Humans are like animals and muscles have not conscience. Muscles contract and relax naturally, this process is control my CNS and environment or movement.

We can think of training a movement and trying to time this movement to our rational  thought process.For example: a tiger does not know how he runs, he just runs. A kangaroo does not know how to land after a jump but he lands.

CNS governs our locomotion and movements, so muscles assist in movement but they don’t generate it.

For example: a baby can’t walk at 4 weeks not because he doesn’t have muscles, it is because the brain is not develop yet.

For example: brain injuries cause movements atrophies but movement with regenerate the brain, koalas will dissolve their brain ones they deposit in the tree where they need to eat because they will stop moving. 

Gravity is the main force that affects our movements, locomotion and muscles. 

For example: if gravity wouldn’t exists, we wouldn’t have foot injuries.