Although you cannot change the ratio of muscle fibers, you can improve what you have. Slow and fast twitch muscle fibers are generally intermingled, with a higher percentage of fast twitch fibers. Through training, you may improve the metabolic efficiency of either type of muscle fiber.Muscles generate power by contraction. Contraction is determined by the types of muscle fibers used and the firing rate of the neurons within the fibers. The voluntary contraction of a muscle begins with the smallest units of slow twitch muscles. These motor units (muscle fiber groups) have the lowest response threshold, create the least amount of tension, and are the most resistant to fatigue. As muscle tension increases, more motor units are recruited from the larger fast twitch fibers. As tension continues to rise, fewer motor units need to be activated because the large fast twitch units contain more plentiful and more powerful muscle fibers. However, because these large fibers are the ones that generate peak tension in the muscle, they fatigue quickly and require more recovery time. For example, whether you walk one mile or sprint one mile, you use the same basic muscle groups over the same distance. However, walking requires less tension in the muscles and therefore relies on the low threshold-low tension motor units so fatigue is slow to develop; whereas, sprinting requires maximum muscle tension on every stride. When muscle fibers must produce maximum tension repeatedly over long periods of time, they fatigue sooner.
Contraction speed is determined by the rate at which the skeletomotor neurons stimulate the muscle fibers. The more frequently the neurons fire, the more tension that is produced within the muscle. At peak tension, the neurons fire so rapidly that the muscle fiber is unable to relax from one stimulation to the next, resulting in generation of maximum force.
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