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I have been told that locking one's knees can lead to fainting or passing out, and if you need to stand for an extended period of time you should always avoid standing and not lock your knees.

I just did a bit of research to see if the same was true for kneeling for an extended period of time, and some of what I read suggested that it's a myth that locking one's knees leads to fainting, it's simply lack of motion in the legs, and kneeling can have the same effect.

Does locking one's knees for an extended period actually lead to loss of consciousness / passing out, and if so, why?

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    This is, of course, purely anecdotal, but I've both knelt and stood for long periods of time without issue, and seen someone faint from locking their knees.. – Seth Jan 25 '16 at 4:28
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    According to what I was taught in Cadets, it doesn't lead to fainting, but makes the results of fainting worse. When you faint with locked knees, you fall over rigidly like 2x4 and are likely to hit your head hard, something that is typically undesirable. With unlocked knees, your knees bend, resulting in you collapsing straight down (like you're going into a deep squat), then falling over, resulting in a much more gentle landing. – Compro01 Jan 25 '16 at 5:23
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    I just read that too @Compro01. Another reason I thought I should ask this on Skeptics :-) – Josh Jan 25 '16 at 5:28
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    @Seth. You have "seen someone faint from locking their knees". How do you know it is the cause? Isn't is just a matter of "you have seen someone locking their knees and faint"? – user22865 Jan 25 '16 at 9:51
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Research shows that while the standard assumption is that knees are locked in standing, actually in reality standing is possible with low leg stiffness without locking the knee. Medically, a knee is locked when a torn part of the meniscus has displaced into a part of the knee or due to a tight Popliteus muscle.

The view that 'knee locking act alone by itself causes syncope' is not backed up by scientific evidence. However, locking one's knees and maintaining a rigid, completely unmoving standing position for long periods of time might interfere with proper blood flow from the legs and cause vasovagal syncope.

It turns out the medical science behind “don’t lock your knees” doesn’t exactly hold up. While the advice to avoid locking knees is widespread – even outside of military communities – the actual cause of fainting is usually loss of blood flow to the brain which can be brought on by any number of things, but the act of locking ones knees has nothing to do with passing out. While the act of locking ones knees and maintaining a rigid, completely unmoving position for a long period of time may interrupt proper blood flow, the act of locking the knees alone does not by itself cause fainting spells.

Syncope also commonly called fainting or “passing out” is a medical term used to describe a temporary loss of consciousness due to the sudden decline of blood flow to the brain. If one is about to faint, there are a lot of accompanied symptoms described here. There are also several types of syncope which have specific trigger events or they seem to occur through other causes such as dehydration, BP medication and other medical conditions. If it occurs very often without a triggering event, it might be a sign of an underlying heart disease requiring medical attention.

Prolonged standing associated with heat or emotional stress can cause gravitational blood pooling in the lower extremities. This might act as a trigger for the fainting spell and may be amplified by the knee locking.

“If dehydration is present the situation is worsened,” explains Dr. Richard Nicholas, chairman of the UAMS Department of Orthopedic Surgery. “The low rate of blood return to the heart in these situations results in hypo-perfusion of the brain and at times the fainting spell. Standing at attention, or with one’s knees locked, may amplify the process.”

Medical advise to counter this knee locking mechanism is to move about or contract the leg muscles which might help in stimulating the blood flow to the heart and improve circulation to the brain.

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    I don't know if it would help at all, but the explanation that always got was that the locked knees didn't so much cause the fainting as keep you from realizing it. If your knees are bent slightly, and your legs start to buckle because you're fainting, you get get jolted back into awareness and know you need to do something. If "locked" into a straight position, the rigidity of the legs makes it harder to notice that you're about to topple over. – Sean Duggan Jan 25 '16 at 13:09
  • @SeanDuggan- Prolonged standing in any position is a triggering event for vasovagal syncope. Even though the knee joints are commonly believed to be locked during standing, research shows that knee joints might have either high or low stiffness which might play a role in stable standing! – pericles316 Jan 25 '16 at 13:20

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