Influence of interelectrode distance and force level on the spectral parameters of surface electromyographic recordings from the lumbar muscles

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Abstract

In order to study the influence of interelectrode distance and force level on the electromyographic (EMG) spectral parameters and on their reliability, bipolar surface EMG measurements were performed on the lumbar muscles of 15 subjects. Two test contractions (45 s) at 40% of maximal voluntary contraction (MVC) were performed, one with 2 cm interelectrode distance and the other with 4 cm, followed by two contractions at 80% MVC with the same change in interelectrode distance. Increasing the interelectrode distance from 2 to 4 cm caused a significant mean decrease (about 8%) in the initial median frequency. It is shown that this shift is of an order of magnitude that may be expected from the bipolar electrode filter factor, and we further conclude that the observed individual variations in the shift are likely to be connected to fluctuations in the shape of the power spectrum and to variations in conduction velocity. No significant change was found for the median frequency slope when changing the interelectrode distance. Increasing the force (from 40 to 80% MVC) also caused a significant mean decrease (about 10%) in the initial median frequency. The median frequency slope became significantly more negative by more than 200%. We conclude, however, that torque fluctuations during the fatigue contractions should have had only minor influence on the standard error of measurement of the initial median frequency and of the median frequency slope.

Introduction

Electromyographic (EMG) measurements of fatigue are affected by unavoidable within-subject variations, but also by factors that are in principle controlled in the laboratory, as for example the electrode arrangement and the muscular force level during a test contraction. These factors influence the frequency characteristics of the EMG signal, and a knowledge of this influence is thus important for the reliability of the method. This knowledge is also of importance when comparing different studies.

It is well known that the EMG power spectrum recorded with a bipolar configuration is influenced by the interelectrode distance. In general, an increase in interelectrode distance shifts the power spectrum towards lower frequencies [1], [17], [33]. This is conveniently expressed as a decrease in the median or the mean frequency. In his theory of the frequency spectrum of EMG signals, Lindström [17] showed that, for a bipolar electrode configuration parallell to muscle fibers of ‘infinite’ length and not located near innervation points, the effect of the interelectrode distance d on the power spectrum P( f ) is that the spectrum is simply proportional to a factor sin2(πf·d/v), where f is the frequency and v is the conduction velocity. (For convenience, a short proof is given in Appendix A). This factor, known as the bipolar electrode filter factor, is periodic in f and vanishes at frequencies f=nv/d, where n is an integer, i.e. at frequencies that are multiples of the inverse of the action potential travel time between the electrodes, causing (for the case of a single conduction velocity), well-defined ‘dips’ in the power spectrum [17]. An increase in d will cause the maxima of sin2(πf·d/v) to shift to lower frequencies; consequently, one might expect the power spectrum to be likewise affected.

In reality, the dependence of the power spectrum on d is somewhat more diffuse and complex, due to factors such as the statistical distribution of conduction velocities, finite muscle fiber length, varying muscle fiber directions and nearness to innervation points. It may be noted that the factor sin2(πf·d/v) is also present in most terms of the more complicated expression for the power spectrum when the electrode arrangement is nearly centered over an innervation point [17]. Extensive further theoretical studies of the EMG signal and EMG power spectrum have been performed by means of direct, detailed computer simulations [4], [5], [11], [15], [16]. In addition, more complex electrode configurations, which allow for a more detailed study of action potentials, have recently been introduced [10]. A linear array of electrodes may be used to study the effect of innervation zones and tendon junctions [21]. According to Dimitrova et al. [5], spatial filtration (e.g. bipolar detection) reduces the effects of excitation and extinction of the action potentials, though without entirely removing those effects. With these complexities in mind, an analysis based on the bipolar electrode filter factor sin2(πf·d/v) appears however to be a useful and convenient first approximation in the present context, and a starting point when discussing the dependence on d.

In accordance with the theoretical estimate indicated above, several experimental studies have shown that lower median frequencies were obtained with larger interelectrode distance on extremity muscles [2], [12] and on back muscles [22], [32]. However, conflicting results have also been reported, showing no difference in the median frequency of the back muscles following a change in interelectrode distance [19].

The median frequency of the power spectrum is considered to be influenced by the force level mainly through recruitment of motor units [28]. At low force level, the recruitment of type I motor units dominates, while type II motor unit are increasingly recruited at higher force level. For larger muscles the recruitment continues up to about 80-90% of the maximum voluntary contraction (MVC), whereafter increased firing rate gives additional force [1], [9]. The muscle fiber diameter plays a role, since fibers with larger diameter have larger conduction velocities [3], [21], which in turn leads to higher median frequencies [29]. It is generally considered that in the back muscles, the type I fibers have larger diameter than type II fibers, in contrast to the case in extremity muscles [20], [27], [30]. On basis of this, Mannion and Dolan [19] argue that, following increased muscular force of the back muscles, the median frequency should decrease when recruitment of type II fibers occurs. However, previous studies are not unanimous and different effects of force level on the median frequency for different parts of the back muscles have been shown. An increased force level has been found to cause a decrease in the initial median frequency [14], [19], [23], [24], no difference [6], [22], [25], [32] as well as an increase [19].

It is also well known that the median frequency decreases during a constant force, isometric contraction, i.e. during fatigue, and that the decrease is typically more rapid at a higher force level. A proposed explanation for this is that the median frequency decrease is mainly due to a decrease in conduction velocity, which, in turn, mainly is due to the accumulation of lactic acid in muscle and blood [3]. Since the type II fibers are glycolytic, lactic acid will accumulate when these fibers are active, which increasingly will be the case with higher force levels. Thus, the median frequency slope should become steeper with higher muscle force. This has been confirmed in a number of studies [6], [13], [14], [23], [24], [31].

For the reliability of EMG measurements of fatigue with a bipolar electrode arrangement, it should be important to estimate the magnitude of differences that occur in spectrum parameters when interelectrode distance or force level is changed. Most studies of the influence of force level and interelectrode distance on the median frequency have been made on extremity muscles. As studies of back muscles show varying results, complementary studies may be needed to obtain more knowledge as regards different parts of the erector spinae. The aim of the present work was therefore to study the effect on the EMG spectral parameters of the lumbar muscles (at L1 and L5) of changing the interelectrode distance from 2 to 4 cm, and of changing the contraction level from 40 to 80% MVC.

Section snippets

Subjects

Fifteen subjects (4 men and 11 women) with no history of periodic low back pain volunteered to participate in this study. Mean (SD) for age, weight and height for the subjects were; women: 28 (8.8, range 22–49) yr, 59 (7.1) kg and 167 (5.5) cm; men: 29 (4.9) yr, 80 (14.3) kg and 182 (3.3) cm. Informed consent was obtained from each subject and the study was approved by the local ethical committee.

Electromyography

Electrical activity was recorded from the erector spinae at the levels of the first (L1) and the

Results

The results are shown in Fig. 2, Fig. 3 and in Table 2, Table 3. In summary, an increase in interelectrode distance from 2 to 4 cm caused a significant decrease by about 8% in the initial median frequency averaged over all measurements. Looking at the individual measurements, 61% showed a decrease in median frequency, 24% showed an increase, and 15% showed a difference smaller than 1 Hz. The median frequency slope was not significantly affected, although a tendency to a less negative slope

Influence of interelectrode distance

The average decrease of about 8% in initial median frequency in our study when increasing the interelectrode distance from 2 to 4 cm agrees principally with the results of Zedka et al. [32] and of Rosenburg and Seidel [22] who recorded EMG from the erector spinae at lumbar levels of L3 and L4 respectively. Zedka et al. [32] found that a change in interelectrode distance from 2 to 4 cm caused a significant decrease in median frequency of about 20% (estimated by the authors from the diagram).

Conclusions

In summary, we found that there was a significant decrease by roughly about 10% in the initial median frequency when doubling the interelectrode distance (from 2 to 4 cm) as well as when doubling the force level (from 40 to 80% MVC). The median frequency slope was significantly and strongly (by about a factor three) influenced by the change in force level but not (significantly) by the change in interelectrode distance. The initial median frequency shifts were of a magnitude that could be

Acknowledgements

We gratefully acknowledge research grants from the Karolinska Institutet, Departments of Physical Therapy and Surgical Sciences, and the Karolinska Hospital, Department of Orthopedics. We also acknowledge the contribution of M. Hovöre, P.T., to the data collection.

Britt Elfving graduated in 1969 from the Stockholm University College of Physical Education and Sports, and in 1976 from the Department of Physical Therapy, Karolinska Institutet, Stockholm. She has worked as a clinical physiotherapist, and since 1983 as a teacher at the Department of Physical Therapy, Karolinska Instiutet. She is currently a Ph.D. student and the topic of her thesis is assessment of back muscle fatigue by electromyography; reliability of the method and validity for low back

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    Britt Elfving graduated in 1969 from the Stockholm University College of Physical Education and Sports, and in 1976 from the Department of Physical Therapy, Karolinska Institutet, Stockholm. She has worked as a clinical physiotherapist, and since 1983 as a teacher at the Department of Physical Therapy, Karolinska Instiutet. She is currently a Ph.D. student and the topic of her thesis is assessment of back muscle fatigue by electromyography; reliability of the method and validity for low back pain patients.

    David Liljequist received his PhD at the Stockholm University in 1981 with the thesis ‘Electron penetration in solids and its application to Mössbauer spectroscopy’. Since then, he has been working with teaching and research in physics. He is presently professor in experimental physics at the Stockholm University. His present main interest is the transport of low energy electrons in liquids and solids, with relevance for the calculation of radiation induced DNA damage. An additional interest during recent years is medical research, where methods and models from physics may be useful.

    Gunnar Németh received an M.D. degree in 1979 from the Karolinska Institutet and a Ph.D. in 1984 also from the Karolinska Institutet. He is Associate Professor in Orthopedic Surgery and Anatomy and Head of the Department of Orthopedics at the Karolinska Hospital. Clinical work is mostly spinal surgery and multidisciplinary pain diagnostics. The scientific field include electromyography, motion analysis, biomechanical joint modeling and clinical trials concerning low back pain. He has published more than 150 papers on these topics. He has, for many years, been a member of the International Society of Electrophysiology and Kinesiology and the International Society of Biomechanics.

    Eva Mattsson graduated as PT in 1966 from the Karolinska Institutet and received a PhD 1989 also from the Karolinska Institutet with the thesis ‘Energy cost of walking’. Since 1997 she is Associated Professor in Physiotherapy. She has been working as a teacher since 1967 and now mostly as supervisor and researcher. Her interests are work physiology, strength- and endurance training in healthy and paretic muscles, and the development of research methods and tests of reliability.

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