Steve: you are right in saying that the Bod Pod measures air displacement. In reality it meesures pressure differences between two chambers and calculates air displacement.
Regarding HW, it relies on Archimedes Principle: The volume of an object submerged in water is equal to the volume of the water that the object displaces. Also, 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. When applied to HW, the difference between a person’s mass in air and mass submerged in water 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, which is based on the temperature of the water.
The equation for body density (Db) is as follows: Db = Ma/[(Ma – Mw)/ Dw]
Where Ma is your body mass in air. Mw is your mass in the water. Dw is the density of the water.
In general, the leaner you are, the more you will sink and the fatter you are the more you will float. Using the standard assumptions, water has a density of .9937 g/ml at a temperature of 36 degrees celsius (what most HW tanks are set at and also what the mean body temperature is), fat has a density of .9 g/ml and lean body mass (muscle,water,bone)has a density of 1.1 g/ml. Thus the more lean body mass the more you sink in water and the more fat mass the more you float.
Hey guys, I’m working on the article, but I was curious if there were any other questions about body composition. I want to include as much info as people want to know. Thanks a lot.
“Therefore, we need a way to check our body comp frequently, reliably and conviently” why??? don’t mean to sound like a prick but WHY??? if, as most on this site are, you are training for looks what difference does it make? if you think you look good and are happy with how you look what does it matter what your bodyfat is? if your overwt. just cut your cals. until you look how you want. if your skinny just add cals and train right untill you look how you want. I don’t understand what difference it make what some machine or calipers say. just my opinion.
Answering these raves and rants is getting exhausting, but worth it.
I measure Body Comp for the following reasons:
To measure the effectiveness of my diet and workout program. WHILE IN THE LONG RUN, “how we look nekkid” is the bottom line (for those interested in Physique Enhancement), you can end up in 8-12 weeks WORSE than you started because you didn’t monitor. In the short term, “how you look” is a poor measure.
2)To measure the ratio of fat to lean body mass recomposition that I’m undergoing. What is it that you’ve “lost”? What is it that you’ve “gained”? Again…at the end of 8-12 weeks, you can be nowhere but frustrated.
Another point. It’s a piss-poor argument when someone says “in the past they did this or did that…without body comps, and this program, or that supplement…and THEY built great physiques!” …right…and there were literally millions more who never reached their potential of either health, fitness or esthetics.
I agree with Mufasa. Measuring my body composition is the only way that I can really look at short term results. I may not be able to see the 5 lbs of muscle I’ve gained or the 5 lbs of fat I’ve lost, but if I measure that loss it provides encouragement to keep training and also lets me better evaluate certain diets, training routines and supplements.