Not all muscle is the same. Within your muscles are smaller materials called muscle fibers. These fibers are the source of your muscular strength, and they play a large role in not only your performance, but your physical limits. These fibers can be either fast twitch or slow twitch in nature, and the percentages that these fibers are present in your muscles can determine whether you are are more able-bodied in anaerobic exercises or aerobic forms of physical activity.
Muscle Composition
Everyone's muscles feature both fast- and slow-twitch fibers. Although the ratios may vary from one person to the next, a typical ratio in a person will consist of 60 percent fast-twitch muscle fibers and 40 percent slow-twitch muscle fibers, according to BrianMac.co.uk. Fast-twitch muscles can move five to 10 times faster than slow-twitch muscles, which sometimes makes it easy to identify visually whether an athlete has a muscle composition dominated by fast- or slow-twitch muscles.
Slow-Twitch Fibers
Slow-twitch muscle fibers are also called oxidative fibers. They are high in mitochondria and myoglobin and feature a large number of capillaries, helping to distribute blood throughout the muscles when you exercise. Slow-twitch muscle fibers are associated with aerobic exercise that is endurance-based, primarily because these muscle fibers are better equipped to maintain oxygen flow to the muscles during exercise. They are also more resistant to fatigue than fast-twitch fibers. Endurance exercise like jogging, running, swimming and bicycling can help encourage the growth and strength of these fibers, and over time your body may transition to a greater percentage of slow-twitch fibers to meet the demands of your workout regimen.
Fast-Twitch Fibers
Fast-twitch muscle fibers commonly found in humans tend to have low amounts of myoglobin and mitochondria and have few blood capillaries in contrast to the content of slow-twitch muscle fibers. These muscles have a quick contraction rate, making them most beneficial to anaerobic exercises and fast, short-term events like sprinting. But these muscle fibers also wear out easily and cannot maintain their workload over long periods of time. You can encourage the development of these muscle fibers by performing intense exercise that is short in duration. Lifting weights and other strength training exercises can achieve this, as can plyometric exercises and running short sprints.
Considerations
An increase in muscle mass experienced through exercise can indicate the synthesis of new muscle growth. But exercise also dilates the fibers in fast-twitch muscles and will cause an increase in size that doesn't necessarily result from muscle growth. Even so, this dilation increases the strength of the fibers themselves. Exercise also prompts your body's production of natural human growth hormones, which bolsters the growth of your muscles and can aid in muscle mass increases.
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