Reliability of EMG measurements for trunk muscles during maximal and sub-maximal voluntary isometric contractions in healthy controls and CLBP patients

Abstract

The purpose of this study was to compare the reliability of trunk muscle activity measured by means of surface electromyography (EMG) during maximal and sub-maximal voluntary isometric contractions (MVC/sub-MVC) over repeated trials within-day and between-days in healthy controls and patients with chronic low back pain (CLBP). Eleven volunteers (six controls and five CLBP patients) were assessed twice with a 1-week interval. Surface EMG signals were recorded bilaterally from six trunk muscles. Intra-class correlation coefficients (ICC) and standard error of measurement as a percentage of the grand mean (%SEM) were calculated. MVC and sub-MVC showed excellent within-day reliability in both healthy controls and CLBP patients (ICC mean 0.91; range 0.75–0.98; %SEM mean 4%; range 1–12%). Sub-MVC for both groups between-days showed excellent reliability (ICC mean 0.88; range 0.78–0.97; %SEM mean 7%; range 3–11%). The between-days MVC for both groups showed trends towards lower levels of reliability (ICC mean 0.70; range 0.19–0.99; %SEM mean 17%; range 4–36%) when compared to sub-MVC. Findings of the study provide evidence that sub-MVC are preferable for amplitude normalisation when assessing EMG signals of trunk muscles between-days.

Introduction

In spite of the large number of pathological conditions that can give rise to back pain, 85% of this population are classified as having ‘non-specific chronic low back pain’ (NSCLBP) as no radiological abnormality is detected [13]. There is considerable evidence documenting the presence of movement and motor control dysfunction in subjects with NSCLBP, although the nature of the dysfunction is highly variable [9], [19], [20], [21], [38], [39], [40], [45]. In both clinical practice and laboratory research settings, there is an increasing need for objective assessment. Surface electromyography (EMG) is a non-invasive technique that allows an objective evaluation of muscle activity. The use of EMG has played a major role in the understanding of trunk muscle activity during specific postures and movements, in both healthy control subjects and patients with LBP [12], [14], [17], [28], [42], [44].

EMG normalisation is the process by which the magnitude of muscle activation is expressed as a percentage of that muscle’s activity during a calibrated test condition. Normalisation of an EMG signal is required if comparisons are to be made between subjects, days, muscles, or studies [24], [27], [34], [53]. A normalisation technique should be reliable across muscles and exertions, as well as across subjects to ensure that EMG signals from the trunk musculature are useful in identifying muscle dysfunction and evaluate treatment outcome. Two fundamentally different methods for EMG amplitude normalisation of the trunk muscles are the utilisation of maximal voluntary contractions (MVC) and sub-maximal voluntary contractions (sub-MVC). The MVC is most commonly used in the scientific literature to normalise the EMG signal [34]. The MVC has the advantage of having a physiological meaning where derived data are expressed relative to the maximum [2]. It has been reported that sub-MVC are more reliable in a pain population [30], [41] and are more sensitive when assessing low levels of muscle activity [2], [41], [48]. However, this approach is limited by the difficulty of establishing equivalent sub-maximal loads for different muscles [2].

There is a need to consider the issue of reliability as a prerequisite of validity when EMG is used in the investigation of patients with CLBP to allow for intra- and inter-subject comparison. Unless the reliability of the normalisation procedure is established, the use of these EMG variables, and any variables derived directly from them, may have limited validity in the assessment of CLBP patients with different patterns of motor control dysfunction. From a comprehensive review of the literature, there were no studies comparing the within-day or between-days reliability of MVC and sub-MVC procedures for trunk musculature in healthy controls and LBP patients.

For individuals with LBP, it is questionable if attempts at maximal efforts are valid [6]. MVC techniques have limited value when evaluating individuals who are suffering from LBP if they are not willing, nor able to generate a ‘true’ MVC. Surprisingly, there is little recognition of MVC normalisation as a limitation in studies that investigate subjects with musculo-skeletal pathology [2], [38]. Previous researchers have reported testing LBP patients at a time of ‘not having LBP’, or having ‘low levels of pain’ [3]. This approach is limiting as it does not always represent a true clinical situation.

Knutson et al. [24] stated that, if normalisation is required, the only way to ensure high quality of EMG data is to select an appropriate normalisation procedure based on its reliability. Between-days reliability becomes critical when assessing EMG parameters that are used as outcome measures [15]. To date, no study has investigated the between-days reliability for sub-MVC of trunk muscles.

McLean [31] reported better test–retest reliability for a maximal voluntary activation when compared to a reference voluntary activation commonly used for normalisation purposes in upper trapezius muscle investigation. In contrast, it has been previously reported that sub-MVC are more reliable than MVC in healthy controls when examining EMG data from biceps femoris [1] and triceps [54] muscles. Sub-MVC have also been reported to be reliable within-day in healthy subjects when assessing EMG for abdominal wall muscles [2], [39]. Lariviere et al. [25] compared the between-days reliability of MVC of back muscles between healthy control subjects and CLBP patients and found them to be poor in both groups.

The reliability of EMG data can be assessed in different ways. Where the aim is to establish the difference between-subjects, reliability is best described as a ratio of variance based on the variability between-subjects as compared to the variance within-subjects [46], [50]. This ratio can be expressed by the intra-class correlation coefficient (ICC) and reflects the relative reliability of the measure. A high ICC (≈1) is associated with a small within-subject variance relative to the between-subjects variance [5]. When the purpose is to determine the absolute error in the measure, reliability may best be expressed as the magnitude of measurement fluctuations in the unit of the measurement, i.e. the standard error of the measurement (SEM) [23], [50]. Therefore, ICC and SEM can be seen as complementary measures of reliability [15].

It appears that the literature concerning trunk muscle normalisation is still in development and that research evaluating the reliability of specific normalisation techniques is required. The purpose of the current study was to evaluate and compare the within-day and between-days reliability of EMG measurements of trunk muscle activity in healthy controls and CLBP patients during MVC and sub-MVC.