Electromyographic signs of shoulder muscular fatigue in two elevated arm positions.

Electromyographic signs of shoulder muscular fatigue were studied in abduction and forward flexion at right angle in the shoulder joint for seven subjects. The myoelectric signal was recorded by surface electrodes for the upper part of the trapezius muscle, the infraspinatus muscle, the middle and anterior part of the deltoid muscle and the biceps brachialis muscle. Bipolar wire electrodes were used for the supraspinatus muscle. By a moving window t-test the duration to the first significant increase of amplitude (RMS) or decrease of mean power frequency (MPF) were determined. The torque produced by the mass of the arm in the two studied elevated arm positions was found to exceed 10 per cent of the maximal voluntary contraction (MVC) for all subjects. For most subjects the supraspinatus and the upper part of the trapezius muscle, revealed significant signs of fatigue after five minutes in the two positions and short time constants of myoelectric amplitude changes. Furthermore electromyographic signs of fatigue developed within a minute for all subjects in the forward flexion (supraspinatus muscle) and in the abduction position (upper part of the trapezius muscle).