Because they are controlled by the autonomic nervous system which is responsible for involuntary control of the heart and digestion (as well as breathing and such). If you had to voluntarily control your digestion or heart rhythm you wouldn't survive very long.
involuntary muscles. Stomach, intestines and your heart are examples.
heart, stomach, intestines
Heart muscles are involuntary.
The muscles of the digestive system are smooth muscles.
You cannot control the muscles in your heart and brain, and also your lungs, because they work automatically.
It is most definitely involuntary. You do not have to make your stomach contract every time you eat something.
skeletal muscles are voluntary muscles.
One example of involuntary muscles would be in the digestive system, where muscles in the stomach churn food.
The heart as both involuntary and striated muscles. The striations are similar skeletal muscles. Heart muscles are involuntary like the muscles seen in the digestive tract, called smooth muscle.
If by involuntary you mean acting of its own accord, the organ the lungs act without you controlling them, even when you hold your breath eventually you will breathe of your own accord. The stomach and intestines are involuntary muscles.
heart, so cardiac muscle. Smooth muscle is also involuntary in the intestines.
involuntary muscles