There are three muscle cell types in the human body.  These muscles are: Cardiac Muscle, Skeletal muscle, and Smooth muscle.  Skeletal muscle is unique as it's found around the bones and skin while cardiac muscle and smooth muscle are found solely in the heart and hollow organs respectively.  Skeletal muscle is also cylinder-shaped, while cardiac muscle is long and branched, and smooth muscle is spindle-shaped. Skeletal muscle cells are the only muscle cells that are multi-nucleated,  both cardiac and smooth muscle have one nucleus.  An involuntary muscle is a muscle that contracts by itself. A voluntary muscle is a muscle that only contacts when the body decides to move. Cardiac muscle and smooth muscle are involuntary, as the cardiac muscle pumps blood and the smooth muscles move nutrients through the body. The skeletal muscle is the only voluntary muscle type and moves at the body's discretion. The different muscle types also require different levels of energy. In order to get the body to move the skeletal muscle cells have a high energy requirement and as a result, the skeletal muscle cells contain the most mitochondria. Cardiac muscle has an intermediate energy requirement and has the second most mitochondria. Smooth muscle cells require the least energy and thus have the least amount of mitochondria in their cells.