Arm movement and EMG mean power frequency in the trapezius muscle: a comparison between surface and intramuscular recording techniques.

In a previous study it has been shown that there is a systematic variation of the EMG mean power frequency recorded under dynamic conditions with surface electrodes from non-fatigued trapezius muscle. The variation was most pronounced in relation to shoulder joint angle. The aim of the present study was to find out whether this variation was caused by geometric factors such as electrode displacement, or by neuromuscular factors within the muscle. A comparison has been made between surface and intramuscular recordings. EMG was recorded with standard surface electrodes and with a monopolar "hooked wire" intramuscular electrode. The recordings were analyzed for calculation of mean power frequency (MPF) and root mean squared (RMS) signal level. For MPF we found a low, but statistically significant correlation (r = 0.30) between surface and intramuscular recordings. For RMS the corresponding correlation was high (r = 0.92). Analysis of variance showed a high proportion of explained variance related to the shoulder joint angle for surface recordings, but not for intramuscular recordings. The results were confirmed in the regression analysis. The results indicate that the observed variation is largely caused by geometric displacement of the surface electrodes and not by intrinsic neuro-muscular factors.