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What are the differences between slow-and fast-twitch muscle fibers, and what role do they play during exercise?
Athletes’ muscles are not uniform throughout their body. Human skeletal muscle is comprised of two different types of muscle fiber, slow twitch (type 1) and fasts twitch (type 2), in approximately equal amounts. Fast-twitch fibers can be further subdivided into type 2a and type 2b fibers.
Slow-twitch muscle fibers have a reddish appearance because of their high myoglobin content and have a relatively slow contractile force. They have a high mitochondria content and are surrounded by numerous blood vessels (capillaries) that bring oxygen and nutrient-rich blood into the muscle. Type 1 muscle fibers produce energy aerobically (in the presence of oxygen) and are very efficient at producing ATP (energy needed for the contraction and relaxation of muscle) from the oxidation of fats and carbohydrates; thus, they are recruited quickly, are remarkably fatigue resistant, and are used exclusively during endurance events (such as marathon running and long-distance cycling).
Fast-twitch fibers, on the other hand, are whitish in color, contract three to four times faster than slow twitch fibers, have a low endurance capacity, and are considerably larger than slow-twitch fibers. Type 2 muscle fibers generate most of their ATP energy anaerobically (without oxygen) from carbohydrates (glycolysis). Fast-twitch muscle fibers are recruited during high-intensity exercise such as sprinting and weight training. There are, however, distinct physiological differences between the type 2a and type 2b fast-twitch muscle fibers.
Type 2a muscle fibers perform similarly to both fast and slow-twitch fibers; they have a relatively higher blood flow capacity, a higher capillary density, and a higher mitochondrial content than type 2b fibers, and they are relatively fatigue resistant. Type 2a muscle fibers have some capacity for oxidative metabolism (utilization of fats and carbohydrates for energy).
They are considered to be an intermediate fiber that is used extensively during activities of moderate to high intensity, lasting only a few minutes, such as a 1-mile run or a 400-meter swim (middle-distance events). Type 2b fibers have a very low blood supply, a low mitochondrial content, and a low capillary density, and they fatigue rapidly during exercise. Type 2b fibers are recruited during very intense exercise, such as a 100-meter sprint or a 50-meter sprint swim, and produce energy exclusively through anaerobic metabolism of carbohydrates.
The amount of force generated by type 2b muscle fibers is considerably greater than by type 2a, and in essence, a type 2b muscle fiber is a power fiber. During exhaustive exercise, such as marathon running, the body preferentially recruits slow-twitch muscle fibers first. At some undetermined point, slow-twitch fibers begin to fatigue from glycogen depletion. The reduction in glycogen content eventually forces the body to transition to the fast-twitch type 2a fibers to maintain performance. When type 2a fibers exhaust their energy stores, the body finally activates the type 2b fibers to keep the muscles fueled. When the type 2b fibers are glycogen depleted, the body experiences volitional fatigue (“bonking”), which causes a significant drop in performance.
The ratio of fast-twitch to slow-twitch fibers appears to be genetically determined at a very young age, possibly the first few years of life. Fiber distribution is inherited and cannot be significantly altered through training. Studies completed on identical twins clearly demonstrate inheritance as a major factor in determining fiber type and distribution; results prove that their fiber types are almost indistinguishable.
Although fiber type cannot be significantly altered through training, recent studies now show that training may have a small but significant impact on type 1 and type 2 fibers. It has been estimated, through research, that approximately 10% of fast- and slow twitch fibers can be altered through training. Regular strength training has been shown to shift a small percentage of type 2a muscle fibers to type 2b. Conversely, shifts of approximately 3% to 4% of type 2b fibers can be converted to type 2a fibers as a result of chronic endurance training. In addition, as an athlete ages, there appears to be an appreciable change in muscle fiber distribution, with a shift from fast-twitch to slow twitch fibers.
An athlete’s success in a particular sport is determined by several factors, including training, cardiovascular function, muscle size, motivation, coaching, and muscle fiber composition. The relative percentage and distribution of type 1 and type 2 muscle fibers usually steer an athlete into a particular sport or activity. Athletes who are endowed with a higher percentage of slow-twitch fiber are more likely to gravitate to endurance-type activities, such as marathon running. Conversely, athletes endowed with a higher percentage of fast-twitch fibers are more inclined to be better at strength or power sports, such as sprinting. Athletes with almost equal ratios of type 1 and 2 fibers tend to be better at moderate- to high-intensity activities, such as the mile run. Most athletes discover what they are better suited to by experimenting with different activities at a young age. Comparatively, world-class endurance athletes have been shown to have as much as 90% or more type 1 fiber in their gastrocnemius (calf) muscles, as opposed to elite sprinters, who have only 20% to 25%; therefore, it would make little sense for an athlete suited for endurance to become a power athlete and vice versa.
Athletes should be encouraged to develop the appropriate fiber type for their sport through a well-developed, scientifically based, sport-specific training program created by a qualified exercise specialist.
Terms:
Myoglobin – Found mainly in muscle tissue; serves as a storage site for oxygen.
Glycolysis – Breakdown of glucose into energy.
Mitochondria – Powerhouses of the cell that burn carbohydrates, fats, and proteins for
