Muscle activity and fatigue in the shoulder muscles of assembly-plant employees.

Subjects who do monotonous and repetitive work in a sitting position often complain of discomfort and soreness in the neck and shoulder regions. Twenty-five subjects had electromyographic examinations of the deltoid, infraspinatus, and trapezius muscles. The recordings were performed eight times throughout a whole workday. In a questionnaire administered before the examinations, 16% of the subjects had reported pain in the neck only, 8% pain in the shoulders only, and 48% pain in both areas. The muscle activity was analyzed by means of the amplitude distribution probability function. The results showed high static contraction levels in the deltoid, infraspinatus, and trapezius muscles and high medium contraction levels of the infraspinatus and trapezius muscles, but acceptable maximum contraction levels for all three muscles. The activity levels did not change during the day and showed no differences related to age or sex. The decrease in mean power frequency during a 2-min contraction sustained at 20% of the maximal voluntary contraction was the same in the morning and in the afternoon. Although the electrical activity in the muscles indicates a high degree of muscle activity, there was no change during the day, ie, no sign of muscle fatigue.