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Type I Muscle Fibers
Type I Muscle fibers are involved in endurance activities. These fibers, also called slow twich fibers, are noted for
their ability to produce energy in the presence of oxygen. Thus, they are primarily aerobic. The main fuel source for this fiber is fats (fatty acids), which allow the muscle to work at a steady rate with noticeable resistance to fatigue. Their color is typically quite red, a result of the high content of "myoglobin", an iron-containing protein that stores and delivers oxygen. Slow twich fibers are not noted for their speed, their anaerobic capacity, or their ability to contract at a fast rate repeatedly, but rather their indefatigability and aerobic capacity.
Type II Muscle Fibers
Type II fibers can be categorized into at least two types: Type II-a and Type II-b. These fibers are adapted for strenght and power activities. The Type II-a fiber is a cross between a slow twitch and a fast twitch fiber in that it is both aerobic (oxidative) and anaerobic (glycilytic). Whereas, it is "faster" that the slow twitch fiber, it is not as well suited for endurance activities. Its color is also reddish, a result of the myoglobin content.
In contrast, the Type II-b fiber Is truly a fast twitch fiber, with very high contraction speeds. These fibers are almost exclusively anaerobic and have minimal capacity for aerobic production of energy. They rely primarily on glycogen within the muscle for energy and are therefore very susceptible to fatigue. Their color is pale, and some consider it "white" because it lacks myoglobin. Type II-b fibers tend to accumulate lactate, which ultimately leads to rapid fatigue if the lactate is not removed.
It should be noted that each of the fiber type has diffrent recruitment patterns, and typically the Type II-b fiber is only recruited for use during maximal effort. The other fibers contract during light as well as moderate activity. Moreover, physical training can lead to changes in the characteristics of the fibers. Thus, endurance training would lead to changes in the Type II-a fiber such that they take on more characteristics of the Type I, or slow twitch fiber.
