The muscular system is fundamental to human life and humans would not be able to live without it. It is more than just a system that allows humans to move their limbs; it's also necessary for actions such as breathing, digestion and the pumping of blood around the body. The other functions of muscles include the movement of body parts, stability and heat production.
There are three types of muscle: skeletal, smooth and cardiac. Cardiac muscles are used exclusively in the heart and move involuntarily. In order to pump blood around the entire body, they're very strong. Smooth muscles are used in the stomach, blood vessels and the intestines. Like the cardiac muscles, they do not need to be instructed. They're called smooth muscles due to the fact they do not have striations. Skeletal muscles are the most common and make up most of the weight of the human body. They are almost always attached to the skeleton, and they move limbs or parts of the body.
A lot of the body's energy is used in the muscles, and this gives off a lot of heat within the body. Smooth muscles also help move food along the digestive tract.