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

Abstract

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° to 165° in steps of 30° 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.

Introduction

Clinical evaluations require movement analysis techniques suitable for detecting modifications and functional compensations due to pathological situations and their evolution with treatment. Surface EMG is a very promising technique in this regard because it can provide, in a non invasive way, information about the global activity of the muscle under study [3], [4], [5], [9], [13], [17], [18], [19], [21], [26]. Dynamic conditions introduce a number of factors of variability which are absent in the isometric condition. During movement, the muscles shift with respect to the skin and the detection electrodes. This phenomenon has been reported in a few studies [11], [15], [16], [22], [28] and implies that the amplitude of the signal can change due to geometrical reasons (geometrical artefacts) and not necessarily as a consequence of a different level of muscle activation. This issue can be of minor importance if only activation intervals (muscle on or muscle off) are considered but has relevance in biomechanical studies focused on the relationships between EMG signal and muscle level of activation and in situations in which the envelope of the signal is of interest. The problem is probably more serious if fatigue indexes are estimated during exercise. In this paper we focus only on the amplitude, the variable most commonly used in clinical activity. Further studies will be focused on other EMG parameters.

Hundreds of works on sEMG analysis of muscle activity appeared in literature in the last 15 years. Although the use of sEMG is widespread in research as well as clinical investigations for evaluating muscle activity during movement, only a few authors addressed the problem of electrode location by adopting suited protocols [4], [10], [11], [12], [19], [23], [25], [27]. Often this issue is ignored or not addressed properly. For example, both in isometric and dynamic conditions, the authors place the electrodes over the muscle belly [7], or on the motor point [6] or over the midline of the muscle without any insight into the effect of such a choice on the estimation of sEMG parameters. In many clinical handbooks, including those of recent publication [13], the recommended electrode position is still based on anatomical landmarks because of lack of alternatives. The validity of such a choice for the vastus medialis (VM) and lateralis (VL) will be discussed in this paper.