Because your brain tells them to
signal from a nerve
Your brain.
Your muscles know when it is time to move because your brain sends electric impulses to your muscles at approximately 210 mph.
your brain sends messages to your nerves
If you move your muscles together all the time, you will be tired.
Instinct, they have special muscles that move the ears.
If your scars heal and if you can move your muscles, you know your cells are working.
All muscles contract when functioning properly. That's all they know how to do.
All muscles can move, but only skeletal muscles are voluntary. The cardiac and the smooth muscles cannot be moved by your will, but skeletal muscles can be.
It takes many muscles to move the leg. It takes about 200 muscles just to walk at an average pace. Forty muscles are needed at a time to help pick up their legs.
The brain will send an electrical message via the nervous system instructing the muscle to move.
The muscles that help you move when you want to are called voluntary muscles.
Actually you can move all your muscles, but the name for the ones you move involuntarily are called involuntary muscles.
Skeletal muscles help you when you want to move. Skeletal muscles are voluntary muscles that you can control.
Not that we know of - in order to move things nature had to develop nerves muscles and bones, if neurons could do it alone think of all the time it would have saved.