Median frequency of the electromyographic signal: effect of time-window location on brief step contractions

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Abstract

The purpose of this study was to determine, for different back muscles, if the median frequency (MF) of the electromyographic (EMG) power spectrum changes according to the position of the time window during a 5 s step contraction. Twenty males with no known back problems were standing upright in a dynamometer allowing lower limb and pelvis stabilization. Trunk extension efforts were performed by pushing on a force platform positioned at the T4 level while the extension moment at L5/S1 was displayed as visual feedback. The EMG signals from four homologous back muscles (multifidus at L5, ilicostalis lumborum at L3, and longissimus at L1 and T10) were collected using active surface electrodes during two 5 s static step contractions performed at five force levels (10, 20, 40, 60 and 80% of the maximal voluntary contraction). The root mean square (RMS) and MF values of the EMG signals corresponding to three 250 ms time windows (beginning, middle and end of each step contraction) were computed. The RMS values of several back muscles increased from the first to the third time window for contractions performed at high force levels only. However, a concomitant decrease in the MF values was observed only for the left multifidus muscle. It was concluded that muscle fatigue does not generally manifest itself during 5 s step contractions through the EMG signal. However, it is recommended to use step contractions lasting less than 5 s and to choose a time window located in the first 1–3 s to completely eliminate the possible effects of fatigue.

Introduction

Because there is a linear relationship between the median frequency (MF) computed from the power spectrum and the average conduction velocity of recruited muscle fibers [1], [2], the MF is at times used as an estimate of the muscular conduction velocity. Moreover, as the muscle fibers' conduction velocity is dependent on the diameter of the recruited fibers, the conduction velocity and the MF can be used to study their patterns of recruitment during isometric contractions performed at different force levels [3], [4], [5], [6]. In back muscle evaluation, the MF is extensively used to contrast the muscle characteristics of patient and control subjects [7], [8], [9], [10].

The relationship between muscle force and MF is generally documented through step contractions with different steady force levels maintained for a few (from 3 to 5) seconds [9], [11], [12], [13], [14]. The MF analysis is done on a limited time window during the step contractions but, generally, authors do not indicate the starting time of the analysis. The choice of the time-window position is determined solely on the quality of the force measures (adequate force level and stability) used to control the level of muscle effort. It is implicitly assumed that no muscle fatigue occurs during the maintained step contraction.

Recent research proposes that the selection of the time-window position for spectral analysis could act as a confounding variable when step contractions are used [15]. There is a decrease in the MF of the electromyographic (EMG) power spectrum during muscle fatigue [4], [14] and this change is more pronounced during high force levels [9], [14], [16]. The step contraction used to evaluate back muscles is usually maintained for less than 5 s, but it would appear possible that significant muscle fatigue might increase in that period of time. Thus, the EMG power spectrum would be compressed toward the lower frequencies especially when the time window is positioned at the end of a high force level step contraction.

The aim of this study was to evaluate, for different back muscles, if the MF of the EMG power spectrum changes according to the position of the time window (beginning, middle and end) during 5 s step contractions performed at different force levels. It was hypothesized that (1) the MF values computed from a time window located at the end of a step contraction would be smaller (indicating muscle fatigue) than the MF values computed from a time window located at the beginning of the contraction and (2) this effect would be more marked at higher force levels.

Section snippets

Subjects and tasks

Twenty males [age: 25 years, standard deviation (SD) 4 years; height: 1.77 m, SD 0.06 m; mass: 72 kg, SD 9 kg) with no back problems or physical disabilities participated in the study. The subjects were excluded if they had experienced back pain in the preceding six months or if they were obese (body mass index exceeding 30 kg/m2 according to Garrow and Webster [17]). They were informed of the experimental protocol and its potential risks, and gave written consent prior to their participation.

Results

The maximal L5/S1 extension moments performed at the beginning of the procotol and used as the MVC in the present study attained −264 ± 50 Nm (mean of two trials/subject).

There was no significant difference between the relative forces (%MVC) attained at each time-window position for each level of force except at 10% MVC (Table 1). In this case, the pairwise contrasts revealed the presence of a significant difference between relative forces at WIN1 and WIN2, but the magnitude of the difference

Discussion

In the present study, the RMS amplitude of the EMG signal of several back muscles increased over the time of step contractions performed at high force levels. These results suggest that muscular fatigue was initiated because force data reveal that the same level of force was maintained on the entire step contraction. To the authors' knowledge, there is no other potential explanations for the RMS increase at the end of the step contractions. It appears unlikely that a derecruitment of motor

Acknowledgements

Christian Larivière was supported by a post-doctoral fellowship from the Institut de Recherche en Santé et en Sécurité du Travail (IRSST) of Québec. This project was a part of a larger project also funded by IRSST.

Christian Larivière received a B.Sc. degree in Physical Education, an M.Sc. degree in Kinanthropology and a Ph.D. degree in Clinical Sciences from the University of Sherbrooke, Quebec, Canada, in 1992, 1994 and 1999, respectively. He is currently a Post-Doctoral Fellow in Biomedical Sciences at the University of Montreal, Quebec, Canada. Since 1991, his research has focused on the development of linked models applied to lifting tasks and on the quantification of lumbar impairments through

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    Christian Larivière received a B.Sc. degree in Physical Education, an M.Sc. degree in Kinanthropology and a Ph.D. degree in Clinical Sciences from the University of Sherbrooke, Quebec, Canada, in 1992, 1994 and 1999, respectively. He is currently a Post-Doctoral Fellow in Biomedical Sciences at the University of Montreal, Quebec, Canada. Since 1991, his research has focused on the development of linked models applied to lifting tasks and on the quantification of lumbar impairments through kinematic, kinetic and electromyographic measures.

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    A. Bertrand Arsenault received a B.Sc. (Physical Therapy), M.Sc. (Kinesiology) and Ph.D. (Kinesiology) from the University of Montreal, Simon Fraser University and the University of Waterloo, respectively. He practised physical therapy for several years before joining, in 1980, the School of Rehabilitation of the University of Montreal and the Research Center of the Montreal Rehabilitation Institute. Since 1980, he has acted as Professor, Director of Graduate Studies and Director of the Physical Therapy Program at the University of Montreal and as a Researcher and Director of the research center. He has been involved in research activities focusing on the evaluation of the musculo-skeletal system of stroke patients as well as of subjects suffering from back and neck pain.

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    Denis Gravel is Professor at the School of Rehabilitation of the University of Montreal and a Researcher at the Research Center of the Montreal Rehabilitation Institute. After receiving his B.Sc. in Physical Therapy in 1970, he practised physical therapy for two years. Then, he completed his M.Sc. degree at the Department of Anatomy of the University of Montreal. From 1976 to 1983, he acted as a Professor at the School of Rehabilitation of the University of Montreal. From 1984 to 1991, he completed his Ph.D. degree in Neurobiology at Laval University. From 1992 to 1996, he was granted from the FRSQ (Fonds de Recherche en Santé du Québec) as a Clinician Researcher. His research interests focus on the evaluation of normal and pathologic motor function using electromyography, biomechanics and dynamometry techniques.

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    Denis Gagnon received a B.Sc. in Physical Education and an M.Sc. in Kinanthropology in 1980 and 1985, respectively, from the University of Sherbrooke, and a Ph.D. in Biomechanics in 1990 from the University of Montreal. He is Professor at the Department of Kinanthropology of the University of Sherbrooke and Director of the Occupational Biomechanics Laboratory. His research interests focus on the study of trunk muscle coactivity strategies during dynamic lifting and on the investigation of back muscle fatigue during static effort in healthy individuals and those with low back pain.

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    Patrick Loisel received his MD degree in 1971 from the Faculty of Medicine of Paris VI (France), and first practised in France. He immigrated to Canada in 1983 and, after postgraduate studies at University Laval (Quebec City), joined the University of Sherbrooke in 1985. He has been a Fellow of the Royal College of Physicians and Surgeons of Canada since 1986. He is now full Professor at the Faculty of Medicine of the University of Sherbrooke and presently involved in research, practice and teaching in work rehabilitation at Charles LeMoyne University Hospital (south shore Montreal). He was the Principal Investigator in a large, population-based randomized clinical trial testing a model of management of occupational back pain (Sherbrooke model). His research work is in the development and validation of programs and tools for the prevention of work disability for musculo-skeletal disorders. He is the Head of the PREVICAP multidisciplinary clinical research team in work disability prevention. He was President of the Scientific Council of the Quebec Research Network of Rehabilitation from 1995 to 1998 and is presently a member of this network and the HEALNet Research Network of Centers of Excellence Program of Canada. He is President of the Quebec Commission for Evaluation of University Education Programs.

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