Type I muscle fibers are used for long-duration, low-intensity movements, while Type II fibers are used for short-duration, high-intensity movements. Type I fibers are red in appearance, have more myoglobin and are more suitable for aerobic activity, while Type II fibers are white in appearance, have no myoglobin and rely on glycolytic enzymes for energy. Type I fibers are slower to twitch and highly resistant to fatigue, while Type II fibers are faster to twitch and less resistant to fatigue. The proportion of these fiber types is determined genetically and can affect athletic performance.
Alternatively known as slow-twitch muscle fibers, type I muscle fibers are found in skeletal muscle, the type of muscle that produces movement by attaching to bone. These fibers are called upon during long-duration, low-intensity muscle movements, unlike type II muscle fibers, which produce short-duration, high-intensity movements. All muscle tissue is made up of a variable proportion of these fiber types, with that proportion determined genetically. As such, some people are better suited to more explosive types of movement, such as running, while others, those who have been shown to possess more Type I muscle fibers, perform better in endurance activities such as distance running.
The functional units of skeletal muscle, muscle fibers or myofibers are elongated muscle cells that have multiple nuclei and give the muscle its striated appearance. Within these cells are bands known as sarcomeres that contain alternating filaments, or rows, of proteins known as actin and myosin. It is the binding action of these proteins pushing against each other that causes the sarcomere to shorten as a unit. When all of the sarcomeres in all of the muscle fibers in a muscle shorten simultaneously, a muscle contraction occurs, causing the muscle to pull on its bones and thus initiate movement around a joint.
Type I muscle fibers differ from type II muscle fibers, of which there are multiple types, in several ways. First, type I muscle fibers are red in appearance because they contain an additional protein called myoglobin to which oxygen binds. The presence of oxygen in type I fibers makes them more suitable for aerobic activity, which requires oxygen for energy. They are also denser with capillaries, which explains their red color, as well as their increased oxygen levels, and with mitochondria, the energy-producing units of muscle cells.
Type II fibers are white in color because no myoglobin is present in these cells, making them anaerobic. These rely on what are known as glycolytic enzymes to supply cells with energy. It is these chemicals that break down glucose, the simplest carbohydrate the body uses for energy.
Another way that type I muscle fibers are distinguished from type II fibers is by their rate of contraction and their level of resistance to fatigue. There is an inverse relationship between these two factors: Type I fibers are slow to twitch and highly resistant to fatigue, while Type II fibers are faster to twitch and less resistant to fatigue. Type IIa fibers, for example, have a relatively high resistance to fatigue and produce only moderately rapid muscle contractions. Type IIb fibers, on the other hand, which contract the fastest of all fiber types, have low fatigue strength. The world’s elite sprinters are likely to have a high percentage of type IIb fibers in their muscles, causing their muscles to produce explosive contractions but tire very quickly, while the best distance runners tend to have a high percentage of type I muscle fibers, allowing them to run at a slower pace for longer periods of time.
Protect your devices with Threat Protection by NordVPN
