. For example, smooth muscles in our stomach and intestine move food through our digestive system. Tiny muscles in our eyes make the pupil bigger or smaller in response to light. Ali muscles contract and relax automatically,without our thinking about it, and so they are called involuntary muscles. The third group, cardiac muscle, is found only in the walls of the heart. These muscles contract and relax automatically in a powerful, regular rhythm to pump blood around our body. Our heart beats an average of about 70 times a minute, every single minute of our lives, without ever resting.