What are muscles for? And what do they actually do? Let's dispense with the conventional explanation first. Muscles are for strength. Perhaps, but that explanation isn't useful. We don't really need more strength. Okay, everyone can invent a situation where more strength would be handy, but for every situation we can also invent a work-around or a way to adapt without increasing strength. I'm not anti-strength here, we should all try to make each other stronger, smarter, richer and better looking. I'm just saying it isn't a need. Being normal isn't a deficiency. Anyway, strength is relative. If you are doing athletic competition against people in your weight class, in most situations more muscle mass is an advantage. But that is a small part of the martial arts world. And to the contrary, it is the minimum promise of most martial arts classes that skill and training can confer more advantage than strength alone.
So if muscles aren't for strength what are they? What do they do?
They pull inward. They shorten. Wow, that was easy. But think about the implications of this. Pushing out from the center isn't actually possible. It is an illusion. A bone can act as a lever moving a distal point in an out-from-the-center direction, but the muscle has to move inward to accomplish that. If you are paying attention to muscles, their only action is inward. Yes, you can also lean in towards another person and lever out from the center in two directions, toward the ground and toward the opponent. But the muscles are still moving inwards.
Bones and ligaments are passive structures. Muscles do all the active work. If a muscle is active on one side of the joint and loose on the other, the ligaments will hold the joint together. But in the longterm ligaments wear out so that isn't a good way to move. That's why stretching can be dangerous. If you have trained yourself to relax muscles, you will be instantly stretching ligaments. So watch out.
Ligaments connect bone to bone, by definition they transfer force from one bone to another. But they also surround the joint capsules. Every joint has a synovial fluid sack which, like cartilage, goes in between two bones. Ligaments keep the synovial fluid sacks from slipping out.
If all the muscles are engaged the body becomes liquid. Wherever a muscle is deficient our "structure" loses some of its liquidity. Anywhere a muscle lacks tone, ligaments have to make up for it. And ligaments don't pull inward, they are pretty static. Engaged muscles always have room to lengthen, ligaments never do.
Muscles pull inward and keep our structure in a liquid state.
Homework: Are the any exceptions? Why?