Fatigue pattern of trapezius muscle in relation to its functional role

The trapezius muscle is very vulnerable to pain and fatigue. This study investigates the pattern of trapezius fatigue during an isotonic and isometric contraction. It looks at the relationship between the fatigue pattern and the muscle´s functional role. Electromyographic (EMG) signals were recorded simultaneously by surface electrodes from the upper trapezius and middle deltoid muscles from 8 healthy subjects. Two different test positions were maintained until the subjective limit of fatigue. In one test position the role of trapezius was as stabilizing synergist, while in the other its role was a prime mover. The RMS and mean power frequency (MPF) values were calculated for every 1 second of the recorded EMG signals. The results show a much more pronounced MPF shift toward lower frequencies when the trapezius is the prime mover than where it is a stabilizing synergist. RMS values show a significant increase when the trapezius is a stabilizer and show a decrease when it is a prime mover. The results challenge the common belief that the RMS value increase during fatigue process and it is hypothesized that the muscle fatigue pattern is directly related to the functional role of the muscle