A muscle is a type of bodily tissue that lies deep underneath the surface of the skin. Different types of muscles serve different purposes within the body:
- skeletal muscle in one way or another attached to the bones of the skeleton and assist the body in movement;
- cardiac muscle is found in the heart (the heart is both an organ and a muscle) and contracts to pump blood throughout the body
- smooth muscle resides within internal organs.
The most physically prominent of these muscles are the skeletal muscles, which are the only of the three muscle groups that can be voluntarily controlled. Through exercise and outside means (such as steroid use), these muscles can be augmented. The enlargement of this type of muscle is also dependent on the amount of the hormone testosterone present within the body (more testosterone generally means the possibility for larger muscles).