Muscle Fibers: Fast and Slow Twitch

Muscles are the instruments that power all movements made by the human body. Muscles are defined as being contractile tissues that are capable of extension and contraction to generate movement. The body uses three different types of muscles for various purposes: skeletal, cardiac, and smooth muscles.

Skeletal muscle, also known as striated muscle, is the type that constitutes most of the muscle mass within the body. Cardiac muscle is the specialized tissue found only in the heart. Cardiac muscle is activated involuntarily through the function of various impulses, including those directed through the autonomic nervous system controlled by the hypothalamus, the regulatory region of the brain. Smooth muscle is the tissue that lines the hollow organs of the body and it is also the subject of involuntary control, the autonomic nervous system.

All types of skeletal muscle are constructed from densely knit fibers, which are provided the nutrients necessary for their function by capillaries, tiny blood vessels extending from the arteries of the cardiovascular system. Muscle fibers are bound into bundles, called fascicles, to form a working unit. The ultimate control over every muscle fiber is exerted by the brain, through the transmissions that it directs to the body through the central nervous system. These transmissions emanate from the brain through the spinal cord, and ultimately through nerve pathways to neurons located within every muscle. The neuron is the local control mechanism that regulates the function of a group of muscle fibers; one neuron may control as many as 2,000 individual fibers. Given their function of the control of physical movement, these devices are known as motor neurons. The speed with which the neurons communicate with their related fibers dictates the characterization of the fiber as either a "fast twitch" or a "slow twitch."; all muscles possess both fast-twitch and slow-twitch fibers.

Fast-twitch fibers and slow-twitch fibers possess the same capacity to generate muscular power. Fast-twitch fibers are activated by their neurons at a rate ten times faster than the rate of activation for slow-twitch fibers. The distribution of fast- versus slow-twitch fibers in the muscles is primarily an inherited characteristic, determined by the genetic coding of each person. While it is common for a person to have muscles with a relatively even distribution of fast-and slow-twitch fibers, some persons inherit a tendency to a significantly greater number of one type of fiber over the other. Such persons tend to excel in the sports best suited to their muscular composition.

Fast-twitch fibers are further subdivided into two sub-categories, fast twitch (IIa) and an intermediate speed twitch fiber (IIb). Fast-twitch fibers are relied on by the body to propel it in short, intense bursts (such as those required in sprinting, weightlifting, or other short duration, explosive movements). Slow-twitch fibers are the units employed by the body to provide the power for endurance activities.

The manner in which the two kinds of fiber are utilized is tied to their construction as well as to the function of the neuron. Slow-twitch fibers possess a greater quantity of mitochondria, the portion of the human cell that acts as a powerhouse within each cell in the production of energy. Slow-twitch fiber cells can process greater amounts of oxygen to assist in the generation of adenosine triphosphate (ATP), the body's fuel for the production of energy. For this reason, slow-twitch fibers are relied on when the muscle must extend and contract repetitively, as in distance running or cycling events.

Physiological studies confirm that extensive endurance training will create an adaptation by the body, in that the intermediate fast-twitch fibers (IIa) may be converted to slow-twitch fibers over time.

The training that will assist in the development of fast-twitch fibers involves the repeated activation of the appropriate muscles. Techniques include isometric training, in which the muscle is held in a resistance-generating position for set periods. The clasping of both hands and pulling them with equal force from each arm is a simple isometric movement. A goal of isometric exercise is to ensure that the targeted muscle is contracted and extended in a disciplined fashion, which encourages an optimal relationship between each neuron and the muscle fiber group. Weight training, particularly the lifting of significant amounts with short rest intervals in each set, also stimulates fast-twitch fiber development.

The best known of the explosive training techniques aimed solely at the development of fast-twitch fiber is plyometrics. These programs that usually emphasize intense jumping and interval sprint training—which often are used by sprinters, hurdlers, basketball players, and other athletes that seek to become more explosive in their movements—are the best-known techniques to develop fast-twitch fibers in the leg muscles. The muscle becomes conditioned to respond to the stimulation provided to the neuron as dictated by the demands of the exercise. When the body senses that the number of fast-twitch fibers available to perform the movements are insufficient, neighboring fibers will be co-opted into assisting the existing fast-twitch fibers.

see also Endurance exercise; Muscle mass and strength; Plyometrics.