There are a variety of measurement techniques for trying to determine body fat and lean muscle mass. All of them provide estimates…some better than others. All have some limitations and potential sources of error to be aware of. So, let’s briefly take a look at some of the most common and how to get the best result from each measurement.
Measurements can be done via site-specific measurements or indirect measurements.
Site-specific measurements include circumference measurements, biopsies, ultrasounds, CT scans, or MRIs. Circumference measurements are pretty crude as you cannot distinguish what caused any changes in measurement (fat loss/gain, muscle loss/gain, more/less water retention, etc.); they may be useful if combined with other measurement models. Most of the other site-specific measurements are generally less accessible for most people as they require a technician to perform them and/or can be costly, so we will focus on indirect measurements.
Indirect measurements of body composition analysis can be categorized into one of three basic models: two component (2C; measurement is broken into two categories: fat mass and fat-free mass), three component (3C; fat mass, fat-free protein mass, fat-free mineral mass), or four component (4C; fat, protein, mineral, and water). 4C models combine several measurement techniques
Note that fat-free mass is not just muscle; it includes all non-fat components of the body – organs, bones, fluids, and muscle.
2C models just divide body composition into fat mass and fat-free mass. They do not measure muscle mass on its own. Changes in fat-free mass between measurements could be the result of changes beyond muscle gain or loss – i.e. water retention changes.
This is the most basic indirect method. It is done by pinching external body fat and using a pair of calipers to measure the skinfold across multiple sites of the body (chest, triceps, abdomen, thigh, biceps, etc.). Some versions just take measurements from three sites; others up to nine sites. Generally, two measurements are taken at each site, and the average is used to enhance accuracy. The values are put into regression equations that generate an estimate of body composition.
If done by a skilled practitioner, studies have found it to provide a valid assessment of fat-free mass. Combine skinfold measurement with circumference measurement to give a bigger picture of progress.
Pros: It is cheap, noninvasive, convenient, and easily accessible.
Cons: Accuracy is highly dependent on the skill of the practitioner and the quality of the calipers.
Underwater Weighing (Hydrodensitometry / Hydrostatic Weighing)
This is considered the best of the 2C models. Underwater weighing is based on the principle that the body’s weight under water is directly proportional to the volume of water displaced by the body’s volume. Given that lean tissue is denser than water, and fat mass is less dense than water, a person with more body fat will have greater buoyancy and thus weigh less underwater than someone with a greater amount of lean tissue.
There are several potential sources of error that can negatively influence results. The primary one is that the person being tested must fully exhale and remain completely motionless throughout testing; not fully exhaling and staying motionless can substantially alter results.
Pros: It is the gold standard of 2C models.
Cons: You need a water tank. Fortunately, there are now companies and services that have the equipment to perform the measurement. Some services even have testing vehicles that allow for setting up mobile testing at various locations.
Although more sophisticated, like the 2C models, 3C models cannot distinguish between water and fat-free tissue proteins.
DXA (Dual X-Ray Absorptiometry) / DEXA (Dual Energy X-Ray Absorptiometry) Scans
DXA divides the body into total bone, lean soft-tissue mass, and fat mass. Particularly in research settings, DXA scans have become popular for their accuracy and a relatively low cost per scan.
However, it also has numerous potential sources of error. Different types of DXA units may provide different results. Accuracy also varies by population as factors like sex, size, and amount of body fat can bias results. Although less prone to water in the body than other measurements, it can be thrown off by food consumption and glycogen status; so, an overnight fast is often suggested.
Pros: It differentiates between bone and lean soft-tissue mass. It can estimate lean mass for the legs, arms, and trunk as opposed to just the entire body. As such it can estimate changes in fat-free mass in those areas.
Cons: DXA is a generally less available option as a licensed radiological technician is often required to operate the unit. Food and exercise restriction before the scan is needed.
Bioelectrical Impedance Analysis (BIA)
BIA emits a low-level electrical current throughout the body and then estimates composition based on the ease of that current’s flow through the body using the individual’s height, weight, and sex as factors. These days, there are medical grade devices as well as less sophisticated consumer scales and hand-held devices that do BIA.
As with most models, there are several sources of error to watch out for. BIA is highly thrown off by hydration. Like DXA, recent food consumption can throw off readings, so an overnight fast is also recommended. The type of unit may also give different results – hand-held units vs scales.
Pros: It is probably the quickest and most convenient method to measure with generally good validity.
Cons: The quality of devices and the algorithms used in the devices can vary. Food and exercise restriction before the measurement is needed.
All the measurements listed have some limitations and potential sources of error with each having pros and cons. Understand the limitations to maximize getting a more accurate reading.
Schoenfeld, Brad. Science and Development of Muscle Hypertrophy. Human Kinetics, Inc. Second Edition, 2021.