Muscular System

The muscular system refers to a person's muscles.  Muscles are bundles of tissue that expand (get larger) and contract (get smaller). By expanding and contracting, muscles can make the body move. Muscles allow organs to work as well as protecting them from damage by things outside the body.

Every time you move, you use muscles. Some muscles operate without you thinking about it while other muscles need to be told to move. All muscles work by shrinking, which makes them pull or squeeze. For example, if a muscle is attached to one end of a bone, when it shrinks, the bone or limb will move. The larger the muscle, the more powerfully it pulls. There are about 650 muscles attached to bones in the body. These muscles that are fastened to bones are called tendons. 

Muscles can pull but they can not push. Instead, they work in pairs that pull in opposite directions. When one muscle pulls, its partner relaxes.

Smooth muscles make things move inside your body, such as pushing food along your intestines.
The Heart muscle is a strong muscle that squeezes and pumps blood around the body. 

You use hundreds of muscles when you run and jump. All these muscles are coordinated by your brain, a bit like how a conductor controls an orchestra. 

Further Reading

Subpages (1): Muscles Activity