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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
    
Your circulation is being cut off when this occurs. –  L.B. Sep 15 at 0:36

3 Answers 3

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
    
I wouldn't recommend deletion of an answer just because it is not exactly what the OP is looking for. The information might be of use to others. –  The Last Word Sep 15 at 4:15

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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Yes, a phenomenon also dubbed "saturday night palsy" when you nursed the bottle a little too closely ;-) –  Raoul Sep 15 at 12:38

Numbness is caused by cutting off circulation (that's why hand surgeons can use a tourniquet to numb your hand for a carpel tunnel release procedure). The painful tingling occurs upon RESTORATION of circulation. It's called REPERFUSION INJURY, and here's the mechanism:

  1. pressure on the back of the leg reduces venous return (i.e. via popliteal vein)

  2. reduced venous return = fluid backup in veins, capillaries, and arterioles

  3. poor circulation ---> hypoxia in tissues

4.a. hypoxia causes cell injury/stress, and reduced ATP synthesis (without O2 cannot run electron transport chain)

4.b. in the cells toxic buildup of anaerobic metabolites, adenosine, oxygen radicals, and nitrogen radicals ensues

  1. restoration of circulation causes reactive hyperaemia (google it for more info)

  2. stressed cells release all built-up nitrogen and oxygen free radicals into tissues ---> causes painful sensation in the extremities.

  3. superoxide dismutase, catalase, glutathione peroxidase, and other radical scavenging enzymes dispose of the radicals, preventing long-term injury

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It would be fantastic if you added some references to your response! –  Bez Sep 14 at 17:33
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Ack, please don’t scare people like that! Tingling when you sit on your leg funny is just a paresthesia, as another answer indicates. The process you’re describing is related to arterial occlusion – venous occlusion (which also isn't what's going on here - ever seen someone with a DVT?) doesn’t generally cause this, and nerve compression certainly doesn't. (The paresthesia itself may be due to compression of tiny vessels surrounding the nerve, but that's a completely different issue than the tissue hypoxia you describe.) –  Susan Sep 15 at 3:31
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(I assume in the parenthetical statement you’re referring to Bier Block, but the tourniquet is not the anesthesia - it is merely retaining a traditional local anesthetic in the arm.) –  Susan Sep 15 at 3:32
    
This answer is inexact in many ways. I respectfully advise you to reference things you are unsure of, especially in things pertaining to the medical field, because people are very easily scared of what others tell them on that subject. –  Raoul Sep 15 at 12:31

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