Fatigue in the shoulder muscles during static work at two different torque levels

Abstract

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 flexors. Twelve clinically healthy women performed two endurance tests of the shoulder flexors (at 50% MVC--the high torque level, and at approximately 18% MVC--the low torque level) until maximum exhaustion. Surface 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 the shoulder muscles throughout the two tests using a 10-graded scale. A significantly higher degree of perceived fatigue was found at the low rather than at the high torque level. In contrast significantly lower MPF were found at the end of the endurance times in the three muscles at the high torque level when compared to the low torque level. At the low torque level MPF of the trapezius was constant throughout the test. In the deltoid the most prominent decrease occurred during the initial 30-40% of the endurance time at the low torque level. It is suggested that the MPF shift mainly reflects peripheral fatigue of the type-2 fibres. The results of the present study question the use of the MPF shift to monitor peripheral fatigue in the fibres active (mainly type-1) at low torque levels.

Muscle recovery from a short fatigue test and consequence on the reliability of EMG indices of fatigue

“…However, this conclusion might not be generalizable to other skeletal muscles that do not have a postural function (as in the trunk) and that may fatigue more rapidly, especially if they are composed of a high percentage of type II fibers (Gerdle and Fugl-Meyer 1992;Gerdle et al 1993).…”

Surface EMG of shoulder and back muscles and posture analysis in secretaries typing at visual display units

“…Different occupational studies found trapezius muscle loads of approximately 5% of maximum voluntary contraction capacity in VDU workers (Feng et al 1997;FernstroÈ m et al 1994;Jensen et al 1993a;Sundelin and Hagberg 1989;Vasseljen and Westgaard 1995b). In isometric contractions sustained at low force, changes in the frequency domain of the EMG signal do not necessarily occur (Caer et al 1993;Gerdle et al 1993;Jorgensen et al 1988;O È berg et al 1994).…”

Worsening of neck and shoulder complaints in humans are correlated with frequency parameters of electromyogram recorded 1-year earlier

“…From studies of sustained static contractions it would appear as if the f mean shift (normalised for dierences in endurance times) is force dependent and smaller at a lower level despite the fact that the contraction is performed until exhaustion (Hansson et al 1992;Gerdle et al 1993;Gerdle and Karlsson 1994). Some groups have suggested that the frequency parameters are not useful as indicators of peripheral fatigue at low torque levels (Sakamoto et al 1982;Wiker 1989), while other groups have reported signi®cant frequency shifts at low torque levels (7%±20% MVC) (Fallentin et al 1985;Christensen and Fuglsang-Frederiksen 1988;Kourinka 1988;BystroÈ m and Sjùgaard 1991;Gerdle and Karlsson 1994).…”