BODY COMPOSITION

Assessment of muscle mass can help to monitor training and nutritional efficacy. Three laboratory-based systems and three field-based systems are evaluated in this article. A guide to the most simple and commonly used method, the skinfold measurement, is also presented.

BY MARK CHEETHAM BSc, MCSP

INTRODUCTION As the preseason approaches, it is the time for baseline testing – a time when physiological and performance data are collected and crunched so that desired improvements can be planned and progress tracked. In terms of sporting success, muscle

mass enjoys a closer relationship with improved performance than does body fat. Therefore, regular assessment of muscle mass can help in the monitoring of an individual’s training and nutritional efficacy. The aim of this article is to describe and evaluate the methods commonly described in the literature. Although it is understood that laboratory methods are generally not a feasible option for most sporting purposes, an appreciation of them is essential.

BACKGROUND

In the realm of body-composition research, the perspective where body mass is comprised of fat mass (FM) and fat-free mass (FMM) is termed a two-compartment model and forms the classic viewpoint. There are also multi-compartment approaches, which involve combining the data from a number of methods and enabling the splitting of the body mass into further divisions. The four-compartment approach, for example, uses the values taken from three different methods to give values relating to the individual’s total body water, FM, total bone mineral content and protein. Although the multi-compartment methods represent the current criterion measures, their complexity precludes them from routine use in performance assessment.

LABORATORY METHODS Hydrodensitometry Also known as underwater weighing

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(UWW), the hydrodensitometry method is based on Archimedes’ principle, which states: “When a body is immersed in water, then it is buoyed up by a force which is equivalent to the weight of the volume of water displaced” (1). Put simply, the individual is weighed

on land and then again underwater, the basic premise being that fat floats and FFM sinks, ie. the less the person weighs underwater, the higher their body fat content. Although the subject is required to exhale completely during the test, residual lung volume (RV) must still be accounted for. Direct measurement is necessary, as estimation of this variable from vital capacity or anthropometry can represent a sizeable source of error. This brings us on to the greatest

source of error in UWW, namely converting body density to body fat percentage (BF%) – or, more specifically, the calculation one chooses. The problems lie in the assumption that FFM is comprised in the same way – ie. has the same density – in all people. In fact, the density of FFM is known to vary, depending upon the person’s age, gender, ethnicity, sexual maturation, level of obesity and physical activity. It is beyond the scope of this article to review the population-specific equations that exist, but it is imperative that the reader appreciates that, apart from the technical proficiency of the tester, choosing the appropriate equation is crucial.

The most commonly applied equations in UWW are those of Siri and Brozek et al. (1). Due to their use of slightly different values of fat density (0.9000 grams per cubic centimetre (g.cm3) and 0.9007g.cm3, respectively), these equations produce slightly different results, but they are usually within 1% of each other.

Despite the level of equipment required, the relatively complex protocol, and the fact that the original equations relied on data from a very limited number of white individuals, UWW is still commonly referred to as the gold standard of body-composition determination. Whether it deserves this title is a topic on which researchers continue to debate.

Dual X-ray absorptiometry The dual X-ray absorptiometry (DEXA) method constitutes a three- compartment approach. Instead of viewing an individual’s composition as simply FM and FFM, DEXA also assesses bone mass. It achieves its measurement as a result of the differing attenuation of X-rays emitted at two different energies by the body’s various tissues. The advantages of DEXA are that it is safe (low radiation doses of short exposure), it requires little compliancy from the individual (unlike UWW), it takes approximately 15min for a whole-body scan, and only negligible effects are produced by hydration status. In addition, the individual’s ethnicity, athletic status and musculoskeletal development do not appear to affect the accuracy of the method. In practical terms, however, the equipment is relatively expensive and complex and requires specialist operation. Not surprisingly, the logistical and financial factors to this method render it an impractical option for the routine assessment and monitoring of body composition in a sports environment.

Air displacement plethysmography The air displacement plethysmography method was developed into a viable method for assessment of body composition in 1995. In principle, it is

sportEX dynamics 2009;21(Jul):24-26