Smooth muscle is different from the better known muscle groups like arms, etc, which are under voluntary control. These are 'automatic' muscles, and form part of the internal muscle groups which process food. These muscles are found in organs like the intestines. The smooth muscle, which is in 'layers' or 'sheets' like the outer layers of intestinal lining. This muscle expands and contracts naturally as part of a process, and acts as a continuum of muscle behavior. (Strictly speaking these muscles don't actually need voluntary controls. Imagine trying to voluntarily control the movements of a mouthful of coffee.)