Involuntary muscles are found in the circulatory system, digestive system and urinary system. Humans do not exert any control over these muscles, which is why they are called involuntary.
Many of the muscles in the body are voluntary, which means a person has to think about using them. The muscles in the arm, for example, do not move unless a person thinks about moving his arm and takes the necessary steps to do so. Cardiac muscle is involuntary, which is why humans do not have to think about making their hearts beat. The MedCell at Yale website says smooth muscle is also involuntary. This type of muscle pushes food through the digestive tract and helps empty urine from the bladder.