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A Sneak Peek into the Pros and Cons of Hydrostatic Weighing

Pros and Cons of Hydrostatic Weighing
A steep increase observed in the trend of physical awareness amidst people has been the motivating factor in the extensive research on the measurement of body composition. Of all the testing methods, hydrostatic weighing has gained substantial attention. Through this post, Buzzle presents the pros and cons associated with hydrostatic weighing.
Anamika Kumari
Last Updated: Jun 3, 2018
Loophole in the Test
Patients suffering from osteoporosis have lower bone densities than others. On the other hand, athletes have denser bones as compared to the rest. Therefore, there lies a possibility of sizable overestimation and underestimation, respectively, of body fat in these cases.
Hydrostatic weighing, or hydrodensitometry, is the most accurate test used to measure the body composition of a subject till date. The other terms used for this test are 'underwater weighing' and 'hydrostatic body composition analysis'. It helps determine the body density of a 'subject' or human body.
This test is based on Archimedes' principle which states that the counteractive buoyant force acting upon any object submerged in a fluid, equals the weight of the fluid displaced by that object.
The actual process of hydrostatic weighing is carried out in three steps.
  1. Measurement of subject weight outside water, Wb.
  2. Measurement of subject weight is entirely submerged in water, Wwb.
  3. Measuring the density of water in which subject is immersed, ρw.
The substitution of these values in the given formula helps estimate the body density. The body density is given by the relation,

Body Density, ρb = ( ρw X Wb ) / ( Wb - Wwb )
The use of hydro-densitometry is invariably done these days to determine the body fat composition in 'subjects' or 'patients'. This is done by estimating the body to be made up of two components―lean mass (comprising bones and muscles) and body fat. A high body fat content makes a body more buoyant because fat is lighter in density. At the same time, lean mass of the body is comparatively more dense than the water and makes the subject sink. Hence, people with higher fat in their body experience a greater weight difference in their weight underwater. Hydrostatic weighing, however, provides information about the body density only.
To find out the percentage fat in the body, either of the two equations, Siri or Brozek, is used.


Percent Fat = [(495 / Body Density) - 450] X 100


Percent Fat = [(4.570 / Body Density) - 4.142] X 100
Having known all about hydrostatic weighing for estimating the body composition of a 'subject', let us now take a look at the pros and cons of this method.
➼ There is no need to obtain the net body volume as this value is not required in the calculation method.

➼ The method has been proven to be of great accuracy over time.

➼ Its relatively higher accuracy makes it the 'Gold standard' for other testing methods of body fat.
➼ The total test time is very short and the entire test takes less than 15 minutes to be conducted.

➼ Talking of accuracy, the least possible error limit is +/-1.5%, which is the least among all the testing methods.

➼ There are no side effects experienced by any patient after this testing procedure.

➼ For 'subjects' on pacemaker support, it proves to be an alternative to the other electrical impedance-based test devices.
➼ The entire setup and equipment required for hydrostatic weighing have high initial cost.

➼ The testing setup, apart from being expensive, takes up quite a lot of floor space too. That calls for a more spacious clinical arrangement.

➼ The equipment required are not readily available to the multitude in general. Only special institutions like universities and medical services have easy access to these equipment.
➼ Once available, it takes specially trained professionals to operate the setup and carry out the testing procedure.

➼ The 'subject' under test is required to exhale all the air and sit idle for the test to be carried out. Also, the air bubbles around the subject needs to be eliminated as far as possible. This situation is not comfortable for most patients.

➼ The different levels of hydration in the 'subject' being tested may affect the error in test results to quite an extent.
➼ There is a possibility of considerable error introduced by the miscalculation of residual volume (RV), or the minimum residual air in the lungs left after complete exhalation. The value of RV has to be assumed to the nearest possible value to obtain more accurate results.
➼ Experimental studies done for the research on the accuracy of the test was based on Caucasian adults. The results for other ethnicity might produce a different percentage of error.

➼ The test does not identify the exact parts of the body where the fat is located.
Other improvised testing methods for body composition are being introduced with each new day. Although, they have their own advantages and disadvantages, the hydrostatic method proves to be the most acclaimed testing so far due to its higher relative accuracy.