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When a human stands in one place, he cannot stand still but we can see the body slightly swings.

Is this due to the Earth rotation or it has something with biology and muscles relaxations?

I have not been able to find a concrete documents on this thing.

sandalone
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    Is the human inebriated? –  Mar 25 '13 at 07:23
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    Is the human nervous? More seriously, the fields from the Earth - gravity plus centrifugal plus Coriolis and other forces - are almost exactly constant in time. So the human may either be able to deal with constant forces or not. If there are swings, the cause is clearly inside the human. Small changes of the tension in the muscles - due to small electric signals or variations of the concentration of sugars etc. - have observable impacts on the location of the bones etc. A way to answer is that the human muscles are surely much more volatile than the Earth's gravity+centrifugal fields. –  Mar 25 '13 at 07:23
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    I was in a big group of people and we listed for a talk standing. After some time I noticed that we all slightly swing. No one was drunk or nervous. they are really small swings, unnoticeable if you don't pay attention to them. –  Mar 25 '13 at 07:28
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    Slone, I agree with you, I am aware of such things, too. Even a perfectly non-drunk person is also unable to walk exactly along a straight path, the inevitable deviations are in centimeters etc. Humans aren't perfect and they're not dead. They swing although less than jellyfish. –  Mar 25 '13 at 07:31

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Although at first sight this might seem to be biology rather than physics, maintenance of human posture is an example of control theory. Indeed a quick Google will find many publications on it - I found a good review here.

A standing human is a dynamical system that is not in equilibrium so it requires a (closed loop) feedback system to maintain it. This means sensors in your body detect deviations from your desired posture and muscles contract to return you to it. The problem is that there is a lower limit to the movements you can detect i.e. you body moves a certain distance before your muscles respond and move it back. The result is that even when we try to stand absolutely still we are actually continually moving then correcting that movement.

Some people are better at maintaining posture than others, so some people can stand stiller than others. Plus the ability declines with age, as I can testify from personal experience :-)

John Rennie
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