Effect of Athletic Training on Muscles and Muscle Performance
Importance of Maximal Resistance Training. One of the car-dinal principles of muscle development during athletic training is the following: Muscles that function under no load, even if they are exercised for hours on end, increase little in strength. At the other extreme, muscles that contract at more than 50 per cent maximal force of contraction will develop strength rapidly even if the contractions are performed only a few times each day. Using this principle, experiments on muscle building have shown that six nearly maximal muscle contractions performed in three sets 3 days a week give approxi-mately optimal increase in muscle strength, without producing chronic muscle fatigue.
The upper curve in Figure 84–5 shows the approxi-mate percentage increase in strength that can be achieved in a previously untrained young person by this resistive training program, demonstrating that the muscle strength increases about 30 per cent during the first 6 to 8 weeks but almost plateaus after that time. Along with this increase in strength is an approximately equal percentage increase in muscle mass, which is called muscle hypertrophy.
In old age, many people become so sedentary that their muscles atrophy tremendously. In these instances, muscle training often increases muscle strength more than 100 per cent.
Muscle Hypertrophy. The average size of a person’smuscles is determined to a great extent by heredity plus the level of testosterone secretion, which, in men, causes considerably larger muscles than in women. With train-ing, however, the muscles can become hypertrophied perhaps an additional 30 to 60 per cent. Most of this hypertrophy results from increased diameter of the muscle fibers rather than increased numbers of fibers, but this probably is not entirely true, because a very few greatly enlarged muscle fibers are believed to split down the middle along their entire length to form entirely new fibers, thus increasing the number of fibers slightly.
The changes that occur inside the hypertrophied muscle fibers themselves include (1) increased numbers of myofibrils, proportionate to the degree of hypertro-phy; (2) up to 120 per cent increase in mitochondrial enzymes; (3) as much as 60 to 80 per cent increase in the components of the phosphagen metabolic system, including both ATP and phosphocreatine; (4) as much as 50 per cent increase in stored glycogen; and (5) as much as 75 to 100 per cent increase in stored triglyc-eride (fat). Because of all these changes, the capabilities of both the anaerobic and the aerobic metabolic systems are increased, increasing especially the maximum oxi-dation rate and efficiency of the oxidative metabolic system as much as 45 per cent.
Fast-Twitch and Slow-Twitch Muscle Fibers. In the humanbeing, all muscles have varying percentages of fast-twitch and slow-twitch muscle fibers. For instance, thegastrocnemius muscle has a higher preponderance of fast-twitch fibers, which gives it the capability of force-ful and rapid contraction of the type used in jumping. In contrast, the soleus muscle has a higher preponder-ance of slow-twitch muscle fibers and therefore is used to a greater extent for prolonged lower leg muscle activity.
The basic differences between the fast-twitch and the slow-twitch fibers are the following:
1.Fast-twitch fibers are about twice as large in diameter.
2.The enzymes that promote rapid release of energy from the phosphagen and glycogen-lactic acid energy systems are two to three times as active in fast-twitch fibers as in slow-twitch fibers, thus making the maximal power that can be achieved for very short periods of time by fast-twitch fibers about twice as great as that of slow-twitch fibers.
3. Slow-twitch fibers are mainly organized for endurance, especially for generation of aerobic energy. They have far more mitochondria than the fast-twitch fibers. In addition, they contain considerably more myoglobin, a hemoglobin-like protein that combines with oxygen within the muscle fiber; the extra myoglobin increases the rate of diffusion of oxygen throughout the fiber by shuttling oxygen from one molecule of myoglobin to the next. In addition, the enzymes of the aerobic metabolic system are considerably more active in slow-twitch fibers than in fast-twitch fibers.
4. The number of capillaries is greater in the vicinity of slow-twitch fibers than in the vicinity of fast-twitch fibers.
In summary, fast-twitch fibers can deliver extreme amounts of power for a few seconds to a minute or so. Conversely, slow-twitch fibers provide endurance, delivering prolonged strength of contraction over many minutes to hours.
Hereditary Differences Among Athletes for Fast-Twitch Versus Slow-Twitch Muscle Fibers. Some people have considerablymore fast-twitch than slow-twitch fibers, and others have more slow-twitch fibers; this could determine to some extent the athletic capabilities of different individuals. Athletic training has not been shown to change the rel-ative proportions of fast-twitch and slow-twitch fibers however much an athlete might want to develop one type of athletic prowess over another. Instead, this seems to be determined almost entirely by genetic inheritance, and this in turn helps determine which area of athletics is most suited to each person: some people appear to be born to be marathoners; others are born to be sprinters and jumpers. For example, the following are recorded percentages of fast-twitch versus slow-twitch fiber in the quadriceps muscles of different types of athletes:
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