue correlated significantly to decrease in maximum force (r = 0.45) and to decrease in endurance (r = 0.5). The electromyographic findings at a force of 2 kg and at 30% of maximum suggested activation of fewer motor units during short contraction in the early postoperative period. During sustained contraction there was more pronounced increase in mean amplitude 20 days postoperatively than before surgery. This may suggest that the decreased muscle performance in the late postoperative period was secondary to muscle fatigue.