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How Many Muscles Are In The Human Body and How do muscles work?

The human body has 639 muscles.

Not all are joined to your bones.

For example, muscles line your blood vessels to keep blood flowing. Your lungs work because of regular muscle movement.
Muscles in the Human Body
Muscles in the Human Body

Some muscles move only when you decide to stand up, sit down, or move in other ways. These are the voluntary muscles that are joined to your bones.
There are other muscles that work without your thinking about them. These involuntary muscles are made of smooth muscle fibers. They mash the food material in your stomach and then act to move it along through your intestines.
Your heart is made of another special kind of muscle called cardiac muscle. It moves in a regular rhythm as it pumps blood to all parts of your body.

How do muscles work?

Muscles are made of tough, elastic tissue.
They are built so that they can contract, which means that they become shorter.
When the muscles whose ends are joined to bones contract, they pull at the bones and make them move.
Muscles are made up of long, thin cells that join together to make muscle fibers.
When the muscle becomes shorter, all the fibers move closer together, making the muscle bulge.
You can see your muscles bulging and relaxing when you move your arms and legs.
If you bend your elbow and clench your fist, the muscles called biceps in your upper arm will bulge.
Muscles can only pull—they can’t push. 
Muscles whose ends are joined to bones work in pairs.
One muscle contracts and pulls the bone one way, and the other contracts to pull the bone back again.

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