February 16


Are Body Composition Analysis Methods at the Gym Accurate?

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When it comes to measuring body composition, you generally have two options available at the gym. There are sometimes BIA - Bioelectrical Impedance Analysis machines and some trainers offer skinfold assessments. Both body composition methods are fairly simple and convenient to do in almost any gym, but are they accurate?


Gym Body Scans - Body Composition Measurement by Bioelectrical Impedance Analysis

You've likely seen these at the gym or even been invited to take part in a scan.

BIA scans use a portable device that basically sends a small electrical signal through your body. The machine measures the amount of resistance that the electric current meets as it passes through you.

Since muscle tissue contains more water than fat, electricity passes more quickly through muscle than fat. After all, water is a good conductor of electricity.

In a more muscular person, the BIA machine will detect the electrical current moving quickly. In someone carrying more fat and less muscle, it'll detect the current moving more slowly.

The machine then calculates body composition, or body fat percentage, based on the detection of the electrical current.

Hydration levels ultimate determine how accurate a BIA body composition measurement at the gym is going to be

Since BIA scanners measure the resistance of an electrical signal and water affects that resistance, the more water your body has, the quicker the signal will travel.

Don't be fooled - this water doesn't necessarily have to come from muscle tissue.

How much you've had to drink, and your hydration levels as a whole, will greatly affect your BIA reading. Likewise, if you've recently eaten, food can affect bioelectrical impedance. Furthermore, different types of food can affect readings in different ways.

And it's not just water that's important. Sodium can be an important factor in electrical conductivity too. If you've had a salty meal recently, or are well topped up with electrolytes, there will be plenty of electrically charged ions in your cells ready to carry that BIA signal quickly.

All of these factors will cause the machine to think you're leaner than you really are. The BIA scanner will detect the current moving quickly and attribute this to muscle mass. Whether that's the case or not. The machine has no way of knowing if the signal is travelling through muscle, well-hydrated cells, or an abundance of sodium ions.

Conversely, the opposite is also true. BIA machines can underestimate muscle mass even in someone with an athletic physique if they are dehydrated. Or lacking electrolytes, etc.

Furthermore, if you've recently trained, or your sports nutrition isn't on point, BIA readings can also become inaccurate.

For example, a heavy training session may use up stores of glycogen in your muscles. If your carbohydrate intake has been low, these stores won't replenish quickly. Since muscle glycogen is stored in association with water, if your muscles contain less glycogen, they contain less water.

And that lack of water may cause the BIA scanner to think you're more fat than you are.

How to make a BIA body composition scan at the gym more accurate

So, to make a quick body composition scan at the gym super accurate, you'd need to:

  • make sure you've eaten the right kind of food
  • but not too much
  • ensure good electrolyte levels in your cells
  • but not overly so
  • empty your bladder
  • have good muscle glycogen stores.

Then, after 6 weeks, if you want another reading to compare, really you should make sure you measure under the exact same conditions! Same meal beforehand, at the same time, same muscle glycogen levels and drink the same amount.

Not very practical I'm sure you can see.

When measuring gym progress, BIA machines aren't accurate when tracking body composition changes over time.

Skinfold measurements to track body composition

As you may already know, I'm a huge proponent of skinfold measuring.

Skinfolds can be extremely useful and accurate when tracking changes to your body composition, in and out of the gym.

But only if you understand how to use them properly.

Essentially, when we measure skinfolds we are measuring the subcutaneous fat at specific spots of your body. That is to say, the layer of fat that's underneath your skin.

We do this at standard sites, based on anatomical landmarks to ensure accuracy and reliability.

We can then total these measurements to give you a 'sum of skinfolds.' An important and useful tool, if used properly.

Using sum of skinfolds to accurately track body composition and gym progress

Once we know your 'sum of skinfolds' we can track it.

That is to say, we can remeasure it over time and compare readings with your previous measurements.

In doing so, we can assess changes in your body composition fairly easily.

If your sum of skinfolds increases over time, you've gained fat. Conversely, if your sum of skinfolds decreases, that indicates fat loss.

Once you convert skinfolds to body fat percentage, you lose accuracy

There are numerous equations that convert skinfold measurements to body fat percentage.

For reliable and accurate date, most professionals recommend you don't do that.

You see, these equations are derived from population-specific formulae. That is to say, each particular group of people require their own set of equations. Different sports, ethnicities, age groups and gender all require different equations.

Even then, these equations are based off assumptions and estimates.

To keep your body composition analysis accurate, it's best to stick with the sum of skinfolds number itself.

Limitations to using skinfolds to determine body composition

The good news is that skinfolds aren't subject to the same fluctuations as BIA.

Hydration, food intake and salt levels have little effect. That being said there are a few drawbacks.

Skinfold measurements can't really give you an accurate reading for absolute muscle mass. That is, they can't tell you how much muscle you have. Generally speaking though, that doesn't matter.

To ensure the measurements are reliable, it's important to use a very specific and precise technique. To avoid errors, it's recommended you have skinfolds measured by an ISAK Accredited Anthropometrist

Regarding reliable technique, skinfolds are best taken before exercise. That means, if you want accurate body composition analysis, it's best to do it before your gym workout.

When you exercise, fluids travel towards the surface of your body. For example, to allow you to sweat. Additionally, blood flow to the muscles and skin increases too. This can cause increases in perceived skinfold thickness.

Does it matter if your gym body composition measurement isn't accurate?

If you track body composition changes using unreliable measurements, it can interfere with your progress. Particularly, if your chosen sport or hobby requires a certain appearance, low body fat or weight category.

For example, bodybuilders may look to maximally increase their muscle mass while at the same time reducing their body fat to as low as possible. A boxer or MMA fighter may not be able to compete if they are over a certain cut off weight. However, if they lose muscle, they lose strength and may not perform as well.

If you don't detect changes early enough, you may not make the necessary adjustments to your diet or training.

Likewise, you may become frustrated if your readings and progress seem all over the place. Despite your hard efforts in the gym, your body composition report may show no change if it's not accurate.

Lastly, unreliable measurements can result in poor body image and low self-esteem. As a result, athletes can fall into patterns of over-restriction and have a poor relationship with food.


If you or your gym buddy is considering body composition analysis, make sure it's accurate!

To give the most reliable data, the recommendations are to:

  • have skinfolds taken under similar conditions each session;
  • use an ISAK Accredited Anthropometrist;
  • ensure the anthropometrist uses the same calibrated equipment each time;
  • interpret results in the context of training and nutrition along with other measurements;
  • for females, understand how hormonal changes and the menstrual cycle may affect measurements.

As an Accredited Sports Nutritionist and ISAK Accredited Anthropometrist, I'm happy to help you with skinfold measuring. If you're looking to safely measure and track body composition changes and live in Perth, get in touch.

Paul Stokes Perth Personal Trainer Sports Nutritionist Group Fitness Instructor Massage Therapist

Body Composition Assessement and Skinfold Measurement in Perth


About the author

Paul Stokes

Paul Stokes BSc (Hons) is a Certified Personal Trainer, Accredited Sports Nutritionist, qualified Exercise to Music Instructor, Precision Nutrition coach, Massage Therapist and teaches 8 of the Les Mills Group Exercise programs.

He currently works in the Oil & Gas industry as a Wellness Coach, imparting his vast knowledge and experience to improve the quality of life of several hundred offshore workers.

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