Body composition, otherwise referred to as kinanthropometry, involves the evaluation of the human physique and components of the body. This includes muscle, bone and fat, addressing the size, shape and proportion of each tissue.
The assessment of athlete’s physique is of interest to both the exercise scientist and coach, when attempting to assess a variety of factors. Body composition measurements are often used when attempting to determine the success of training programs, monitoring growth, determining characteristics of elite athletes and those within weight category sports. The method of assessing body composition is largely dependent ‘on available resources, testing conditions and the application of the results as a clinical and research outcome’ (Burke and Deakin, 2006). Typically, when measuring the physique of athletes, the primary objective is to estimate the value of subcutaneous adipose tissue, using a standardized and valid assessment method.
Methods of body composition measurement
Techniques used to assess body composition have been categorized into three approaches (Martin and Drinkwater, 1991). Direct assessment or Level I is the first approach, based on cadaver analysis. The remaining two approaches, used to assess athletes, require certain assumptions to predict body composition, and are therefore defined as indirect.
Direct Assessment (Level I)
The most invasive and considered a ‘direct measurement’ of body composition is via cadaver analysis. By performing adult dissections it allows for comprehensive understanding of the gross tissue masses of skin, adipose tissue, muscle, bone and organs. A clear limitation to direct analysis is the requirement for subjects to be deceased, therefore rendering this practice useless when working with athletes. It has also been questioned that data extracted from previous cadaver investigations have focused predominantly on an elderly population, therefore less is known regarding body composition via dissection in younger subjects. Nonetheless, previous studies have provided clearer insights into the human physique and without which, alternative, indirect techniques would estimate results to a lower degree of accuracy.
Indirect Assessment (Level II)
Level II methods of body composition are typically used within a research application due to the expensive and sophisticated equipment required. The Level II methods are also used to validate the doubly indirect method (Level III).
Hydrodensitometry (underwater weighing) is used to assess body density, where results can be extrapolated to calculate body fat quantities. It is assumed that when using this method the body is split into 2 compartments, a fat mass (FM) and fat-free mass (FFM). The densities of both compartments are the same for every individual and therefore this provides the methodology for predicting body fat. The theory of hydrodensitometry is based upon the principle of buoyancy or relative floatability and underwater weighing has been considered the ‘gold standard’ for validating other methods of body composition analysis. Despite this, measurement error can be problematic when performing hydrodensitometry. Air trapped in the lungs and gastrointestinal tract are the primary sources of error using this method, and failing to account for residual lung volume will underestimate whole body density because the air volume will contribute to buoyancy. This omission would consequently overestimate the percentage of body fat when converting body density.
Volume displacement techniques have advanced further in the last 15 years, with the creation of the BOD POD. This pod uses an air-displacement method to measure body volume. A large benefit to this method is that subjects do not need to be submerged underwater, and therefore a wider range of subjects can be sampled. However, both techniques, either water or air displacement methods, are expensive and not easily implemented on a regular basis with athletes outside a research capacity.
Dual energy X-ray absorptiometry (DXA) scans are able to measure soft-tissue such as fat and lean tissue. This high technology procedure uses two distinct low-energy x-ray beams to penetrate bone and soft tissue to a depth of approximately 30 cm. Bone mineral content, total fat mass and free-fat mass can all the quantified through DXA. A disadvantage to the DXA scan is that very heavy subjects (over 120 kg) can fail to scan due to the x-ray beam unable to penetrate the full body mass. Additionally, subjects over 2 metres may not fit into commercially available scanners.
Doubly Indirect Assessment (Level III)
Bioelectrical impedance analysis (BIA) is a quick and non-invasive method for estimating body composition. A small alternating current flows between two electrodes attached to the dorsal surfaces of the foot and wrist when lying flat. The current passes through hydrated fat-free tissue and extracellular water at a quicker rate than fat or bone tissue due to a greater electrolyte level, therefore less electrical resistance. Fat free mass can be calculated after normalising for stature, age and gender. Alternative forms of BIA devices are also available such as leg-to-leg scales as these also act as weighing scales. Additionally, arm-to-arm devices are available, however, when measuring across identical limbs and not one to another (arm-to-leg), it is suggested that these values are less accurate as the current is not passing across the full length of the body. Hydration level can influence the results as hypohydration and hyperhydration will alter the body’s electrolyte level and in turn affect the current flow. In athletes it is important to control the level of hydration prior to measuring.
A common practice within sport science is the use of a skinfold measurement (anthropometry). This is non-invasive, inexpensive and easily accessible. The skinfold method involves a procedure where it is possible to measure the subcutaneous fat of individuals to predict total body fat. A skinfold calliper is used to measure the thickness of a compressed, double layer of skin and the underlying subcutaneous adipose tissue. The sum of several skinfold sites on the body can be used to predict fat mass and body fat percentage. It is important that when interpreting the skinfold results that the correct equation is used to reflect the subjects. Equations have been generated specific to population groups, reflecting age and ethnicity, however the application and interpretation of these equations require. Additionally, extensive training is required to enable accurate and reliable measures of skinfold sites, while correct protocol is adhered too. The repeated use of a skinfold calliper on a specific site will result in a decreased measurement due to the force through the calliper, compressing the adipose tissue temporally.