Examination of EMG normalisation methods for the study of the posterior and posterolateral neck muscles in healthy controls

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Abstract

The purpose of this study was to examine the reliability of normalisation methods used in the study of the posterior and posterolateral neck muscles in a group of healthy controls. Six asymptomatic male subjects performed a total of 12 maximum voluntary isometric contractions (MVIC) and 60%-submaximal isometric contractions (60%-MVIC) against the torque arm of an isokinetic dynamometer whilst surface and intramuscular electromyography (EMG) was recorded unilaterally from representative posterior and posterolateral locations. Reliability was calculated using intra-class correlation coefficient (ICC), relative standard error of measurement (%SEM) and relative coefficient of variation (%CV). Maximal torque output was found to be highly reliable in the directions of extension and right lateral bending when the first of three MVIC contractions was excluded. When averaged across contraction direction, high reliability was found for both surface (MVIC: ICC = 0.986, %SEM = 7.5, %CV = 9.2; 60%-MVIC: ICC = 0.975, %SEM = 10, %CV = 13.7) and intramuscular (MVIC: ICC = 0.910, %SEM = 20, %CV = 19.1; 60%-MVIC: ICC = 0.952, %SEM = 16.5, %CV = 13.5) electrodes. Intramuscular electrodes displayed the least reliability in right lateral bending. The use of visual feedback markedly increased the reliability of 60%-MVIC contractions.

Introduction

There is an increasing volume of work examining the function of the neck musculature through electromyography (EMG) studies in a variety of tasks and activities. Examples of such research have included analysis of rear and side impacts in motor vehicles (Kumar et al., 2004, Magnusson et al., 1999), examination of high-performance combat pilots (Green and Brown, 2004, Oksa et al., 1996), analysis of the effect of subject awareness in whiplash-like perturbations (Seigmund et al., 2003) and long duration, low stress occupational tasks such as computer use (Sommerich et al., 2000, Straker and Mekhora, 2000).

Surface EMG is commonly used to record muscle activation of the neck however, this technique may be subject to cross-talk from adjacent muscles. There have been concerns raised over the use of surface electrodes to examine the function of splenius capitis and semispinalis capitis as cross-talk from trapezius, levator scapulae and sternocleidomastoid may lead to questionable results (Sommerich et al., 2000). Furthermore, it has been reported that splenius capitis was involved in both the anterolateral and posterolateral stabilisation of the neck (Keshner et al., 1989) but these results could be disputed as Mayoux-Benhamou et al. (1995) used intramuscular electrodes and determined that splenius capitis was active in extension and ipsilateral extension and inactive in flexion and contralateral flexion and rotation. Therefore, intramuscular EMG using the fine-wire technique may be required to examine the function of the deep neck musculature (Keshner et al., 1989, Mathiassen et al., 1995, Wittek et al., 2001).

Normalisation of EMG signals is performed so that muscle activation can be compared between muscles and individuals both within- and between-days. Amplitude normalisation of EMG signals may be performed via a maximum voluntary isometric contraction (MVIC) or alternatively, by a sub-maximal voluntary isometric contraction (sub-MVIC). The MVIC is most commonly used in the scientific literature to normalise the EMG signal (Mirka, 1991) and specifically for the neck musculature (Choi and Vanderby, 1999, Kumar et al., 2001, Moroney et al., 1988, Schuldt and Harms-Ringdahl, 1988). However, there are situations such as working with symptomatic patients or when low levels of muscle activation are being examined where a sub-MVIC normalisation is required (Netto et al., 2006).

A high degree of reliability is required for EMG data normalisation in studying the musculoskeletal function of the spine. The reliability of using sub-MVIC as a normalisation technique has been examined in the lumbar spine (Allison et al., 1998, Dankaerts et al., 2004), superficial anterior muscles (sternocleidomastoid and anterior scalenes) of the neck (Falla et al., 2002) and anterior, lateral and posterior muscles of the neck (Netto et al., 2006).

The reliability of EMG normalisation procedures for intramuscular electrodes used for the neck region has yet to be examined. Therefore, the purpose of this study was to examine the reliability of normalisation methods used in the study of the posterior and posterolateral neck muscles in a group of healthy controls. Specifically, MVIC and sub-MVIC methods were examined for both surface and intramuscular EMG methods.

Section snippets

Subjects

Six male subjects with mean (±SD) age (31.3 ± 7.5 years), height (179.3 ± 4.3 cm) and mass (81.0 ± 8.2 kg) were recruited as subjects for this study. Exclusion criteria were; current cervical injury, history of dizziness, fainting or symptomatic irregular heartbeat, exertion-induced aggravation of a severe headache, diagnosis, symptoms or risk of carotid or coronary artery disease, current diagnosis of high blood pressure or severe limitations in pulmonary function capacity (Sommerich et al., 2000).

Peak torque in extension and lateral bending

It is important to determine the reliability of the torque measurements when examining the reliability of EMG normalisation procedures as they influence the subsequent EMG values due to the EMG–force relationship. The reliability of torque values for MVIC’s were calculated for all three trials and also when rejecting the first trial (i.e., for trials 2 and 3). Rejecting the first trial produced superior repeatability as indicated by all three indices of reliability (Table 1). Also, the

Reliability of peak torque measurements

In order to improve the reliability of generating a MVIC or sub-MVIC, methods that produce variations in torque output must be controlled for. This can be achieved by adhering to standardised positioning of subjects and by immobilising the trunk when measuring from the cervical musculature. This prevents trunk and leg musculature assisting in generating torque. In the literature, high reliability of chosen testing apparatus has been reported through the use of adequate restraining devices (

Conclusions

An isokinetic dynamometer was used to reliably test subjects when the head was positioned in neutral and contractions were made in the directions of extension and right lateral bending. Maximal torque output was found to be highly reliable in the contraction directions of extension and right lateral bending when the first MVIC was excluded. When averaged across contraction direction, high reliability was found for both surface and intramuscular electrodes however, intramuscular electrodes

Acknowledgements

Financial support for this study was provided by the Edith Cowan University Early Career Researcher Grant Scheme.

Angus Burnett received his PhD from the University of Western Australia in 1997. He was a senior lecturer in Biomechanics at the School of Exercise, Biomedical and Exercise Science at Edith Cowan University in Western Australia from 2000 until 2005. He is currently a Research Fellow at the School of Physiotherapy, Curtin University of Technology. His primary research interest is in Spinal Biomechanics in Clinical Rehabilitation, Ergonomics and Sports and Exercise.

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    Angus Burnett received his PhD from the University of Western Australia in 1997. He was a senior lecturer in Biomechanics at the School of Exercise, Biomedical and Exercise Science at Edith Cowan University in Western Australia from 2000 until 2005. He is currently a Research Fellow at the School of Physiotherapy, Curtin University of Technology. His primary research interest is in Spinal Biomechanics in Clinical Rehabilitation, Ergonomics and Sports and Exercise.

    Jonathon Green is currently enrolled in a PhD at the School of Physiotherapy at Curtin University of Technology in Western Australia. He completed Honours’ degree in Sports Science at Edith Cowan University in 2004 after obtaining a B.Sc. in Applied Chemistry at Curtin University of Technology. His current research interests are in motor control impairment in chronic neck pain patients.

    Kevin Netto obtained a Diploma in Mechanical Engineering from the Singapore Polytechnic in 1992. He completed a B.Sc. in Sports Science with Honours’ in 1999 at Edith Cowan University where he is currently completing his PhD in Biomechanics. He is currently a lecturer in Exercise and Sports Science at Charles Darwin University in the Northern Territory. His research interests lie in the areas of cervical spine biomechanics, neck injuries sustained in unique ergonomic settings and sports biomechanics.

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