DPT-303.3-PT Essentials – Static Testing – Body Composition

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In simple terminology, a person’s body weight can be divided into two compartments, fat mass (FM) and fat-free (FFM), or lean mass. This is the principle behind body composition assessment.  Fat mass includes internal and external fat (adipose tissue). Fat-free mass includes muscle, bone, and internal organs.  These structures added together make our total weight. Previously height/weight tables were calculated to give sensible guidelines for clients who want to achieve a healthy weight.  However, these tables are severely limited in the information they provide as they give no assessment of how total weight is composed.  In other words, they do not assess the proportion of total weight that is fat.

Total weight indications are a useful starting point in assessing health status, but the composition of a client’s body must also be assessed. Trainers and testers must understand the difference between weight loss and fat loss, and the ideal level clients should be looking to achieve.  The following table highlights how weight loss can be achieved.

All forms of weight loss apart from fat loss (water, CHO, bone, muscle or connective tissue) could potentially be detrimental to health.  Therefore, to use changes in body weight as the only measure of body composition is naive.  It is essential that the trainer can accurately measure both fat weight and lean body mass.

It is impossible for muscle to ‘turn’ to fat, or vice versa. They are completely independent components within the body. Going from an active to a sedentary lifestyle, will normally mean that fat mass will increase, whilst independently the FFM will decrease due to the lack of resistance training.

Methods for Body Composition Assessment

Several methods exist for measuring body composition.  All have advantages and disadvantages and no one method can be considered ‘best’ for all clients and all trainers.

Underwater weighing:

Underwater or hydrostatic weighing is regarded as the most accurate method of determining body composition.  It is the ‘gold standard’ by which all other methods are judged.  This method uses the Archimedes’ principle of displacement.  A person is weighed underwater as well as on land.  The densities of bone (3.0 gm/cc) and muscle tissue (1.06 gm/cc) are higher than water (1.00 gm/cc), while fat is less dense than water (0.90 gm/cc).  Thus, a person with more bone and muscle mass will weigh more in water and have a higher body density and lower percentage of fat.  By using a standard formula, the volume of the body is calculated and the individual’s body density determined. From body density, percentage body fat can be calculated using various formulae.  Underwater weighing is normally only used in laboratories for research purposes. It is far too impractical and ‘client unfriendly’ to be considered as a fitness testing procedure.

Body Mass Index:

The Body Mass Index (BMI) is worked out using the following equation:

BMI (kg/m²)         =       Weight (kg) / Height² (m)

As BMI uses weight only and does not consider body composition, it is useless for measuring fat loss.  However, it can be used as a quick method to give some idea of whether or not a client is overweight. The BMI values for obesity are:

This method is particularly inaccurate for body builders and strength athletes who have a higher than normal amount of muscle mass.  Body builders with very low levels of fat are often classified as ‘obese’ by this method.  BMI should not be used as a means of monitoring changes in body composition.  

Weight method of assessment:

  • calibrated scales
  • client to remove shoes and heavy clothing
  • make sure all re-tests are scheduled for the same time of day

Height method of assessment:

  • check calibration of equipment
  • client to remove shoes and stand tall
  • take the highest part of the head as the score (be aware that height decreases as the day goes on)


Circumferential measurements:

Circumferential measurements are quick and easy to take. The operator requires little skill. Formulae exist to convert circumferential measurements to body composition (hence % body fat and LBM) but these are not especially accurate.  

Circumferential measures are useful to get a rough idea of a client’s body composition (perhaps when taking preliminary details by telephone) but are only recommended as a means of monitoring changes in body composition as a last resort. 

Circumferential method of assessment:

  • take measurement at exact location
  • keep tape in one plane
  • if possible, take measurements under clothing (if not, record details of clothing)
  • take the average of three readings

Circumferential sites

With the subject’s arm relaxed, a horizontal measurement is taken at the mid-point between the acromion process and the olecranon process.

With the subject standing erect, a measurement is taken at the maximum circumference.

With the subject’s abdomen relaxed, a horizontal measurement is taken at the level of the narrowest part of the torso above the umbilicus.  The measurement is taken after a normal expiration.

With the subject’s abdomen relaxed, a horizontal measurement is taken at the level of the umbilicus, the navel.  The measure is taken after a normal expiration.

With the subject standing straight, a horizontal measure is taken at the maximum circumference.

Waist : Hip ratio:

The pattern of body fat distribution is recognised as an important predictor of the health risks of obesity (Van Itallie, 1988).

Fat stored in your abdominal region (as opposed to your legs, hips and arms) is considered to be a greater risk factor for CHD.  This seems to be because abdominal fat is more easily mobilised and is transported directly to the coronary arteries.

Health risk increases with waist:hip ratio, and standards for risk vary with age and gender. For example:

High risk> 1.0> 0.85
Moderate risk0.90 – 1.00.80 – 0.85
Low risk< 0.90< 0.80

Adapted from Van Itallie, (1988)

In preference to the ‘waist : hip ratio’ many now favour the simpler and more reliable method of waist circumference measurement. 

Waist circumference is independent of height. 

Lowered Risk
Men < 94 cm ( < 37 inches)Women < 80cm ( < 32 inches)

Risks of morbidity (disease) greatly increase with the following levels (these denote the recognised action levels whereby intervention becomes necessary).

High Risk
Men > 102 cm ( > 40 inches)Women > 88cm ( > 35 inches)

N.I.H. (1998) Clinical guidelines on the identification, evaluation, and treatment of overweight and obese adults

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