Why does hanging straight up, with your feet off the ground, cause your legs to fall asleep?

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Suppose I’m hanging off monkey bars, my legs will fall asleep after a while. This was also seen in the movie “Sherlock Holmes” where Sherlock replicates a fake hanging, his legs begin to fall asleep after hanging for a while. Why is that? I mean you are still straight up, does the blood not circulate the same as when you’re feet are planted in the ground and you’re standing up?

In: Biology
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Blood circulation drops a bit, but most of the time it’s probably nerve compression. If you apply pressure to a nerve it stops conducting, so you go numb and lose power in your leg muscles below where the compression occurs.

(Not a doctor, so this may be wrong).

The calf muscles are involved in pumping blood. They are sometimes even called the “second heart”. I’d imagine that if you are not putting your weight on your legs, your calf muscles stop contracting, and so stop pumping blood around. The pooling of the blood in your legs and lower circulation makes the legs feel like they have gone to sleep.

It goes even further than that. Not only can your legs fall asleep, you can actually pass out due to the drop in blood pressure. Under normal use, your legs are moving or even just moving a bit to support your weight, which helps the blood flow due to muscular contraction. If your legs are just hanging, the muscles are doing nothing and the blood can pool. Since your legs hold such a large quantity of blood, it can cause a drop in blood pressure that can be very dangerous.

In the military, they told us not to lock our knees while standing at attention for long periods because it could ‘pinch off the blood flow’ and cause you to pass out. That is not what is really happening, as nothing is actually pinched off, but I did witness countless people passing out and falling flat on their face while standing at attention.

The same thing can happen if you are suspended in the air in some sort of harness, similar to what you are talking about. Left in that condition long enough, it can even be fatal.

Edit: the reason locking your knees can cause this is that when in that position your skeleton is sort of supporting itself without your leg muscles having to do much of anything.