The effects of skinfold thickness on the selectivity of surface EMG

We investigated the effects of skinfold thickness and electrode orientation on the ability to record selectively from a localized region of a muscle using arrays of surface electrodes. EMG activity elicited by electrical stimulation and by voluntary contraction of the biceps muscle was recorded from subjects with skinfold thicknesses ranging from 2 mm to 21 mm. The selectivity of the surface electrodes increased as the skinfold thickness decreased; action potentials were more rapidly attenuated and underwent less low-pass filtering. As a result, the EMG recorded during a voluntary contraction at one site became less highly correlated with that recorded at neighboring sites as skinfold thickness decreased. We were able to determine the axis of action potential propagation (muscle fiber direction) through comparison of the amplitude and delay of cross-correlation peaks from pairs of colinear electrodes oriented at angles to one another, although the thicker the skinfold the lower the resolution. It was clear that the ability to localize EMG signal sources deteriorated as the amount of subcutaneous fat between the surface recording site and the active muscle fibers increased.