Human muscles are categorized by both their structure and function. The three main types of muscle are skeletal muscles, cardiac muscles, and smooth muscles. Besides these three muscle groups, human muscles can also be categorized as voluntary or involuntary.
Skeletal muscles are voluntary human muscles, which means that one must actively think to cause movement of these muscles. Both cardiac muscles and smooth muscles are categorized as involuntary muscles. Involuntary human muscles function and continue to function without thought or active control. It is impossible to control involuntary muscles.
Smooth muscles can be found in various locations within the organs of the human body. A few examples include the muscles of the stomach and intestines, bladder, and in blood vessels throughout the body. Muscles categorized as smooth muscles can also be found in hollow organs, such as the uterus and airways. Smooth muscles are structured in alternating layers which run in different directions, and their functions require that they have the ability to stretch. These muscles are also able to withstand and hold tension for extended periods of time.
Striated muscles, or skeletal muscles, are the most numerous within the human body. Their function is to connect bones. The many layers of skeletal muscles join bones across a connecting joint, and the lengthening and shortening of skeletal muscles is the basis of movement for bones.
There are at least 640 individual skeletal muscles. A few common examples include the abdominal muscles, the hamstring muscles located on the back of the upper leg, and the frontalis muscle, the intricate muscle of the forehead. Besides movement, the contracting and relaxing of skeletal muscles performs a variety of additional tasks for the human body including maintaining posture and producing heat.
Cardiac muscles are located in the heart. These muscles have very specific functions and characteristics. Cardiac muscles can stretch like a smooth, organ muscle, but can also contract with a high amount of force like a striated muscle. The muscles of the heart function involuntarily to continuously pump blood throughout the body. Cardiac muscles are twitch muscles and are unable to perform slow, prolonged movements.
Both cardiac muscles and skeletal muscle cells are striated. The striations, appearing as alternating bands of dark and light, run across the muscle fiber, perpendicular to the length of the muscle fiber. These stripes are in fact red and white stripes caused by the variation of high and low myoglobin levels in the muscle fiber.