MedlinePlus explains that the human body has three different types of muscles: skeletal, cardiac and smooth. These muscle groups have different functions, are located in different parts of the body and have different structures.
Cardiac muscle fibers have a striated appearance, and they are found in the walls of the heart, as explained by MedlinePlus. Skeletal muscles are also striated, but they are found throughout the body attached to the skeleton and ligaments. Finally, smooth muscle fibers are located in the walls of the body’s visceral organs, such as the intestines and stomach. Smooth muscle fibers are named such because they lack the striations that are found on cardiac and skeletal muscles. Smooth muscle cells are spindle shaped rather than cylindrical. Humans do not have conscious control over smooth muscles or cardiac muscles, but they can control skeletal muscles. Often, skeletal muscles are also known as voluntary muscles, whereas the other two types are known as involuntary muscles.
One interesting exception is the diaphragm, which moves the lungs to pump air in and out of the body. According to the National Center for Biotechnology Information, the diaphragm behaves as two different muscles. One part functions as a smooth muscle while the other portion functions as a skeletal muscle. This allows the diaphragm to operate on both a voluntary and involuntary basis.