An individual muscle fiber may also be called a myofiber or a muscle cell. Each muscle fiber is a single, narrow cell that spans the entire length of a muscle. There are three different kinds of myofibers found in the body: skeletal, cardiac and smooth.
Myofibers are composed mostly of cylindrical bundles of proteins. There are two different types of protein filaments found in each muscle fiber: actin and myosin. When these filaments slide past one another, the muscle contracts. Each myofiber is surrounded by a membrane called a sarcolemma. Groups of muscle fibers are bundled together and encapsulated in connective tissue to form an entire muscle.
The cells that compose skeletal muscles, which are the muscles that connect to the bones and allow individuals to run, bend their arms and turn their heads, are voluntary, which means that each person decides when to contract them. Most of the muscles in the body are skeletal muscles, and they comprise an average of 40 percent of a person's body mass. Cardiac muscle cells, which are found in the heart, and smooth muscle cells that line the intestines and blood vessels, are involuntary, which means that the brain automatically controls their contractions.