Almost half the body's weight is muscle. Muscles are the part of our body that allow us to move. They are made up of special tissues that can contract, or shorten, when they receive a signal from the brain. The muscles are attached to bones by stretchy tissue called tendons. When the muscles contract, they pull on the tendons which pull on the bones and cause our limbs to move.

There are more than 640 muscles, and they hardly ever work alone. Muscles can get shorter and pull, but they cannot push. So most muscles are arranged in opposing teams. One team pulls the body part one way, then the other team pulls it back again. As each team pulls, the other team relaxes and gets stretched. Muscles band together to form muscle groups which work together.

Voluntary muscles, such as your arms
and legs can be controlled by your thoughts.
All this muscle action is controlled by your
brain, which sends and receives signals
through your nervous system. 

   
Muscle actions can be voluntary or
involuntary. Involuntary muscles,
such as the heart, diaphragm and
intestines, are automatically controlled
by the brain. You don't have to think
about making them work. For example the
heart beats between 60 and 80 beats every
minute without you having to think about it.