A comparison of ultrasound and electromyography measures of force and activation to examine the mechanics of abdominal wall contraction

Abstract

Background

Ultrasound imaging is a valuable tool which, when applied appropriately, has the potential to provide information regarding the mechanics of abdominal muscle contraction. Typically, changes in muscle thickness are obtained and interpreted. However, the link between ultrasound measures of muscle thickening and EMG measures of activation is not clear.

Methods

Five healthy males performed a series of abdominal muscle contractions while surface EMG and trunk posture were monitored and ultrasound images of the internal oblique and external oblique were captured both at relaxation and upon contraction. Ramped isometric flexor and extensor moment contractions were also assessed and compared between EMG and ultrasound.

Findings

No definitive relationship between increases in muscle activation and corresponding measures of thickening was observed. Correlations between the two measures, across all contraction types, were 0.14 for internal oblique and −0.22 for external oblique.

Interpretation

The lack of clear association between abdominal muscle activation and thickening may be due to the composite laminate-like structure of the abdominal wall, with force being transmitted between obliquely oriented muscle layers. Thus, ultrasound alone may not be a valid measure of muscle activation or force in the unique architecture of the abdominal wall.

Introduction

In recent years, ultrasound imaging has become an increasingly popular tool to assess the contraction of the abdominal wall muscles (e.g., Ferreira et al., 2004, Hodges et al., 2003, Misuri et al., 1997, Rankin et al., 2006, Whittaker, 2008). Muscle thickness obtained from ultrasound is often interpreted as an indicator of muscle force generation. In addition, with appropriate considerations, electromyogram (EMG) amplitude can be linked to muscle force. The relationship between ultrasound measured muscle thickness and EMG-based muscle activation has not be definitively tested, but has yielded interesting findings within a limited scope of abdominal contraction types. For example, Hodges et al. (2003) documented very little change in muscle thickness beyond activations greater than 20% MVC, and both John and Beith (2007) and Coghlan et al. (2008) demonstrated decreases in external oblique (EO) thickness during activation. These discrepancies motivated this comparative study of ultrasound and EMG measures of abdominal wall muscle activation.

Proper activation and contraction of the abdominal wall is considered important for several reasons. First, abdominal muscles generate forces to produce moments in flexion, lateral bend and axial twist (Gatton et al., 2001, Marras and Mirka, 1990, McGill, 1991, McGill, 1996, Pope et al., 1986, Thelen et al., 1994). Second, properly coordinated abdominal muscle contraction is necessary to maintain a stable spinal column (Brown and Potvin, 2005, Brown et al., 2006, Cholewicki and McGill, 1996, Gardner-Morse and Stokes, 1998). Finally, reports have highlighted the functional role of the abdominal muscles in pressurizing the abdominal cavity, which also can have a stiffening effect on the lumbar spine (Cholewicki et al., 1999, Cholewicki et al., 2002, Cresswell and Thorstensson, 1989, Essendrop and Schibye, 2004). For these reasons, it is essential that assessments of abdominal muscle function and contraction be carefully considered, whether via ultrasound or EMG.

The morphology of the abdominal wall muscles creates a composite laminate-like structure. The external oblique (EO), internal oblique (IO), and transverse abdominis (TrA) are broad sheet-like muscles that overlay one another, have muscle fibres that are obliquely oriented with respect to each adjacent layer, and are tightly bound together through networks of connective tissue. These connective tissues play a mechanical role, enabling force and stiffness to be transmitted between the muscular layers (Brown and McGill, 2009), and likely influencing the resulting contraction dynamics and muscular deformations. Various approaches and magnitudes of torso contraction will differentially recruit muscles around the trunk (McGill et al., 2003), thereby affecting the force and stiffness generated in muscle synergists and antagonists, in fascial connections, and in the pressurized abdomen. Muscle thickness not only relies on a muscle’s own force generation, but also on the forces generated in neighbouring muscles, particularly when its fibre orientation is oblique to its neighbour, due to the transmittal of force via intervening connective tissues (Huijing and Baan, 2003). All of these factors play a role in determining how the abdominal muscles will shorten and thicken upon contraction, thus creating a complex network from which to assess muscle function.

In addition, as abdominal muscles shorten, the site being measured on the muscle will move to a new location within the image (or, depending upon the amount of shortening, potentially outside of the image). A majority of the research that has been conducted has employed a standardized static location within the image to measure changes in muscle thickness with contraction, thus not accounting for potential shortening of the muscle. This may lead to error in the estimation of thickness changes as the observed muscle section shortens.

Considering the aforementioned affects that abdominal muscle morphology and measurement location may have on ultrasound measures of abdominal muscle thickening, this study was designed to evaluate the concordance of conclusions about contraction dynamics obtained from both ultrasound and EMG measures.