Which of the following muscle is also known as an involuntary muscle that supports the heart's cyclic contraction and relaxation?

Cardiac muscles, also known as involuntary muscles, support the heart's cyclic contraction and relaxation. 

The heart is composed of cardiac muscle, which is a unique type of muscle that is only found in the heart. Unlike skeletal muscles, which are under voluntary control, cardiac muscles are involuntary and are regulated by the cardiac conduction system. 

The contraction of cardiac muscles allows the heart to pump blood throughout the body, while their relaxation allows the heart to refill with blood.