Effect of Fiber Type on Activity Level
The white meat of a chicken’s breast is composed mainly of fast-twitch fibers. The muscles are adapted to contract rapidly for a short time but fatigue quickly. Chickens normally do not fly long distances. They spend most of their time walking. Ducks, on the other hand, fly for much longer periods and over greater distances. The red, or dark, meat of a chicken’s leg or a duck’s breast is com-posed of slow-twitch fibers. The darker appearance is due partly to a richer blood supply and partly to the presence of myoglobin, which stores oxygen temporarily. Myoglobin can continue to release oxygen in a muscle even when a sustained contraction has interrupted the continuous flow of blood.
Humans exhibit no clear separation of slow-twitch and fast-twitch muscle fibers in individual muscles. Most muscles have both types of fibers, although the number of each type varies in a given muscle. The large postural muscles contain more slow-twitch fibers, whereas muscles of the upper limb contain more fast-twitch fibers.
Average, healthy, active adults have roughly equal numbers of slow- and fast-twitch fibers in their muscles and over three times as many type IIa as type IIb fibers. In fact, athletes who are able to perform a variety of anaerobic and aerobic exercises tend to have a balanced mixture of fast-twitch and slow-twitch muscle fibers. However, a world-class sprinter may have over 80% type II fibers, with type IIa slightly predominating, whereas a world-class endur-ance athlete may have 95% type I fibers.
The ratio of muscle fiber types in a person’s body apparently has a large hereditary component but can also be considerably influ-enced by training. Exercise increases the blood supply to muscles, the number of mitochondria per muscle fiber, and the number of myofibrils and myofilaments, thus causing muscle fibers to enlarge, or hypertrophy (hı̄-per′trō-fē). With weight training, type IIb myosin myofilaments can be replaced by type IIa myosin myofilaments as muscles enlarge. Muscle nuclei quit expressing type IIb genes and begin expressing type IIa genes, which are more resistant to fatigue. If the exercise stops, the type IIa genes turn off, and the type IIb genes turn back on. Vigorous exercise programs can cause a limited number of type I myofilaments to be replaced by type IIa myofilaments.
The number of cells in a skeletal muscle remains somewhat con-stant following birth. Enlargement of muscles after birth is primarily the result of an increase in the size of the existing muscle fibers. As people age, the number of muscle fibers actually decreases. However, there are undifferentiated cells just below the endomysium called satellite cells. When stimulated, satellite cells can differentiate and develop into a limited number of new, functional muscle fibers. These cells are stimulated by the destruction of existing muscle fibers, such as by injury or disease, or during intensive strength training.
Muscle contractions are classified as either isometric or isotonic. In isometric (equal distance) contractions, the length of the muscle does not change, but the amount of tension increases during the contraction process. Isometric contractions are responsible for the constant length of the body’s postural muscles, such as the muscles of the back. On the other hand, in isotonic (equal tension)contractions, the amount of tension produced by the muscle isconstant during contraction, but the length of the muscle decreases. Movements of the arms or fingers are predominantly isotonic contractions. Most muscle contractions are a combination of iso-metric and isotonic contractions in which the muscles shorten and the degree of tension increases.
Concentric (kon-sen′trik) contractions are isotonic contrac-tions in which muscle tension increases as the muscle shortens. Many common movements are produced by concentric muscle con-tractions. Eccentric (ek-sen′trik) contractions are isotonic contrac-tions in which tension is maintained in a muscle, but the opposing resistance causes the muscle to lengthen. Eccentric contractions are used when a person slowly lowers a heavy weight. Substantial force is produced in muscles during eccentric contractions, and muscles can be injured during repetitive eccentric contractions, as some-times occurs in the hamstring muscles when a person runs downhill.
Muscle tone is the constant tension produced by body musclesover long periods of time. Muscle tone is responsible for keeping the back and legs straight, the head in an upright position, and the abdomen from bulging. Muscle tone depends on a small percent-age of all the motor units in a muscle being stimulated at any point in time, causing their muscle fibers to contract tetanically and out of phase with one another.