Why can’t we stand perfectly still? Why do we rock/move back and forth, even just a little?
In: Biology
Many reasons, here are a few.
The first is that the shape of your body is not statically stable. Think of balancing a pencil point-down on your finger. It takes work to keep it balanced because it’s in an unstable configuration. Some animals have a more stable configuration (like elephants which can sleep standing up) but it’s a trade-off. We’re more agile because of the way we’re built.
Second is the way your brain controls muscles. Think about when you lift something heavy. Your brain isn’t telling your muscles ‘go to a 45° angle’, rather your brain is saying ‘flex a little harder’ and your muscles try to. There’s not perfect feedback and your brain has to send repeated signals to maintain a certain position. Some motor controls in robots use a similar system. The fan in your PC does too (own). This system is extremely useful and not prone to issues, but doesn’t lend itself to keeping muscles at a specific tightness for long periods of time.
There needs to be a positive feedback on your present state of balance. Tipping one direction or another creates the muscle signal and inner ear signal for your brain to apply correction in the opposite direction.
Not a human body expert, but I know that any object needs at least three constraints to remain still in all conditions. For example, you can make a table with 3 legs, but not 2 legs. Since humans are bipedal (2 legs), your muscles would need to constantly fine-tune the balance so you don’t fall over. This basically adds the needed 3rd “constraint”.
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