Elsevier

Clinical Biomechanics

Volume 16, Issue 3, March 2001, Pages 263-266
Clinical Biomechanics

Brief report
Paraspinal muscle EMG fatigue testing with two methods in healthy volunteers. Reliability in the context of clinical applications

https://doi.org/10.1016/S0268-0033(00)00113-3Get rights and content

Abstract

Objective. Comparison of the accuracy of surface electromyogram for back muscle endurance assessment with two different tests.

Design. Test–retest measurements in 16 healthy volunteers on two separate occasions for each test under controlled conditions.

Background. Back muscle endurance is considered important in low back pain rehabilitation. Reliability of paraspinal muscle endurance assessment is a pre-requisite for accurate and meaningful clinical applications of the technique.

Methods. All participants performed each test twice. A direct comparison was made between two popular fatigue testing methods, the modified Biering–Sørensen and a 60% maximum voluntary isometric contraction in the upright position during which time fatigue was assessed from the electromyogram spectral and amplitude analysis.

Results. Reproducibility of initial median frequency was excellent for both tests. Normalised median frequency slope values were more reliable with the 60% maximum voluntary contraction upright test. The clinical applicability of these measures in detecting significant differences after patient rehabilitation is recommended. Root mean square had very large between-day error for both tests.
Relevance

This paper provides information on the accuracy level of power spectral analysis for use as an endurance outcome measure in back muscle rehabilitation studies.

Introduction

Assessment of the endurance capacity of low back muscles is important in identifying potential risk factors for back pain development. As increased fatigability of the paraspinals is associated with the presence of low back pain (LBP) [1], monitoring of the effect of various training regimes in endurance enhancement seems appropriate. However, existing maximum performance tests are problematic when applied to LBP populations, as maximum exertion is linked to fear of activity and pain tolerance [2]. Electromyographic (EMG)-based methods of fatigue assessment overcome these problems by monitoring time-dependent changes in the signal under short duration submaximal contractions [1]. Median frequency (MF) decline in particular has been described as a valid fatigue indicator as in non-LBP subjects it correlates well with endurance time and this relationship is better for lordotic back postures like the one in the Biering–Sorensen test [3].

Results of previous studies on the reliability of MF-slope, irrespective of the postures studied, are contradictory, with not all studies reporting high levels of reproducibility [1], [3], [4], [5]. The same studies have found the initial median frequency (IMF) to be very reproducible. Also, the EMG amplitude depicted by the root mean square (RMS) parameter of the raw signal has been shown to demonstrate higher relative increase with prolonged contractions for LBP populations [6]. The reliability of this measure has also not been consistently described [4].

The purpose of the study was to establish a reliable technique for EMG assessment of endurance for the back muscles. The level of accuracy of these methods also needs to be quantified in a meaningful manner for a confident monitoring of paraspinal endurance enhancement in patients after therapeutic exercise interventions. Associations between improvement in endurance characteristics with better function and fewer relapses is an area worth investigating in back pain research.

To this end, a direct comparison was made between two popular paraspinal muscle fatigue tests, the modified Biering–Sørensen (B–S) and a 60% maximum voluntary isometric contraction (MIVC) from an upright position.

Section snippets

Subjects

Sixteen non-LBP subjects (7 male, 9 female) gave informed consent to participate in this study. Their mean (SD) anthropometric characteristics were age 25.7 (4.5) years, height 169 (10) cm, body mass 66.7 (14.2) kg and body mass index 22.9 (3.0) Kg/m2.

Testing protocol and experimental design

Muscle endurance measurements were made in a custom-made isomyometer device similar to a previously described one from an upright trunk position at 60% MVIC [1], as well as with a modified B–S test [3]. In the B–S test, subjects have to hold their

Results

There were no significant systematic between-session differences identified. All reliability indices are summarised in Table 1. These are separately presented for each electrode site and for upper and lower lumbar sites (merged right and left) for both tests, as no significant side-differences were identified. IMF had the highest reliability and the least error associated with each measurement. The clinical applicability of this parameter is equally good for both tests with the SDD ranging from

Discussion

Reliability is essential in order to establish the most stable measures for subsequent fatigue assessment monitoring. Issues like testing equipment, muscles monitored, data collection and analysis, contraction periods vary across studies and influence EMG parameters.

In line with other investigators, spectral parameters (IMF, MF-slope) were more reproducible than amplitude (RMS) changes for both tests [4]. MF-slope data were more reproducible at the 60% MVIC upright test, with the normalised

Conclusions

IMF and MF-slope EMG-based fatigue indices had adequate level of reliability in the 60% MVIC test from upright. The clinical applicability of MF-slope to detect training-induced paraspinal muscle endurance changes in LBP populations with this test is supported, on the basis of relatively good between-day variability, provided that changes attributed to the intervention match or exceed 33–34%.

Acknowledgements

The authors received grant support from the Greek Scholarship Foundation (IKY) for this study. Also, technical support from Mr Ian Bell, John Daly and Dave Kruup is greatly appreciated.

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