Geometrical factors in surface EMG of the vastus medialis and lateralis muscles.

Surface EMG signals detected in dynamic conditions are affected by a number of artefacts. Among them geometrical factors play an important role. During movement the muscle slides with respect to the skin because of the variation of its length. Such a shift can considerably modify sEMG amplitude. The purpose of this work is to assess geometrical artefacts on sEMG during isometric contractions at different muscle lengths. The average rectified value (ARV) of 15 single differential signals was obtained by means of a linear array of 16 bar electrodes from the vastus medialis and lateralis muscles. The knee angle was changed from 75 degrees to 165 degrees in steps of 30 degrees and voluntary isometric contractions at a low, medium and high force level were performed for each angle. The ARV pattern was normalized with respect to the mean activity to compare signals from different joint angles. From the data collected it was possible to separate the geometrical changes from the changes due to different intensities of activation. In three out of five subjects, we found (within the resolution of our measures) a 1 cm shift for the vastus medialis muscle while no shift was observed for the other two subjects. For the vastus lateralis muscle a 1 cm shift was found in two out of four subjects. Such a shift produces the main contribution to geometrical artefacts. To avoid such artefacts the innervation zones should be located and the EMG electrodes should not be placed near them.