FATIGUE IN THE SHOULDER MUSCLES DURING STATIC WORK AT 2 DIFFERENT TORQUE LEVELS

The present study aimed to investigate whether and when a shift in the mean power frequency (MPF) of the electromyogram (EMG) occurs at low torque levels during a maximum endurance test of three shoulder flexor s. Twelve clinically healthy women performed two endurance tests of th e shoulder flexors (at 50% MVC - the high torque level, and at approxi mately 18% MVC - the low torque level) until maximum exhaustion. Surfa ce EMG were obtained and MPF and signal amplitude (RMS) were computed for the trapezius descendens, the anterior part of the deltoid and the infraspinatus. The subjects also rated the perception of fatigue in t he shoulder muscles throughout the two tests using a 10-graded scale. A significantly higher degree of perceived fatigue was found at the lo w rather than at the high torque level. In contrast significantly lowe r MPF were found at the end of the endurance times in the three muscle s at the high torque level when compared to the low torque level. At t he low torque level MPF of the trapezius was constant throughout the t est. In the deltoid the most prominent decrease occurred during the in itial 30-40% of the endurance time at the low torque level. It is sugg ested that the MPF shift mainly reflects peripheral fatigue of the typ e-2 fibres. The results of the present study question the use of the M PF shift to monitor peripheral fatigue in the fibres active (mainly ty pe-1) at low torque levels.