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Submerging yourself to get body fat %?

  1. Jan 26, 2006 #1

    Pengwuino

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    Gold Member

    So exactly how did that whole idea of dunking yourself in a vat of water measure your body fat??? It sounds like some sort of prank!!
     
  2. jcsd
  3. Jan 26, 2006 #2
    Because you can calculate density. If we have mass, volume, and density, and we know the densities of fat vs normal organs and muscle, we can calculate your overall composition based on your density.
     
  4. Jan 26, 2006 #3

    Astronuc

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    Staff: Mentor

    Protein is slightly heavier than water, and fat is lighter than water.

    My body fat is low enough (5-7%), that my density is much greater than water, and my neutral bouyancy (with some air in my lungs) is probably a meter or so below the surface. My father on the other hand has always been able to float near the surface without effort.

    I noticed in a swimming class that I took in university, the women had little difficulty floating near the surface, whereas I had to use my arms or legs for some minimal thrust.

    See - http://www.annecollins.com/body-fat-calculators.htm

    http://mb-soft.com/public2/bodyfat.html

    Proteins have densities of ~1.2 - 1.4 gm/cc - http://www.proteinscience.org/cgi/content/full/13/10/2825
     
  5. Jan 26, 2006 #4
    In general, a healthy woman will have a higher % body fat than a healthy man (for fairly obvious reasons).
     
  6. Jan 26, 2006 #5
    Ouch that is a low bodyfat % to just walk around with. You must be a very active man or have a tweaked metabolism??
     
  7. Jan 26, 2006 #6

    russ_watters

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    Actually, no - because the air in your lungs gets compressed as you sink, there is no neutral buoyancy point. You either sink or float. That's a big issue in scuba diving - maintaining depth or sink/rise rate requires continuous adjustments of buoyancy. Neutral buoyancy points only exist for objects that are more incompressible than water.

    Anyway, I've been that way before, but I haven't been swimming in a while, so not sure if I am now....

    It was always a big problem on swim tests, though...
     
  8. Jan 26, 2006 #7

    Astronuc

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    One sinks until the density of the sorrounding water equals one's density, which I agree will have to do with the amount of air in one's lungs. I have participated in submersion density tests where I had to exhale as much air as possible. Even with a lung fully expanded with air, I am about 0.5 m or so underwater.

    I used to swim a lot as a teenager, and I used to swim underwater with a single breath for several minutes.

    Somewhat of both. I used to be much more active - running, football, cycling (primary mode of transportation in university), etc. Now I sit at a computer most of the day, and I have let my self go. :frown:

    Metabolism is probably higher than average, but I also moderate caloric intake accordingly.

    Actually my body fat has kind of crept up to about 7-9%, but I am starting to be more active and train for something I plan to do in about 2 years. I also hope to resume Tae Kwon Do and some other arts like Shotokan and/or Kempo.

    I am about the weight and size I was when I left high school more than 30 years ago, but down from my maximum. :biggrin:
     
  9. Jan 27, 2006 #8

    FredGarvin

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    It's actually considered to be the most accurate methods of body fat measurement. All of the other forms of measurement are compared to it. Guys like Covert Bailey have dunked thousands of people to develop databases of information and correlations.
     
  10. Jan 27, 2006 #9

    russ_watters

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    Well, right - except that the volume of air in your lungs changes with the surrounding pressure too, but faster than the density of the water changes - which causes your buoyancy to decrease as pressure increases.

    It isn't generally noticeable in a pool, but in scuba diving even only down to 20 feet, there is a noticeable acceleration as you go up or down. It's actually a little scary. When decending, you are told to counterweight yourself to be slightly positive at the surface, requiring a dolphin-dive and kicking to get down, but once you are down a few feet, you go negative and start accelerating. Then you start inflating your buoyancy compensator to keep your rate of decent constant.

    Once down at the bottom, deep breathing will cause you to rise or fall, and the biggest challenge for a new diver is maintaining depth. A small lapse and concentration and in just a few seconds you can find yourself zooming to the surface or the bottom. Instructors generally follow above the students so that if anyone loses control they can grab-em and keep them from shooting all the way to the surface.
    I suspect that another cause - such as surface turbulence - makes it appear that you have an equilibrium point. I am pretty dense too ( :uhh: ) and I have noticed the same phenomena.

    About.com has several articles about scuba diving: http://scuba.about.com/cs/skindivehowto/a/buoyancy.htm
    If your buoyancy stayed positive or increased with depth, you'd never get down.
     
    Last edited: Jan 27, 2006
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