Body composition is an inexact science. The only truly valid way to measure body composition is by dissection and I’m very sure no one wants to participate in a validation study comparing method X to dissection. Therefore, you need to find a method that is reliable. Reliability is the how well a a measurement can be duplicated over and over again. Accuracy is really not as important as you may think. First of all, body composition measurement is something we use to track our progress in the gym. Even if our measurements are a few percent off from what our real body composition are, we can look at the changes that are occuring to our fat mass and muscle mass.
Therefore, we need a way to check our body comp frequently, reliably and conviently. Contrary to popular belief, hydrostatic weighing is not nearly as accurate as you may think. We have cadaver dissection data on 51 cadavers, but it is really not that comprehensive. Most equations that are used today for converting your body density measure from hydrostatic weighing to %body fat relied on 3 male cadavers ages 25, 35, and 46. It also can be quite unreliable. For example, if I was to measure your body composition first and then have you sit in a sauna and lose 1 kg of water and then measure you again; that 1 kg water loss would show up as a loss of .5 kg fat free mass and .5kg fat mass, even though it should be a loss of 1kg fat free mass. Also, unless you work at a facility or lab with a hydrostatic weighing tank you won't be able to be measured frequently enough and you also cannot do it yourself. Other methods such as DXA, Bod Pod, and various other nuclear techniques are also expensive, need multiple people, and cannot be done frequently.
So that leaves the typical body builder with 3 realistic choices. The best choice of these methods will be the one that is reliable (gives you the same measure day in and day out assuming that you are not actively changing your body comp), the one that can be done alone and the one that is least expensive.
Choice 1 is girth measurements and %body fat equations derived from these measurements. This does not work well for bodybuilders due to the constant increasing in girths due to increased muscle mass. It works well for other things just not keeping track of body composition.
Choice 2 is biolectrical impedance (i.e. the Tanita scales and hand held analyzers). This can be ok, but only if you measure yourself at the same time under the same conditions. Fluctuations in your hydration markedly affect your measurements. So this can work, but measure yourself at the same time of day wearing the same amount of clothes each time.
Choice 3 is skinfold measurements. This is probably the most misunderstood measurement. All skinfold measurements have been derived by regression equation compared to hydrostatic weighing. But whereas hydrostatic weighing is markedly affected by day to day fluctions in hydration, skinfold measures of %body fat are not that affected by hydration. Skinfolds are also inexpensive (Accumeasure makes a decent one that is less than $15). They can be done alone (use the 3 site equation at the web site) or be done with your training partner (use the 7 site equation). After you practice a bit, you will find that your measurement technique will get pretty good and thus you won’t have a lot a variability if you are only measuring yourself or your training partner. For the equations go to this great site that I got from a Kelly Baggett post: Bodyfat Calculators push page
For women out there, you are going to need a partner to do both equations (3 and 7 site).
In conclusion, most methods are expensive and really don't allow the bodybuilder to utilize them well because they are also inconvenient. Skinfolds allow you to track your body comp as often as you want for a minimal expense. And my final advice: pick one method or one skinfold equation and stick with it. Don't ever mix methods unless you want frustration and bad data. Good luck. - Jason N.