InnerBody explains that the human body contains three muscle types, which each perform a different function. The most obvious muscles are the skeletal muscles, which attach to tendons and move voluntarily, according to the Cleveland Clinic. Skeletal muscles produce body movements and maintain posture, allowing humans to sit, stand and walk. The Cleveland Clinic describes smooth muscles as those that line some of the body’s organs, and cardiac muscle as those that control the heart.
In contrast to skeletal muscles, which are under voluntary control, cardiac and smooth muscles move involuntarily. Smooth muscles line the walls of many internal organs, such as the stomach and intestines, according to InnerBody. The smooth muscles, which are also called visceral muscles, help to pass food through the digestive system and other similar functions.
The University of California, Clermont, explains that the diaphragm is a smooth muscle that is at least partially under conscious control. While humans do not have to think to breathe, the diaphragm can be controlled, such as when people hold their breath.
All three types of muscles vary in appearance. Skeletal and cardiac muscles both possess striations when viewed microscopically. InnerBody states that these striations indicate that the muscles are strong. By contrast, smooth muscles lack such striations and are comparatively weak.