Why locking your knees could cause you to faint.
In: Biology
It has to do with blood flow.
Unlike your arteries, which are driven by your heartbeat, your veins are much more passive, with no force driving the blood back to your heart. Instead, your veins rely largely on your muscles to squeeze the veins, sort of like a tube of toothpaste or Gogurt.
Locking your knees allows you to stand without using your leg muscles quite as much, which is good for the fact that it saves energy, but bad because without your muscles engaging, you’re losing out on return blood flow back to your heart and brain — and that’s where the risk of fainting comes in.
If you Google this you would find that locking knees does not actually cause you to pass out. What causes you to pass out is blood pooling in the lower extremities.
What does happen is that when you lock your knees, you prevent the muscles that would move, to be still and the blood does not return as normally.
If you faint while your knees are locked, you tend to have a worse injury than when the legs are not locked.
Blood pooling in the legs causes the fainting by reducing blood flow to the brain. Temperature also plays a factor.
I have vasovagal syncope, or as my daughter says, “you get passy outy a lot!”
I have discovered there are a lot of reasons why people pass out: altitude, overexertion, dehydration, pressure changes in the abdomen, alcohol intake and the knee locking thing.
The knee locking thing has to do with the gates in your leg veins: when you contract leg muscles the valves in the veins open to allow blood to flow back towards the heart. As I understand it, each valve has two opposing flaps that act like gates opening one way. That stops blood from moving back down.
When you lock your knees, the muscles sometimes relax and the gates can open, effectively dumping blood into your legs. Down you go!
Conversational aside to guys: peeing standing up at night can have the same effect for some. It’s called micturation syncope. The pressure change causes a misfire of the vagus nerve and those same gates open. A bunch of dudes die every year from falls when this occurs because there are so many hard surfaces in the bathroom.
The veins in your legs rely on your skeletal muscles to push blood back up efficiently. It’s a great system as long as you flex your leg muscles whenever you stand. Locking your knees lets you stand without using muscles. Blood has trouble making it out of your legs. Your blood pressure (in your head) drops. You pass out.
We were told that it had more to do with the mechanics of falling.
Should you happen to find while your knees are locked you fall over like a tree. Significantly higher injury risk than with loose knees where you’re supposed to just crumble like a cookie.
Not sure if that’s true at all or even half the answer, but it’s a fun thought
I was a groomsmen and the guy next to me locked his knees. He (6’4”) dropped onto me (5’3”) and almost started dominoes, but I somehow kept my feet until he popped back up. “That’s what they mean don’t lock your knees!” He said after the ceremony was over 🤦🏻♂️
When I was in basic training, I was in charge of the domino flight. On the parade grounds at Lakeland AFB, TX, one hot Aug afternoon, my guys started passing out and falling like dominos. I could only stand there and watch.
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