Which type of muscle is not under voluntary control?

Smooth muscle is the correct answer because it operates involuntarily, meaning it is not under conscious control. It is found in various structures in the body, such as the walls of hollow organs like the intestines, blood vessels, and the bladder. The primary function of smooth muscle is to facilitate automatic movements, such as peristalsis in the digestive tract or the contraction and relaxation of blood vessels.

This involuntary characteristic is crucial for maintaining bodily functions without the need for direct conscious thought. In contrast, skeletal muscle, which is striated, is under voluntary control, allowing for deliberate movements. Cardiac muscle, although striated like skeletal muscle, is also involuntary as it functions automatically to pump blood throughout the body.