The measurement of body composition is an inexact science, and outside the laboratory, it becomes an even more inexact science. Many times, I've checked into the T-mag forum and seen a thread started by some frustrated guy who just got his body fat measured. The last time he got pinched by his trainer, he was 10% body fat and now his brand new bioimpedance scale says that in reality, he's 20% Crisco.

Confusion about body composition measurements is one of the most common problems seen by beginner, intermediate and advanced bodybuilders. One month you get measured by hydrostatic weighing, the second month you get your skinfolds pinched, and the third month you grab on to your handheld bio-impedance device. What's the end result? A lot of measurement variability and frustration. Am I the only one here who sees the problem?

In an effort to find an accurate, reliable and convenient way to measure body composition, most people shoot themselves in the foot by mixing methods. So before I go any further, if you take just one lesson home from this article, let it be this: Pick one method and stick with it for the duration of your training career. Later in this article, I'll explain the various methods and their pros and cons, but always remember to pick one method and stick with it (unless you want to be the next frustrated poster in the T-mag forum)!

Unfortunately, the only really accurate way to measure body composition is by dissection. I'm absolutely sure no one wants to participate in a validation study comparing the new "superduperfatologic measuring device" against dissection. The next best option is to get into a research study that uses a multi-compartment model, which combines measurements of body density from hydrostatic weighing, total body bone mineral from dual energy x-ray absorpitometry, and total body water from deuterium oxide dilution. I can assure you that it's not very likely and also not very practical. Sure, you get an accurate measure of your body composition, but what are you going to do next time?

If you plan to be in the iron game for a while, composition measurements are a must. The mirror is nice and pictures are helpful, but numbers, (i.e. pounds of fat and muscle), tell the true story of how well your diet, training and supplementation are working. In order to get these numbers, you need to find a method that's both convenient and reliable. In other words, it should be a method that's both a) readily available and b) can be duplicated over time.

What about accuracy? This isn't nearly as important as you may think (although it is vexing when a tubby comes up to you and says he's holding tight at 5% body fat). In fact, it's probably the least important consideration (after convenience and reliability) when tracking body fat measurements. Let me explain.

Let's consider how we count our calories. We rely on labels to tell us how much protein, fat and carbohydrates are in the foods we commonly eat. I've yet to meet a person who walks around with a bomb calorimeter and literally burns up an equivalent portion of everything he eats in a day just so that he can have an accurate measure of his daily calories. That would not only be expensive, but that would take the "excessive" nature of bodybuilding to a whole new level (as if overgrown men parading around in thongs isn't enough already). We rely on food labels because they don't change; they're reliable day-in and day-out. By tracking our calories in a diet log, we can see if we're eating too much or too little in accordance with our observed weight changes.

Would we have to go back and change the last three years of food logs if we found out that our daily can of tuna was really 200 calories rather than 175? NO! What if we found out that in reality, all food labels are low by 10%? It wouldn't matter, because we've been living in a world where we adjusted to the fact that the labels are low by 10%. It really does not matter if the label is in reality off by a little as long as the label itself is consistent and we've adapted our food intake to suit our goals. So, why are people so worried about getting the most accurate body-composition measurements possible? You need to find a technique that can track your body composition changes over time. It doesn't matter if it measures your body fat 5% higher than what dissection of your body would show, as long as it is always 5% higher. Are you still following? Keep going; it only gets clearer.

First of all, body-composition measurement is merely a tool you use to track your progress in the gym. Even if your measurements are a few percentage points off from your actual numbers (and your methodology is reliable), you should be able to monitor your progress via the changes you see in both your fat mass (FM) and fat free mass (FFM). This becomes incredibly important for anyone who desires to drop fat and/or to pack on muscle. You better know the composition of the weight that you're dropping or adding. How else are you going to know how to adjust your diet, supplementation and/or cardio? If you're losing three pounds of lean body mass a week, it's pretty obvious that it's time to increase your total calories or time to start adding Methoxy-7, MAG-10, or 4-AD-EC to your supplement arsenal.

So whether your goal is to lose fat, gain muscle or both, you're going to have to track those changes. Don't be bogged down in what you've heard about the accuracy of various body composition measurement tools. Simply find a method that allows you to check your body composition frequently, reliably and conveniently.

For the purposes of this article, I'll focus on the methods you're liable to have access to. These include:

Hydrostatic (underwater) weighing

Bod Pod

Girth measurements

Bioelectrical impedance


There are many other methods including dual energy x-ray absorptiometry, total body electrical conductivity, radioisotope tracers, 40K counting, and nuclear imaging. But who has the time, money and access to these machines? Not many, and in reality, they don't offer much of an advantage over other available methods.

Is that a hydrostatic weighing tank or is there a hot tub in this lab?

If you walk down the hall in the university's exercise physiology lab, you'll probably find an old school, redwood-frame hot tub. Well, it's only a hot tub for those lucky grad students with keys to the lab. For everybody else, it's a hydrostatic weighing tank. The tank is usually heated to about 97-98 degrees, so it'll be consistent with total-body temperature. Other than being a comfortable place to take a soak and regenerate your sore muscles, what important purpose does this tub serve?

Hydrostatic weighing (HW) relies on Archimedes principle to measure body volume (Vb). Archimedes's principle states, "the volume of an object submerged in water is equal to the volume of the water that the object displaces" (1) and "a body immersed in fluid is acted upon by a buoyancy force, made evident by a loss of weight equal to the weight of the displaced fluid" (2). When applied to HW, the difference between a person's mass in air (MA) and mass submerged in water (MW) is the weight of the water displaced. The volume of this displaced water is obtained by dividing the mass of the displaced water by the density of the water (DW), which is based on the temperature of the water (3). The equation for Vb is as follows:

Vb = (MA - )/DW

Still with me? If not, drink a serving of Power Drive and then go back and reread the last paragraph. Okay, ready? Next, an additional volume that contributes to Vb must be taken into account when using HW. This additional volume, which does not contribute to overall tissue density, is the amount of air in the lungs after a maximal expiration. It is referred to as the residual volume (VR). The VR must be measured and subtracted from the Vb for the calculation of body density (Db) (3).

Db = MA/(- VR)

The Db is then plugged into an equation used to predict percent body fat (%BF). An example of this is:

%BF = [(/ Db) - ] * (4)

This conversion of body density to %BF is where many of the problems with HW begin. Contrary to popular belief, HW isn't nearly as accurate as you may have heard. To demonstrate this potential for inaccuracy, let's look at the four basic assumptions regarding HW (5):

The separate densities of the body compartments are additive.

The densities of FM, FFM and all FFM components are known and constant for all persons.

The components of FFM are proportionally constant in all persons, therefore,

Proportional differences from the "reference body" are solely due to differences in FM quantity.

Assumption #1 is true. All the components of the body add up to 100%. Assumption #2 presents a few problems, but they're minor in comparison to the significant problems seen in Assumption #3. Recall that Assumption #3 states: the components of FFM (water, fat-free muscle, mineral, and to a lesser extent, glycogen) are proportionally constant in all persons. These assumptions were derived from just three male cadavers ages, 25, 35, and 46 whose data was merged to form the "reference body" (4).

We currently have data from 51 cadavers collected over the past 150 years. The majority of the cadavers were males and females that were very old or died from chronic disease, and their average ages were 56.6 and 75.5 respectively. However, FFM composition was determined on only 5 of these cadavers. Are you ready for something that makes no sense? These five cadavers don't agree with the above "reference body" assumptions, even though the "reference body" was derived from 3 of these 5 cadavers (6,4). Therefore, assumption #3 and #4 can't be true.

If you believed all the above assumptions, you'd still believe that the only proportional difference between you, your grandmother, and Ronnie Coleman is that the three of you only differ in your proportions of body fat. Think about it! Do Ronnie Coleman's bones grow in proportion to his huge increases in muscle mass and water? No! I could go into much more detail, but you get the point. Assumptions #3 and #4 can be problematic.

The problems seen with HW's accuracy also spill over into its reliability as a technique. The most common change among bodybuilders is fluid variation. If you've been in the iron game for any amount of time, you know the importance of adequate hydration. In other words, you drink a ton of water and experience a fairly large shift in fluid balance from hour to hour. Why is this a problem? A one-liter change in total-body water (TBW) (which is very easy to do and quite common) can lead to a 0.7% change in percent body fat (7).

Other problems with HW include that fact that it's expensive; it's problematic to have it done frequently; it's inconvenient; and few places have the equipment to measure residual volume. This lack of equipment is significant, as a variation of 600 ml in the calculation of VR can produce a change of up to 8% in the final %BF calculation (8). As you can see, there are several reasons why you should avoid HW.

Nanu! Nanu! Welcome to the Bod Pod

Anyone remember the TV show, Mork and Mindy? When you see the Bod Pod, images of Robin Williams shouting "Nanu! Nanu!" will instantly come to mind. The Bod Pod is a large egg-shaped device that also measures body volume. Instead of using water displacement, air pressure changes are used to determine the body volume of the subject contained in the egg. Body weight is divided by body volume, giving you body density.

Body density is plugged into the same equations that HW uses. Therefore, all of the "reference body" problems that were seen with HW will be seen with the Bod Pod. The Bod Pod is also sensitive to changes in ambient room temperature and pressure. Basically, it needs its own room to work optimally. The Bod Pod is also expensive, inconvenient and very uncommon. Plus, you have to wear a speedo and swim cap while you're in there.

The Old Fashioned Tape Measure

Checking girth measurements and deriving %BF equations from these measurements is the cheapest method used to determine body composition. Trouble is, it doesn't work well for bodybuilders because no differentiation is made between increases in girths due to fat gain versus muscle gain. The 20-inch arms belonging to Krispy Kreme patrons are 20-inch arms alright, but are quite different from the "Great Guns" earned after Ian King's 12-week program.

Variations in girth measurements can also happen without a change in fat or muscle. Fluctuations occur frequently with fluid variations and/or changes in muscle glycogen content. Another consideration is the amount of tension on the tape measure. There are a few different models that are designed to control the amount of tension with each measurement and these would be a must for any type of reliable girth measurement.

Overall, this is a technique that has minimal research behind it due to its weakness as a good predictor of percent body fat. For these reasons, I don't feel it is necessary to include an example equation. It may work well for other things like assessing risk for coronary heart disease or fitting pants, but it's not ideal for tracking body composition.

Bioelectrical Impedance – Is This Going To Shock Me?

Another popular method is bioelectrical impedance (i.e. the Tanita scales and hand held analyzers). Bioelectrical impedance devices measure the body's resistance to an electrical current. Unless you have a pacemaker or some other electrical implant, you won't feel a thing. If you look closely at a Tanita™ scale, you'll see four metallic electrodes. Two of those electrodes generate a voltage and the other two measure the residual voltage that remains after the current passes through the body.

The ratio between the starting and ending voltages is called impedance. To determine how to convert impedance values into a %BF, validation studies were done with HW or better yet, multi-compartment body composition models. The impedance value was combined with things like weight, height, and sex to create a complicated algorithm that converts all these factors into a percent body fat. (9) Okay, that may sound reasonable, but here are some problems that we run into when we use these scales.

The problems are primarily caused by changes in total-body water (TBW). TBW is assumed to be about 73% of your FFM. Any changes to this quantity of water or to its electrolyte content will change your impedance and thus change your %BF value. So, assume that this device is measuring your TBW and that TBW is 73% of your FFM. It then calculates FFM and subtracts that mass from your body weight, leaving you with a measure of your fat mass (FM). You divide FM by body weight and that gives you a percent body fat value. If you remember the problems with HW Assumption #3, they all still apply here.

Bodybuilders and athletes are nightmares to deal with when it comes to assumptions regarding fluid balance. I drink at least two gallons of fluids a day. The people used in these studies probably drank less than a half-gallon per day. The reason this method may be better for the bodybuilder or athlete is that it's convenient, requires no third-person assistance, and isn't very expensive (aside from the initial $100-150 investment).

Impedance measurements can also be altered by changes in skin temperature, skin moisture, body posture and much more. To maximize reliability, you need to find the time of day when your skin temperature, TBW, and skin moisture are the most consistent. If you don't work out in the morning, then it's pretty easy. If you have a Tanita™ scale, wake up, go to the bathroom and hop on with no clothes on or with the same type of clothing on every single time. It's okay to measure in your underwear, but remember: clothes add to your weight and can artificially change your percent body fat. After you have this measurement, get another measurement after you've taken your morning shower and completely dried off (hair also). Track the numbers over a couple of weeks and then just use the one (before shower or after) with less variability.

Tips for Bioimpedance Measurements

Take 3-5 measurements each time you step on the scale and average the measurements except for the first. (Drop the first measurement because the metal electrodes are cold and often it takes most people a minute or two to adjust to not wearing any clothes.)

Measure yourself every day and then average the measurements over the course of the week for tracking purposes. Day to day variations will drive you nuts, but a weekly average will be very reliable. I've had great results with this technique and it correlates very well with my skinfold data.

In summary, bioelectrical impedance may not be the best choice, but with a little manipulation, it can work very well.

Next week, in Part 2 of this article, Jayson Norcross will give an in-depth tutorial on how to perform the most accurate body composition method currently available: the good ol' skin-fold test.