The human body is made of more than 650 different muscles. Our bodies function and move by contractions^* of these muscles. The contractions pull on whatever is attached to the muscle. Muscles never push. They only pull.

Involuntary muscles cannot be controlled.

Voluntary muscles can be controlled.

Muscles are made of muscle fibers. Exercise makes these muslce fibers grow larger or tear. Tearing increases the number of muscle fibers. Both processes make a muscle stronger.