ISOTONIC AND ISOELECTRIC ENDURANCE TESTS FOR THE UPPER TRAPEZIUS MUSCLE

As an alternative to a conventional endurance electromyogram (EMG) tes t for assessment of muscle capacity in the upper trapezius muscle (iso tonic test, IT), an isoelectric test (IE) has been investigated. Nine subjects performed the two endurance tests. The tests were performed w ith a straight horizontal arm in the plane of the scapula. In IE, EMG amplitude was fed back to the subject and the subject was instructed t o maintain a constant EMG activity equal to the level with the arm uns upported. Subjective ratings of local fatigue were obtained during the experiments. The EMG recordings from both tests were analysed for the root mean square value as well as according to the mean power frequen cy (MPF) technique. All the subjects endured maximal 15 min IE while i n IT the median endurance was 11.9 min. Average subjective ratings of perceived fatigue increased more in IT compared to IE. The average nor malised MPF curve from IT increased while that from IE was unaffected. There was a significant difference between the MPF results at 6 and 8 min. It is noteworthy that the MPF was higher in IT in spite of a hig her accumulated biomechanical load. It was concluded from these studie s that the interpretation of decreased MPF as an indicator of increase d local muscle fatigue is doubtful at low contraction levels. It is su ggested that an IE is a more appropriate method for the functional eva luation of low threshold motor units of the upper trapezius muscle in research into occupational disorders.