Our muscles, which make up about half of our body mass, control the way the body moves. Muscles work together all the time, whether we are actively playing sports, or quietly reading and writing. Muscles lie in bands just beneath the surface of the skin. A muscle is made up of thousands of fibers bundled together within a protective sheath that consists of blood vessels and nerves. These nerves can be up to 12 inches (30 centimeters) long. A muscle becomes stronger when you work it, which is why people who regularly exercise have more defined muscle tone than those who do not exercise.
There are about 660 muscles in the human body. The three types of muscle tissue are skeletal, cardiac, and smooth. The main and most unique characteristic of muscle tissue is its ability to contract, or shorten, making some type of movement possible. Skeletal muscles hold the bones together, and are often called “voluntary” muscles because the brain controls them. The cardiac muscle, which is found only in the heart, contracts to send blood from the heart into the arteries. The brain sends signals to the cardiac muscle to speed up or slow down its contractions, called the heartbeat. Smooth muscles, located in the internal organs such as the stomach and intestines, help these organs or tissues do their job, such as help you digest and eliminate your food.