Co-activation alters the linear versus non-linear impression of the EMG–torque relationship of trunk muscles

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Abstract

The use of electromyographic signals in the modeling of muscle forces and joint loads requires an assumption of the relationship between EMG and muscle force. This relationship has been studied for the trunk musculature and been shown to be predominantly non-linear, with more EMG producing less torque output at higher levels of activation. However, agonist–antagonist muscle co-activation is often substantial during trunk exertions, yet has not been adequately accounted for in determining such relationships. The purpose of this study was to revisit the EMG–moment relationship of the trunk recognizing the additional moment requirements necessitated due to antagonist muscle activity. Eight participants generated a series of isometric ramped trunk flexor and extensor moment contractions. EMG was recorded from 14 torso muscles, and the externally resisted moment was calculated. Agonist muscle moments (either flexor or extensor) were estimated from an anatomically detailed biomechanical model of the spine and fit to: the externally calculated moment alone; the externally calculated moment combined with the antagonist muscle moment. When antagonist activity was ignored, the EMG–moment relationship was found to be non-linear, similar to previous work. However, when accounting for the additional muscle torque generated by the antagonist muscle groups, the relationships became, in three of the four conditions, more linear. Therefore, it was concluded that antagonist muscle co-activation must be included when determining the EMG–moment relationship of trunk muscles and that previous impressions of non-linear EMG–force relationships should be revisited.

Introduction

Examinations of issues in spine and torso mechanics are often assisted by the use of electromyographic techniques; thus, assumptions must be made regarding the relationship between EMG activation magnitudes and muscular force output. Much of the research concerning EMG–force/torque relationships in the spine literature has focused on that of the extensor musculature. The form of the relationship has been most often identified as non-linear (e.g. Stokes et al., 1987; Thelen et al., 1994; Potvin et al., 1996; Sparto et al., 1998; Staudenmann et al., 2007) although some have determined it to be linear (e.g. Seroussi and Pope, 1987; Dolan and Adams, 1993). Despite the increasing attention paid to the importance of well-coordinated abdominal muscle contraction in ensuring optimal spine health (e.g. van Dieen et al., 2003; Cholewicki et al., 2005; Urquhart et al., 2005; Lee et al., 2006), a very limited amount of work has been done investigating the EMG–torque relationships of the abdominal muscles, yet it too has identified a distinct non-linear form (Stokes et al., 1989, rectus abdominis; Thelen et al., 1994, rectus abdominis and external oblique), with a decline in the rise of the moment as EMG increases.

In determining the nature of the EMG–torque relationship, it appears that there has been a lack of consideration of the additional moment which must be overcome due to antagonist muscle co-activation. Co-activation of muscles acting both agonist and antagonist to a dominant moment is highly prevalent during trunk exertions (Lee et al., 2007; Ross et al., 1993; Thelen et al., 1995; van Dieen et al., 2003). Therefore, it is hypothesized that this activation may alter the perceived EMG–torque relationship of trunk muscles, as the torque produced by agonist muscle groups will be continuously underestimated as a function of the comparative amount of antagonist co-activation. The purpose of this paper is thus two-fold: (1) to examine in more detail the EMG–torque relationship of the abdominal musculature and (2) to re-examine the EMG–torque relationship of the extensor musculature with and without accounting for the additional resistive moment that must be overcome due to antagonist muscle co-activation.

Section snippets

Methods

Eight healthy males (mean/S.D. age=24.9/4.7 years, height=1.79/0.03 m, mass=82.0/9.1 kg) with no history of back problems, volunteered from the University population. Each read and signed a consent form approved by the University Office of Research Ethics.

Effect of antagonist muscle activity

When determining the linearity in the EMG–torque relationship without consideration of antagonist muscle activity, relationships ranged from nearly linear (Extensor Upright) to varying degrees of the non-linear form reported previously in literature, with a declining increase in moment as EMG increased across its spectrum from zero to 100% of maximum (Extensor 50, Flexor Upright, Flexor 50) (Table 1).

Accounting for the additional resistive moment generated by the antagonist muscle groups

Discussion

The primary result of this study was that accounting for antagonist muscle activity influences the relationship between trunk EMG and its generated torque. Specifically, antagonist muscle activity creates an additional resistive moment that has to be overcome by the agonist muscle groups; ignoring this gives the impression of a non-linear relationship between the agonist EMG and the externally generated moment. The true nature of the trunk EMG–torque relationship was found to be more linear

Conflict of interest statement

Neither author has any affiliations that have influenced the content of this work.

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