Skeletal muscle tissue is one of three types of muscle tissue commonly found in the body, along with cardiac and smooth muscle tissue. Skeletal muscle tissue generally allows for physical movements of all sorts. Skeletal muscles usually function in pairs, and their control is typically voluntary. Skeletal muscles are made up of skeletal muscle fibers, or cells. There are typically two types of skeletal muscle fibers found in humans, each one with unique properties.
Skeletal muscles are considered striated, meaning one can find alternating bands of dark and light crossing the width of the muscle fibers. Skeletal muscles are generally to be found attached to the skeleton, usually by tendons. Skeletal muscles typically function in pairs. When one muscle is relaxed, the corresponding muscle usually contracts. This process of alternating contracting and relaxing creates physical movement.
Skeletal muscles can generally be contracted and relaxed at will to perform voluntary functions, such as movement. They also control activities such as urination, and defecation. Skeletal muscles are also considered crucial to some involuntary functions, like breathing.
The cells that make up skeletal muscle tissue are long and fibrous. There are believed to be two types of muscle fibers common to the human species, type I and type IIb. Each type typically has different characteristics. The different muscle groups of the body usually consist of different concentrations of each type of muscle fiber, depending on the functions of each individual muscle group.
Type I muscle fibers generally contract slowly. They typically do not succumb to fatigue as quickly as type IIb muscle fibers might. Large amounts of myoglobin, the protein that carries oxygen to cells, are usually found in type I muscle fibers. As a result, these muscles are considered able to produce large amounts of energy quickly. Type I muscle fibers are often found in their highest concentrations in the skeletal muscle tissue of the back, legs, and neck, as these muscles must work constantly to maintain posture and facilitate movement.
Type IIb muscle fibers typically contract quickly. They do not generally receive the larger supplies of oxygen that type I muscle fibers receive. Type IIb muscle fibers are believed to fatigue more quickly than type I muscle fibers. This type of skeletal muscle fiber is often found in the highest concentrations in the arms and shoulders, as these muscles are generally used less frequently than those of the back and neck. Skeletal muscle tissue, however, is usually made up of a mix of both types of fibers, with the average person possessing 40 percent type I fibers and 60 percent type IIb fibers.