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  #1  
Old 07-19-2009
andreasl33 andreasl33 is offline
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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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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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  #7  
Old 07-23-2009
elskbrev elskbrev is offline
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Default .

Interesting. I was wondering why I felt more bouyant in the 10' pool at the YMCA last June than in the 4' pool where I swim now through August. So, it was all in my head. Thought it might be water chemical composition, temperature or something. (YMCA pool--81 F, 4' pool--86 F)
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Old 07-23-2009
shuumai shuumai is offline
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I've found that when I relax my knees more, I can breathe easier and kick with more power. Perhaps I've just been holding too much tension in my legs by trying to keep them more straight. I'm sure the easier breathing comes from a slight change in body angle. That should cause more drag, but I noticed that I was moving faster when I reached the wall. Having a slightly upward body angle might work with my lack of flexibility in...basically, my entire body! I don't know. For all I know, I my head might have been too deep before.

I also noticed that my right knee doesn't relax as much as the left knee when doing breaststroke. When I do relax more, the whole stroke flows better.

In both cases, I do get a sense of better floatiness and ease.
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Old 07-23-2009
atreides atreides is offline
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Quote:
Originally Posted by elskbrev View Post
Interesting. I was wondering why I felt more bouyant in the 10' pool at the YMCA last June than in the 4' pool where I swim now through August. So, it was all in my head. Thought it might be water chemical composition, temperature or something. (YMCA pool--81 F, 4' pool--86 F)
Interesting point. I thought the same last year when I was taking lessons. I started out in 3.5 Ft pool and constantly thought my chest was going to hit bottom when I pushed off in a superman glide. This pool sloped from a very shallow "play" pool to about a 15 yd, 3 lane lap pool. The real lap pool in the complex was a consistently 4 ft ,25 meter pool. When I took lessons there even the instructor commented that I seemed more buoyant. According to Archimedes, buoyancy is all about the amount of water displaced versus the weight of the displacing object. If the water displaced weighs more than the displacing object, the object floats. If it doesn't, it doesn't. But maybe amount water present gives us the illusion of buoyancy. If a more shallow pool, you may displace the same amount of water but because there is less water, you are lower in the pool. In a larger body of water, the fraction of the water that is displaced is less (more water) and therefore the higher you float. Just a theory.
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Old 07-23-2009
shuumai shuumai is offline
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The overall amount of water shouldn't make a difference. One can float in a bath tub. (Though, at some small level, I wonder if there is a difference. If so, perhaps even the angle of the walls could have a small effect.)

I have a tendency to swim relative to the depth of the pool. I do that much less now though. For example, if the pool slopes from 1.5m to 3m, I might swim a little deeper in the deep end. If I swim underwater, I tend to maintain a distance from the floor instead of staying level, but that's partly to avoid surfacing too early. At first, my stroke would fall apart in deeper water, especially butterfly. Relaxing into buoyancy, thinking "forward," and just focusing on the stroke helps. Now the experience is more like, "Hey, look how far away the bottom is now." hehe
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