Muscle cells are composed of fibers or filaments that have the ability to contract and relax resulting in movement. When muscle cells contract, this results in a change in its shape. The filaments slide past one another resulting in movement and ATP is the fuel muscles need to function. They are important for activities like responding to stimuli via reflex actions, for voluntary movement, and for involuntary movement such as peristalsis of the digestive tract. The heart is made of cardiac muscle. It does not have to be consciously controlled along with smooth muscle which is responsible for involuntary movement. The striated or skeletal muscle is responsible for voluntary movement.