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It's nice to know when you're making progress in the gym. Especially if you're in it for the long-term, you want to know if things are going in the right direction. Is your hard work paying off?
While scale weight can be a useful tool for a quick data point, it only measures total body weight. The scale doesn't differentiate between muscle, fat, water, or other stuff. Plus, the scale is not consistent. On any given day, the scale can swing between 5-10lbs depending on fluid / hydration levels.
In an effort to quantify biometric changes during a fitness program, researches have spent years looking for the best ways to measure body composition. How much muscle is built during a strength program? How much fat is lost during a HIIT / cardio program? How much does water fluctuation influence body composition? These questions all lead to tests, studies, results, and more questions.
However, over the last 50 years, scientists have come up with a few accurate and reliable ways to measure body composition. While it's true that some tests may be more accurate and reliable than others, all of the tests listed here are considered "industry standard". Some tests may be more accessible for the average fitness enthusiast, other tests may be more accurate yet require a medical facility. Most tests have some trade-off between accessibility, reliability, and accuracy.
But before we look at the different body composition tests, I want to make sure we're all on the same page about what makes a good test. A few keywords:
Validity: Does the test measure what it's supposed to measure? Does a body composition test actually measure percent body fat, or is it counting the number of hairs on your head?
Accuracy: Does the test closely reflect the reality of what's being measured? If a test yields a result of 20% body fat, is that true? Or could it be 25%? Or 15%?
Margin of Error: How much wiggle room is there for the results? Margin of error can be increased exponentially if a test is difficult, or if humans are involved. Is the test easy to mess-up? Some tests have a margin of error of +/- 5 percent. Others may have a +/- of 2 percent. This can be due to a technician being needed to do a test, the subject not wearing the right clothes, or slight miscalculations in the math.
Accessibility: How easy and practical is this test? Does it require hours of preparation, or can you do it in a few seconds? Some tests require absurdly expensive lab equipment, while others require a piece of plastic. There's often a trade-off between accessibility and accuracy. Some tests have a good balance of both.
In general, the body composition tests I'm listing below all fit these parameters within a reasonable margin. That is, they all work well.
HOWEVER
These tests don't play well with each other. You shouldn't compare the results of different tests. Often, to get the "best result possible", many fitness enthusiasts MIX test results and try to compare the different results. This can lead to frustration because each test has a different validity rating, accuracy rating, and different margin of error.
If done correctly, test #1 may yield a result of 15% body fat, while test #2 yields 22%, and test #3 yields 12%.
That's a pretty big discrepancy! Which is it, 22%, or 12%?
This is why I suggest you pick a single method and STICK TO IT. If you do different tests, don't compare the results. Only compare the results within the same test.
Now let's look at the actual tests, the pros and cons, and decide which ones work best. It's a long list, so grab a beverage and buckle up.
THE TESTS
BMI
Body Mass Index is a super quick and sometimes useful method of measurement. Simply take your mass (in kg) and divide it by your height (in meters squared). Simple! This test is easily done, easily repeatable, and has nearly no margin of error, which is why the medical community uses it. However, BMI doesn't factor the difference between lean mass or fat mass, which makes it a little controversial.
BMI makes a few assumptions. First, it assumes that most people fit into the average bell curve of human anatomy. Unusually muscular people, or someone on the tail-ends of the bell curve, may yield an invalid result. For example, body-builders and muscular celebrities like Dwayne Johnson or Terry Crews are considered "obese" when using BMI.
Unless you're unusually muscular (and most people are not), BMI can be a useful tool for a quick assessment. I wouldn't put too much stock into it, but it's a simple and easy calculation.
WAIST CIRCUMFERENCE
Another simple and quick method of measurement, the waist circumference can be surprisingly valid. It won't give you a body fat percentage, but there are certain health standards for age and gender. Here's a nifty calculator.
Simply take a measuring tape and measure the circumference at the belly button level.
The margin of error here can be big because many people do it incorrectly. They either don't hold the tape correctly, squeeze the tape too tight, or have too much slack in the tape. Some people will type in their clothing size instead of the tape, which is also not correct.
Regardless, losing fat means this measurement is likely to go down over time. Take it for what it's worth.
CALIPERS
A more specialized test, body fat calipers are one of the more common and easily accessible measuring tools. They're cheap and simple to use. If done correctly, they're also extremely valid for measuring body fat, and are one of the more accurate measurements available to the average consumer.
Caliper tests work by pinching an area of skin and sublayer of fat, separating it from the muscle. The thickness of that area is measured (usually in centimeters or millimeters) and factored into an equation.
There are different equations you can use: A 3-site, 4 site, 7 site, 9 site, etc. The most common sites are hips, belly, and triceps. The more sites you use, the greater accuracy the final number will be.
The pros of this test are its validity and accuracy. When done properly, this test holds up against some of the more rigorous and expensive body fat percentage tests. However, many sites around the body are needed to achieve this accuracy. Plus if you get the site wrong, your measurements will be off. A second person or technician is needed for the hard-to-reach sites. This is where it gets a little awkward, because the technician will be pinching lots of ticklish spots including the triceps, thigh, upper back, armpit, chest, and so forth. It's not a test you'll do often.
Next, the margin of error on this test is okay, but ONLY IF the technician measures correctly. If the subject is flexing or "showing off" for the technician, the results will be skewed. The subject has to be completely relaxed and as jiggly as possible. This test also only measures the fat under the skin (subcutaneous), not the deeper layers of fat between organs (visceral fat).
The final margin of error is that someone with a higher body fat percentage will be difficult to measure, because too much material can be pinched under the skin and it becomes difficult to measure the actual thickness. Thus, the accuracy of this test improves as body fat percentage decreases.
Bio-Electrical Impedance Analysis (BIA)
This test is a mouthful to pronounce, but is one of the more common and easily accessible tests. It's also fairly valid and accurate - if the conditions are right.
You've likely done this test before. If you've ever stepped on a scale with metal edges, it will report various numbers, including body fat percentage. Other devices, like a handheld BIA, require you to enter your current weight, height, age, and gender into the calculation, and will report your estimated body fat percentage.
The theory behind this test is that the speed of an electrical current is impeded by different tissues. For example lean tissue, water, bone, or fat will all impede the speed of the current at different rates. A device will send a small (imperceptible) electrical current through your body and measure the speed. The BIA will then calculate the speed of the current, compared with your height, weight, age, and gender, and calculate your estimated body fat percentage.
This is a quick, easy, and fairly accurate method. It only takes a few seconds. However it has one major drawback - the analysis is HEAVILY influenced by hydration levels. Being dehydrated or overhydrated can affect the report by 2 to 5 percent. That's huge!
Hydration levels fluctuate throughout the day, and can vary widely between pre- and post- workout. This means you can use the BIA to measure pre-workout, and again post-workout, and have a drastically different bodyfat percentage between the two numbers. Post-workout usually provides a lower number due to dehydration.
Conversely, drinking a large amount of fluids immediately before measuring can INCREASE the percent fat results. This is likely due to the device measuring the excess water and assuming it's fat cells.
When using the BIA, make sure you're consistent with all the factors: proper scale weight, hydration levels, times of day, pre- or post-workout, and so on. Also understand that a 2-5% swing in either direction can be "normal", while nothing has actually changed.
Hydrostatic Underwater Weighing
Of all of the valid body composition tests, this one may be the most absurd. It's also one of the more accurate ones - if done correctly. Basically, dunk yourself underwater and exhale while being weighed. Simple in concept.
A technician will fill up a tank of water and measure your weight twice: Once outside of the tank, while dry, and again while underwater. Comparing these two numbers will yield approximate body mass. Because fat floats, Archimedes' principle of water buoyancy becomes extremely relevant. We can then estimate body fat percentage based on how much of your body is floating. Surprisingly, this test is pretty accurate. There are a few cons, however.
First, it's probably not something you'll do regularly. You'll schedule a visit to the local university, change into swimwear, dunk yourself underwater, and exhale every ounce of air you have in your lungs while submerged. That last part is the kicker. The technician is trying to read the scale (which is floating up and down) while you're exhaling ALL of your air out. This means the technician only has a few seconds to accurately measure your underwater mass before you drown.
The second con is that we operate under the assumption that all air volume was expelled underwater. This often isn't the case. Most people don't feel comfortable blowing bubbles underwater until they pass out. That's understandable.
Additionally, as a former grad student myself, I can say with authority that reading a moving scale while a subject is struggling to hold still underwater is more difficult than it sounds. If you're off by a few pounds, you'll throw off the test results.
All-in-all, not a very accessible test, and the margin of error is large enough to pass on this one.
BodPod
Air-displacement plethysmography, or the BodPod, works nearly identically to the underwater weighing method. The principles and equations are all the same. The BodPod, however, uses a relatively comfy egg-shaped chamber to measure air displacement - rather than dunking you under water.
A few of the "problems" with underwater weighing go away when using the BodPod. Mainly, drowning is no longer an actual concern. A big drawback, however, is that BodPod tests can be expensive because the equipment is expensive. You won't find many of these at your local gym. Some high-end facilities, like the Walton Life Fitness Center, have one and it's relatively cheap.
However you still have to schedule a time, and you still have to wear a speedo.
DEXA
Dual-Energy X-Ray Absorptiometry is about as tough to pronounce as it is to actually do. You'll need a medical table, two high-energy x-ray beams, and a technician who can operate it all.
This test shoots your body with two different x-rays: one to measure bone density, and another to measure soft-tissue. These x-rays are absorbed differently and the technician can get a read on your bone mineral density, lean body mass, fat mass, and what you ate for lunch.
While this test may be insanely in-accessible for most people, it's also one of the most accurate tests possible. This can read your body composition with a margin of error less than 1%. It can even detect the differences between individual limbs. Plus you don't have to do anything. Just try not to fall asleep on the table.
Unfortunately, this test is hard to find. You'll need access to a medical facility or human performance lab which has one. It's also (usually) extremely expensive. These machines start at ~$50,000 refurbished. But don't worry, they pass that price on to you, the consumer.
3D Scan
More and more high-end facilities are using advanced 3D-imaging to determine body composition. Companies like Fit3D are leading the pack with readily-available scanners across the US, and many gyms are getting in on the action.
Like a DEXA, the 3D scan is surprisingly accurate. Maybe not as accurate as a DEXA, but not far off. A typical 3D scanner will have you stand semi-naked on a rotating platform while it takes 3000 high-res pictures, measures your weight, scans body dimensions, and does a BIA test. With these images, it composites a 3-dimensional image of your body showing everything from your biceps circumference to your posture.
It then emails you a rather embarrassing full report.
This report consists of weight, height, body dimensions, BMI, body fat percentage (via bio-electrical analysis cross-referenced with body dimension calculations), posture analysis, and even foot-pressure placement. Yeah, it measures how you stand and WHERE in your feet you balance your weight.
It's a pretty neat machine.
It has a few drawbacks, namely that not every gym has one. However some companies make home-models that are not too insanely-expensive. Another major drawback is that you basically have to be naked, or as naked as possible, for the machine to work correctly. If you're wearing loose clothing, the images won't pickup your body dimensions. This will throw off the calculations.
If this test is available to you, it's probably the most accurate and comprehensive option. It's also not terribly expensive, but you probably won't do it every week.
CONCLUSION
If I had to rank all of the above valid tests, I would put them in this order starting with the best:
Bioelectrical Impedance Analysis (BIA) - Quick, very accessible, kind of accurate.
3D Scan - Maybe accessible, extremely detailed and very accurate.
BodPod - Maybe accessible, very accurate.
Calipers - Very accessible, somewhat accurate if done correctly.
DEXA - If you can find one, do it. Probably expensive.
Waist Circumference - No bodyfat percentage, but easy and accessible.
BMI - No bodyfat percentage, but easy and accessible.
Hydrostatic Weighing - Not very accessible, kind of accurate, don't hold your breath.
Just remember that whichever test you choose, only compare the results within the same test. The BodPod results can't be compared to the caliper results, and the BIA test may have a large margin of error compared to the 3D scan. For your sanity, pick a test method and stick with it!
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