MadSci Network: Anatomy

Re: Why does standing with your 'knees locked' make you pass out?

Date: Tue Oct 26 14:20:17 1999
Posted By: Sean Moore, M.D., emergency medicine, smbd jewish general hospital
Area of science: Anatomy
ID: 940709915.An

Well the short answer is that it does not cause you to pass out.  By 
locking the knees, you diminish the use of the muscles in your legs which 
normally increases the blood return from the legs.  As blood pools in the 
veins of your legs, your heart has less blood returning to be sent to the 
brain.  By unlocking your knees, you are forced to use the muscles in the 
legs more for the position of standing.  There is no artery that gets 
blocked in the knees and causes the phenomenon.

Admin Note - It takes quite a long time of standing with the knees locked before
one begins to feel light-headed and eventually pass out.  This is affected by
temperature, with fainting more likely on hot days.  But in any event, the key
is standing with knees locked for long periods, not simply locking your knees.

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