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Anyone who swims or learns how to swim must have experienced this simple observation. When the muscles in our bodies are relaxed, we tend to float; whereas stiffness/tensed body tend to drown.

So what physical principle explains this observation?

This question is not about the increased buoyancy from air inside the lungs. My own guess would be that relaxing the body increases the surface area, but does it increase enough to cause such a noticeable effect?

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  • $\begingroup$ My observation is that when I push air toward the bottom of my lungs, my center of buoyancy moves closer to my center of mass, which helps me float horizontally. Probably relaxation accomplishes the same thing. $\endgroup$
    – S. McGrew
    Jun 8 '18 at 14:03
  • $\begingroup$ Increased surface tension? $\endgroup$
    – Anurag B.
    Jun 8 '18 at 14:10
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    $\begingroup$ When I was young and skinny, it was impossible for me to float in fresh water. I could not do it even with a full breath of air. $\endgroup$ Jun 8 '18 at 14:24
  • $\begingroup$ @S.McGrew: I am not really sure about that. Hence, the question. $\endgroup$ Jun 8 '18 at 14:27
  • $\begingroup$ @AnuragBaundwal: I'm looking into it. But my initial suspect says that there shouldn't be a huge difference in the surface tension of water around a tensed/relaxed arm. $\endgroup$ Jun 8 '18 at 14:27
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Increasing surface area has no effect on buoyancy. Changing volume does have an effect. When I was young and skinny, I could sink to the bottom if I just let most of the air out of my lungs. But I still couldn't float properly even with full lungs until I learned to push the air in my lungs closer to my center of mass. If you don't have enough lung capacity to fill your lungs with air and thereby reduce your mass-to-volume ratio to less than that of water, you will sink. If your buoyancy is slightly positive and your center of mass is below your center of buoyancy (tends to be somewhere in the lung area), then your feet will sink and the top of your head will stay just above the water surface.

Most people tense their stomach muscles when they're not relaxed. If you relax your stomach muscles, your diaphragm can move downward (that is, toward your navel) and thereby move your center of buoyancy the same direction. Alternatively, if you stretch your arms above your head (that is, away from your navel), you're moving your center of mass upward toward your center of buoyancy. Full lungs, arms above your head, and relaxing your belly muscles should make you float just fine.

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  • $\begingroup$ Your response kind of explains the mechanism of floating, and that is fine. What I'm more interested in is that this floating while relaxing can also be observed for individual body parts like left arm, right leg. Clearly, these body parts are not involved in air management? $\endgroup$ Jun 10 '18 at 5:28
  • $\begingroup$ @topologically_astounded, Are you saying that a relaxed arm floats better than a non-relaxed arm? Or that relaxing one's arm makes it easier to float the whole body? $\endgroup$
    – S. McGrew
    Jun 10 '18 at 13:23
  • $\begingroup$ From my personal observations alone and with no reference (that I know of) to back my claim, yes that's exactly what I'm saying. A relaxed arm in my experience, in water with no current found itself floating upward, as opposed to a tensed arm! I wonder if, and how much does, that sound weird? $\endgroup$ Jun 11 '18 at 8:14
  • $\begingroup$ It sounds weird. A proper experimental study would need to measure what "tensed" means to an arm. I can only guess that it includes tension in the shoulder and chest muscles that control the arm's position relative to the body. This would affect the arm's ability to float freely. On the miniscule chance that tensing affects the arm's volume and thus its density, one could try to measure the volume before and after tensing. If you can hold your breath long enough: float vertically, see how much of your head sticks out, then tense without moving and see what changes. $\endgroup$
    – S. McGrew
    Jun 11 '18 at 13:10

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