For example, standing on the edge of a cliff. Why do our knees buckle? What causing our legs to shake and go weak?
Shouldn’t our instincts do the opposite so we don’t have an accident by falling?
In: Biology
This question was already asked and here is a [good response to it](https://www.reddit.com/r/explainlikeimfive/comments/40taii/comment/cyx2zkr/?utm_source=share&utm_medium=mweb3x&utm_name=mweb3xcss&utm_term=1&utm_content=share_button)
Many of the feelings we experience when scared are there to protect us from harm. Either by causing us to run away or fight off the danger. However some of those primary reactions have unneeded or unintentional side effects. The primary reaction is to send your blood to the major organs. The heart the lungs and the brain. The secondary side effect of doing this empties our limbs of much of the blood needed to keep them stable. So when you blood rushes to you heart lungs and brain there isn’t enough blood in your limbs to feed them oxygen and so they tremble.
The second theory is that by drawing the blood to your core it pulls your center of gravity in to your core. It also makes your limbs lighter. Thus easier to run. Basically try running while carrying something out in front of you versus carrying it on your back close to your core. The second option is much easier to run with because it carries the weight close to your core.
Also the blood that rushes to your core is then highly oxygenated and then rushed back to your limbs thus giving you a rush of power to either run or fight.
Basically we have some ideas why it happens but it could be several different things. The overarching idea is to either protect the self or a side effect of trying to protect the self.
All the stuff about “blood shunting away from your limbs” is BS. A sympathetic nervous system response aka fight or flights preferentially shunts blood TO your skeletal muscle, heart, and lungs. It shunts away from your digestive system.
See: https://en.m.wikipedia.org/wiki/Fight-or-flight_response under Function Of Physiological Changes
Adrenaline surges, blood flow increases bigly, high muscle tension, blood pressure shoots through the roof, nervous system engages into hyperdrive. Your legs are trembling because you are full of energy.
Shaky legs make us feel unsteady and unsafe. Feeling unsafe makes us want to move away from the edge of the cliff. Moving away from the cliff makes it less likely we’ll have an accident.
So trembling legs might be good for survival.
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