Co-activation alters the linear versus non-linear impression of the EMG–torque relationship of trunk muscles
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.
References (23)
- et al.
Constraining spine stability levels in an optimization model leads to the prediction of trunk muscle cocontraction and improved spine compression force estimates
Journal of Biomechanics
(2005) - et al.
Mechanical stability of the in vivo lumbar spine: implications for injury and chronic low back pain
Clinical Biomechanics
(1996) - et al.
The relationship between EMG activity and extensor moment generation in the erector spinae muscles during bending and lifting activities
Journal of Biomechanics
(1993) - et al.
Active trunk stiffness increases with co-contraction
Journal of Electromyography and Kinesiology
(2006) - et al.
Less is more: high pass filtering, to remove up to 99% of the surface EMG signal power, improves EMG-based biceps brachii muscle force estimates
Journal of Electromyography and Kinesiology
(2004) - et al.
The relationship between trunk muscle electromyography and lifting moments in the sagittal and frontal planes
Journal of Biomechanics
(1987) - et al.
Effects of EMG processing on biomechanical models of muscle joint systems: sensitivity of trunk muscle moments, spinal forces, and stability
Journal of Biomechanics
(2007) - et al.
Identification of dynamic myoelectric signal-to-force models during isometric lumbar muscle contractions
Journal of Biomechanics
(1994) - et al.
Abdominal muscle recruitment during a range of voluntary exercises
Manual Therapy
(2005) - et al.
Effects of different levels of torso coactivation on trunk muscular and kinematic responses to posteriorly applied sudden loads
Clinical Biomechanics
(2006)
Delayed trunk muscle reflex responses increase the risk of low back injuries
Spine
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2018, Journal of Manipulative and Physiological TherapeuticsCitation Excerpt :This decrease in activity of the abdominal surface musculature, such as the RA muscle, has already been studied with EMG.28 Although the use of US is a valid and less invasive measure of detecting changes in muscle morphology, which in turn is related to muscle activity and can be used to infer muscle activity,29 there are many factors that can influence muscle activity, and the relationship among activity, strength production, and muscular rigidity in many cases is still unclear.30 It is therefore necessary to continue investigating the relationship between EMG and RUSI.
Training-related changes in the EMG-moment relationship during isometric contractions: Further evidence of improved control of muscle activation in strength-trained men?
2015, Journal of Electromyography and KinesiologyCitation Excerpt :This observation could indicate that, for strength-trained athletes, priority is assigned to a form of optimized control of muscle activation and regulation that could contribute to enhance the performance and the energetic efficiency of the muscle contraction (Baratta et al., 1988; Carolan and Cafarelli, 1992; Dal Maso et al., 2012; Häkkinen et al., 2000): compared with untrained participants, the increase in agonist muscles activity in strength-trained athletes closely resembled that of the net joint moment, while the smallest increase in the antagonist muscles activity could correspond to that strictly necessary to assure integrity and active stability of the knee joint related to the increase in the net joint moment. This strength training-related difference in the co-activation of the knee agonist–antagonist muscle pair must also be considered to further understand the linear, or the possibly non-linear, underlying relationship EMG and the net joint moment (Brown and McGill, 2008). The impression of linearity of the EMG–moment relationship between strength-trained athletes vs. untrained participants may be, at least partly, attributable to lower antagonist muscle co-activation which has been associated with specific cortical adaptations induced by regular strength training that could exert a specific encoding of antagonist muscles (Dal Maso et al., 2012).
Validity of trunk extensor and flexor torque measurements using isokinetic dynamometry
2014, Journal of Electromyography and KinesiologyCitation Excerpt :We observed a significant and strong correlation between EMG RMS of rectus abdominis muscles and trunk flexor torque (r > 0.99; Fig. 5a). Our results are in agreement with previous analysis conducted on rectus abdominis and obliquus muscles (Brown and McGill, 2008). In the same way, the progressive increase in isometric trunk extension strength was positively correlated to trunk extensor muscle activity (Fig. 5b).