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  #1  
Old 07-19-2009
andreasl33 andreasl33 is offline
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andreasl33
Default Why do tense muscles make the body sink (or do they)?

I have heard that many times, but could not quite grasp the physics behind the statement

"Tense muscles make you sink."

What makes a body float or sink is its density (or mass/volume). Why would tense muscles increase the density of the body and make it sink? After all, virtually any matter our body is made of, is virtually incompressible for practical purposes. 80% of the body is water, which cannot be compressed by the power tense muscles can provide, the same goes for fat or bones. The only thing that is compressible by weak forces (that or muscles can generate) is air. But our body contains air only in the lungs. And as long as the muscles around the lungs don't tense up, I can see no reason why the above statement would be true.

Might there be some other reason? Tense muscles are not able to perform the movements as precisely as relaxed muscles and therefore one sinks? Or we get tired earlier, which slows us down and makes us sink?

Any comments welcome...

Last edited by andreasl33 : 07-23-2009 at 10:54 PM.
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Old 07-21-2009
Rhoda Rhoda is offline
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Quote:
Originally Posted by andreasl33 View Post
I have heard that many times, but could not quite grasp the physics behind the statement.
"Tense muscles make you sink."
...Tense muscles are not able to perform the movements as precisely as relaxed muscles and therefore one sinks? Or we get tired earlier, which slows us down and makes us sink?

Any comments welcome...
All of the above might contribute. Perhaps when you are relaxed you let the water support you more, whereas when you are tense you fight it.
Water is 880 times denser than air. That's good news and bad news. It's dense enough to support you if you let it, and dense enough to win every time if you fight it.
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Old 07-21-2009
atreides atreides is offline
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"A factor you CAN control is how relaxed you are. Tense muscles often keep your body a bit lower in the water, since many people breathe more shallowly and rapidly when tense (hence less air in the lungs for floating)."
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Old 07-21-2009
andreasl33 andreasl33 is offline
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Good point, which makes sense. I would not have imagined that tense leg muscles, for instance, will influence your breathing, but there may be subtle connections. Where is the quote from?
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Old 07-21-2009
atreides atreides is offline
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atreides
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Quote:
Originally Posted by andreasl33 View Post
Good point, which makes sense. I would not have imagined that tense leg muscles, for instance, will influence your breathing, but there may be subtle connections. Where is the quote from?
http://www.relaxnswim.com/physics/buoyancy.htm
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  #6  
Old 07-21-2009
andreasl33 andreasl33 is offline
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Nice link. Thanks for sharing.
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