THE INFLUENCE OF ELECTRODE POSITION ON BIPOLAR SURFACE ELECTROMYOGRAMRECORDINGS OF THE UPPER TRAPEZIUS MUSCLE

The effect of electrode position on the upper trapezius muscle on the myo-electric signal amplitude was investigated with special reference to arm position and estimate of force output. Previously, a depression of the electromyogram (EMG) signal has been reported midway between t he seventh cervical vertebrae (C7) and acromion (Veiersted 1991, Eur J Appl Physiol 62:91-98) although this electrode position has been reco mmended (Zipp 1982, Eur J Appl Physiol 50:41-54). Ten healthy subjects performed maximal shoulder elevations with the arm in vertical, abduc ted and flexed positions and they performed a dynamic movement test. T he myo-electric signal was recorded along the length of the right uppe r trapezius muscle by a 16-channel bipolar array electrode and was int egrated with a 0.2-s time resolution. A region just lateral to the mid point between C7 and the lateral edge of acromion was found with high and stable amplitudes (% coefficient of variation equalled 5.6). At th e midpoint a dip in the amplitude profile appeared which was slightly displaced by arm abduction or flexion probably due to sliding of the s kin relative to the muscle. A linear EMG-force relationship was found in the region with high signal amplitudes, whereas the more lateral an d the dip region showed highly variable EMG-force relationships. Thus, it was found that when using bipolar surface electrodes with an inter electrode distance of 2 cm a centre position 2 cm lateral to the midpo int between C7 and acromion provided good repeatability and high signa l yield.