Muscles aid in movement, stability, posture, circulation, digestion and breathing; they also assist in the production of heat. Forty percent of the human body is comprised of muscles, and the three main types are skeletal, smooth and cardiac muscles.
The skeletal muscles in the human body cover the skeleton, and they allow people to stand erect, walk, bend and move around. These muscles are controlled by the individual and contract and extend at will. Muscles use a large amount of the body's energy when they contract. This increases the metabolic rate, which, in turn, produces a vast amount of heat. This is especially important for individuals living in cold climates.
Smooth muscles work involuntarily. These muscles line the various internal organs, and they contract rhythmically on their own. They cause food to pass through the digestive tract, aid in breathing and control the flow of blood in the arteries.
The cardiac muscles are some of the strongest muscles in the body. They are found exclusively in the heart, and, like the smooth muscles, work involuntarily. These muscles cause the contraction of the heart, which results in blood being pumped through the circulatory system and throughout the human body.