06 January 2021

There are numerous body composition measurement methods around, and as a trainer it is important to be able to differentiate between the different profiling methods and select the one which is most applicable to your client’s needs.

Let’s start by defining the key terms that relate to body composition.

  • Body composition – the ratio of fat mass to fat free mass
  • Fat mass (FM) – the net fat in the body
  • Fat free mass (FFM) – the net tissue, which is not fat (muscle, bone, blood, organs, minerals)

There are numerous different ways to test body composition, and these vary in accuracy, cost, and availability. All of the profiling methods available are estimate measurements, which mean assumptions are made regarding the rations of lean mass, fat mass, water, protein, and other minerals. The assumptions made by testing methods will result in certain limitations, and it is important to match the measurement method to the client to get accurate results. 

Dual-Energy X-Ray Absorptiometry (DEXA)

DEXA scans are quick, non-invasive, and considered the gold standard for measuring body composition. The DEXA scan uses two energy rays; during the scan, each ray passes through different tissues at different speeds. Through this the DEXA scan measures FM, FFM, and bone mineral content, making it also the gold standard for assessing risk for osteoporosis. The scan does expose the individual to a small amount of radiation energy, but this is 100 times lower than traditional x-ray methods. A further benefit of DEXA is the compartmentalised analysis of the body; the scan will identify the distribution of different tissue throughout the body, which is unique to DEXA scanning.

The DEXA scan is not without limitations, the main one being the cost and distribution of machines. Due to the complexity of the machines, the cost to the client is high, at around $150 NZD per scan. There is also the possibility for misrepresentation in FFM measurement through changes in hydration status, muscle glycogen levels and muscle creatine levels.

Whole Body Plethysmography (BODPOD)

The BODPOD functions through air displacement, using a two-compartment model which measures the air displacement from the compartment containing the subject, to the other. The BODPOD uses standard assumptions for FFM and FM along with body weight and volume measurements which are used in the calculation of density.

Similar to the DEXA scan, this method is quick and non-invasive, and additionally it does not require the individual to be exposed to radiation. However, the BODPOD has been shown to display a 2.2% difference to DEXA results, but this difference greatly increases as individuals’ bodyfat increases (1). For this reason it is not generally used as a tool for measuring changes in bodyfat, which is something trainers should be aware of.

Skinfold calipers

Skinfold calipers involve taking a pinch measurement of the skin and subcutaneous fat tissue. Pressure is applied through the calipers, with the adipose tissue compressing the subcutaneous fat in a standard way, and the thickness of the skin assumed to be negligible. A measurement of the fold is then taken in millimetres (mm). There are then numerous calculations which use different sites to calculate body fat.

The skinfold calipers have a unique set of positives; they are extremely cheap and portable, which the majority of measurement protocols are not. This has made them the most common measurement protocol in the health and fitness industry; however, as with all methods there are limitations, with these being quite considerable in certain circumstances. The main limitation of skinfold calipers is the accuracy is highly dependent on the individual taking the measurement; certain sites and folds must be accurately located, marked, and measured for the results to be valid. Skin fold measurements taken just a small degree away from the correct site have been shown to produce a significant difference in measurements (2). There is a governing body for this method, the International Society for the Advancement of Kinanthropometry (ISAK), and all individuals taking these measurements should be registered to ensure accuracy.

Hydrodensitometry (underwater weighing)

Hydrodensitometry, which is more commonly referred to as underwater weighing, works similarly to the BODPOD, but instead of measuring air displacement it uses water displacement. Body density is calculated through this displacement of water with similar assumptions being made regarding the density of tissues.

This method has many benefits; it is easy to conduct, doesn’t expose the individual to radiation, and is highly accurate when done correctly. The results of the measurement are exceptionally dependent on the performance of the subject, and unfortunately, due to the nature of this testing method performances can greatly vary (3). The process itself has a high degree of discomfort and it can be challenging to get the subject to expel all the air in their lungs.

Bioelectrical Impedance Analysis (BIA)

BIA uses a low-frequency electrical current which passes through the body; this current will pass through different tissues at different speeds. The BIA measures total body water, which is used to calculate FFM. The ease of measurement, cost, and portability have made BIA one of the primary body composition measurement methods in recent years.

There are a large variety of different BIA measurement devices, and it is important to look out for certain features when choosing a device. Firstly, the machine uses certain measurements based on FFM to calculate body fat, so the more information that is inputted, the more likely you are to receive an accurate outcome. Entering at least gender, height, weight, and age is essential in ensuring accurate results. BIA is extremely sensitive to hydration status (4), so it is essential to standard hydration and other practices prior to measurement.

Choosing the right option for your clients

The key here for Personal Trainers is understanding the methods and selecting the best option for your clients’ needs. Generally speaking either DEXA, skinfold calipers, or BIA would be selected. The DEXA scan would only be used in rare circumstances due to the practicality of getting a test and the cost for the client (serious bodybuilders would be the type of client who would suit this method). The caliper and BIA methods are both applicable for a wide range of clients, so it’s best to look at the options available to you; for example is there a good BIA machine close by, or is the person using the calipers suitably qualified? These are all things to consider.

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Written by George Pollitt


1. Ball, S. D., & Altena, T. S. (2004). Comparison of the Bod Pod and dual energy x-ray absorptiometry       in men. Physiological measurement, 25(3), 671.

2. Norton, K., Whittingham, N., Carter, L., Kerr, D., Gore, C., & Marfell-Jones, M. (1996). Measurement             techniques in anthropometry. Anthropometrica, 1, 25-75.        

3. Katch, F. L. E. D. M., Michael, E. D., & Horvath, S. M. (1967). Estimation of body volume by         underwater weighing: description of a simple method. Journal of Applied Physiology, 23(5),     811-813.

4. Kushner, R. F. (1992). Bioelectrical impedance analysis: a review of principles and applications. J      Am Coll Nutr, 11(2), 199-209.