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. Durin g movement the muscle slides with respect to the skin because of the variat ion of its length. Such a shift can considerably modify sEMG amplitude. The purpose of this work is to assess geometrical artefacts on sEMG during iso metric contractions at different muscle lengths. The average rectified value (ARV) of 15 single differential signals was obt ained by means of a linear array of 16 bar electrodes from the vastus media lis and lateralis muscles. The knee angle was changed from 75 degrees to 16 5 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 pat tern was normalized with respect to the mean activity to compare signals fr om different joint angles. From the data collected it was possible to separ ate the geometrical changes from the changes due to different intensities o f activation. In three out of five subjects, we found (within the resolution of our measu res) a 1 cm shift for the vastus medialis muscle while no shift was observe d 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 contr ibution to geometrical artefacts. To avoid such artefacts the innervation z ones should be located and the EMG electrodes should not be placed near the m. (C) 2000 Elsevier Science Ltd. All rights reserved.