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Most people have experienced the temporary loss of feeling and tingling in their leg resulting from sitting in an abnormal position for a short while. Usually you get a loss of feeling in your leg while it is being compressed/constricted at some point and then the tingling sensation as the pressure is removed. But what is actually happening? I understand the the blood vessels are probably constricted from the pressure, but how does this lead to loss of feeling and later the strange tingling sensation? Are there any other things that extended compression on the leg does to cause this? What exactly are the requirements to get the sensation of a one's leg or other limb falling tingling in that manner?

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Whilst it's not particularly academic or explanatory, this article does give you a basic explanation in layman's terms: health.howstuffworks.com/human-body/parts/question552.htm - I too would be interested to know the details, though. –  Polynomial Apr 12 '12 at 18:54
    
This happens to arms, too. It hapens to my arms a lot during a sleep. I have feeling nerves are involved here not blood vessels. –  Andrei Apr 13 '12 at 13:19
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2 Answers 2

The explanation in the link Polynomial posted is essentially correct.

Whenever there is a reduced or blocked blood supply (ischaemia) to your extremities, the 'five P's' can occur: pulselessness, pain, pallor (colour), paresthesia (numbness) and paralysis (or weakness).(1).

The numbness and weakness happen after the blood flow have been reduced for a particularly prolonged period.

Cells in our body require a blood supply to stay alive (think about a stroke or heart attack for example). So a reduced supply can cause them to function abnormally or after a time (depending on the cell or tissue type) die.

So with a 'sleeping leg', staying in an awkward or particular position where arterial blood supply is blocked or reduced to the leg, the muscle, nerve tissue etc all lack supply hence causing sensory disturbance and weakness.

The possible buildup of metabolites could also contribute to the symptoms (pain).

Hope that helps!

  1. Miller's Anaesthesia - Miller.
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The linked explanation talks about squeezed nerves, and not about blocked blood supply. So I'm afraid your answer is actually incorrect: the arterial blood supply isn't blocked! –  Michael Kuhn Apr 23 '12 at 13:33
    
Check out the 3rd paragraph in that article Biocs - "This pressure can also squeeze arteries, stopping them from carrying nutrients to body cells." –  Zoidberg Apr 24 '12 at 2:51
    
The article states: "The pressure squeezes nerve pathways" and "This pressure can also squeeze arteries". Your answer does not mention nerves at all, whereas the article gives most weight to nerves. –  Michael Kuhn Apr 24 '12 at 5:52
    
Cheers Biocs. Good point. I haven't found an academic source in any of the textbooks or material I have access to regarding the nerve nerve compression mentioned in the article. I'll check it out and edit it accordingly. –  Zoidberg Apr 24 '12 at 9:51
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The feeling you describe is called "paresthesia," and according to the NINDS info page, it happens "when sustained pressure is placed on a nerve."

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