Estimating maximal EMG amplitude for the trapezius muscle: On the optimization of experimental procedure and electrode placement for improved reliability and increased signal amplitude.

The amplitude of electromyographic (EMG) surface recordings on the upper trapezius muscle and its reproducibility during maximal voluntary contractions were examined. At different electrode positions 1.5-3.0 cm lateral to the midpoint between the seventh cervical vertebra (C7) and the acromion the mean maximal EMG amplitude estimates were within 90% of the highest EMG amplitude that was recorded, when the estimate was based on the highest value of two shoulder elevations and two arm abductions. Near the midpoint between C7 and the acromion the mean maximal EMG amplitude was only 42% of the highest value. The repeatability coefficient based on two estimates of the maximal EMG amplitude obtained with 2 h interval indicated that 95% of the differences in the recorded amplitudes should be less than 22% of the mean maximal amplitude. Increasing the averaging period during signal processing from 0.6-2.0 s decreased the maximal amplitude estimate by 8%. It is concluded that electrode positioning is an important factor to consider and that more than one shoulder elevation and arm abduction should be performed when estimating maximal amplitudes.