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Old 02-18-2005, 06:38 PM
Hermitian Hermitian is offline
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Why do you pass out when you lock your knees?

I've always heard that if you lock your knees, you will eventually pass out. A few questions:

1. Why? Evidently it has something to do with restricting blood to the brain. How can your knees prevent blood from getting to your brain?

2. Does everyone do this? I have locked my knees for long times before and had no problems. Are some people just more prone to this problem?


I've been wondering this one for years, and am eager to find out!

Thanks guys!
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  #2  
Old 02-18-2005, 07:10 PM
Zsofia Zsofia is offline
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Well, I've certainly seen it happen: an unfortunate bridesmaid during the vows once. She knocked over a candelabra on her way down, too, and the carpet caught on fire a bit. Of course, there was also the excitement of the day, and it was a little warm in the church. She was incredibly embarassed.
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Old 02-18-2005, 07:29 PM
Qadgop the Mercotan Qadgop the Mercotan is offline
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Blood will pool in the legs when someone stands. Walking, or other activities which flex the calves help to push this blood back up towards the heart and back into the circulation.

Stand at attention long enough, and you run the risk of pooling too much blood volume in the legs for the proper blood pressure to be maintained. Voila, syncope!

QtM, MD
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Old 02-18-2005, 07:30 PM
Qadgop the Mercotan Qadgop the Mercotan is offline
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To elaborate further, it pools in the veins in the legs for the most part. And no, not all will faint if they do this. But a significant number of people can. It also depends on other factors, like whether they're a bit dehydrated to start with, temperature, humidity, hormone levels, etc. etc.
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Old 02-18-2005, 07:38 PM
Ike Witt Ike Witt is offline
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Quote:
Originally Posted by Qadgop the Mercotan
Stand at attention long enough, and you run the risk of pooling too much blood volume in the legs for the proper blood pressure to be maintained. Voila, syncope!
How long does one have to stand still? I realize that it will be different for everyone, but are we talking 5 minutes or 5 hours?
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Old 02-18-2005, 08:00 PM
Qadgop the Mercotan Qadgop the Mercotan is offline
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Quote:
Originally Posted by adam yax
are we talking 5 minutes or 5 hours?
Yep. Some would pass out in less than 5 minutes, others would last longer than 5 hours (but it'd be a bitch of a torture).
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Old 02-18-2005, 08:31 PM
Strinka Strinka is offline
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If you'll pass out if you stand still long enough, then why did early industrial factories not have chairs? They (the factory owers) forced their workers to stand for 14+ hours a day, presumably to reduce costs, but I would think that it would be a lot more prfitable to buy chairs and have workers able to work for 14+ hours than have your workers pass out.
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Old 02-18-2005, 08:38 PM
Qadgop the Mercotan Qadgop the Mercotan is offline
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Quote:
Originally Posted by Strinka
If you'll pass out if you stand still long enough, then why did early industrial factories not have chairs? They (the factory owers) forced their workers to stand for 14+ hours a day, presumably to reduce costs, but I would think that it would be a lot more prfitable to buy chairs and have workers able to work for 14+ hours than have your workers pass out.
As long as the person standing moves their legs, they use the muscles which pump the blood back up to the heart. It's only when standing rigidly still (which generally requires a person to hold their knees rigid) that this pumping action ceases and the risk of fainting rises drastically.
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Old 02-18-2005, 11:49 PM
AskNott AskNott is offline
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It's easy to think of the heart as the only pump, but it's not correct. There are two or three others. The veins have one-way check valves, and blood can back up behind them. Flexing the muscles in the legs will squeeze the veins, and blood is forced back to the heart. The abdomen has some extra blood in the vessels in the intestines, and you can pump that by either taking a very deep breath or pulling in the abdominal muscles. Either breathing deeply, or pulling in hard on your abs will squirt an extra dose of blood into the system.
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Old 02-19-2005, 04:53 AM
Richard Pearse Richard Pearse is offline
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Yeah I've seen someone standing to attention faint after 10mins or so and smash a number of teeth. Of course the rest of us are instructed NOT to help or catch anyone who falls (don't want anyone to break ranks chaps), but to wait for the ambulance staff who are often present during military parades. There's a bit of a technique to appearing to be stiffly at attention but in actual fact be wiggling toes, flexing muscles etc to avoid the blood pooling.

All those things they tell you to do to avoid deep vein thrombosis in an aeroplane are what you should do if standing up for a long time.
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Old 02-19-2005, 09:30 PM
Vlad/Igor Vlad/Igor is offline
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When I was younger, I had orthostatic hypotension, and would see stars if I stood up too quickly. I learned to keep my calves and quads flexed for 30 seconds or so unless I started walking. I still do this when I stand up, especially at church.

Vlad/Igor
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