When comparing the same systems for endurance, the relative values are the following:
Phosphagen system 8 to 10 seconds
Glycogen-lactic acid system 1.3 to 1.6 minutes Aerobic system Unlimited time (as long as nutrients last)
Thus, one can readily see that the phosphagen system is the one used by the muscle for power surges of a few seconds, and the aerobic system is required for prolonged athletic activity. In between is the glycogen-lactic acid system, which is especially important for giving extra power during such intermediate races as the 200-to 800-meter runs.
What Types of Sports Use Which Energy Systems? By considering the vigor of a sports activity and its duration, one can estimate closely which of the energy systems is used for each activity. Various approximations are presented in Table 84-1.
Recovery of the Muscle Metabolic Systems After Exercise. In the same way that the energy from phosphocreatine can be used to reconstitute ATP, energy from the glycogen-lactic acid system can be used to reconstitute both phos-phocreatine and ATP. And then energy from the
Energy Systems Used in Various Sports
Phosphagen system, almost entirely
100-meter dash Jumping Weight lifting Diving
Phosphagen and glycogen-lactic acid systems
Glycogen-lactic acid system, mainly
400-meter dash 100-meter swim Tennis Soccer
Glycogen-lactic acid and aerobic systems
800-meter dash 200-meter swim 1500-meter skating Boxing
2000-meter rowing 1500-meter run 1-mile run 400-meter swim
10,000-meter skating Cross-country skiing Marathon run (26.2 miles, 42.2 km) Jogging oxidative metabolism of the aerobic system can be used to reconstitute all the other systems-the ATP, the phos-phocreatine, and the glycogen-lactic acid system.
Reconstitution of the lactic acid system means mainly the removal of the excess lactic acid that has accumulated in all the fluids of the body. This is especially important because lactic acid causes extreme fatigue. When adequate amounts of energy are available from oxidative metabolism, removal of lactic acid is achieved in two ways: (1) A small portion of it is converted back into pyruvic acid and then metabolized oxidatively by all the body tissues. (2) The remaining lactic acid is reconverted into glucose mainly in the liver, and the glucose in turn is used to replenish the glycogen stores of the muscles.
Recovery of the Aerobic System After Exercise. Even during the early stages of heavy exercise, a portion of one's aerobic energy capability is depleted. This results from two effects: (1) the so-called oxygen debt and (2) depletion of the glycogen stores of the muscles.
Oxygen Debt. The body normally contains about 2 liters of stored oxygen that can be used for aerobic metabolism even without breathing any new oxygen. This stored oxygen consists of the following: (1) 0.5 liter in the air of the lungs, (2) 0.25 liter dissolved in the body fluids, (3) 1 liter combined with the hemoglobin of the blood, and (4) 0.3 liter stored in the muscle fibers themselves, combined mainly with myoglobin, an oxygen-binding chemical similar to hemoglobin.
In heavy exercise, almost all this stored oxygen is used within a minute or so for aerobic metabolism. Then, after the exercise is over, this stored oxygen must be replenished by breathing extra amounts of oxygen over and above the normal requirements. In addition, about 9 liters more oxygen must be consumed to provide for reconstituting both the phosphagen system and the lactic acid system. All this extra oxygen that must be "repaid," about 11.5 liters, is called the oxygen debt.
Figure 84-2 shows this principle of oxygen debt. During the first 4 minutes of the figure, the person exercises heavily, and the rate of oxygen uptake increases
Rate of oxygen uptake by the lungs during maximal exercise for 4 minutes and then for about 40 minutes after the exercise is over. This figure demonstrates the principle of oxygen debt.
Effect of diet on the rate of muscle glycogen replenishment after prolonged exercise. (Redrawn from Fox EL: Sports Physiology. Philadelphia: Saunders College Publishing, 1979.)
more than 15-fold. Then, even after the exercise is over, the oxygen uptake still remains above normal, at first very high while the body is reconstituting the phospha-gen system and repaying the stored oxygen portion of the oxygen debt, and then for another 40 minutes at a lower level while the lactic acid is removed. The early portion of the oxygen debt is called the alactacid oxygen debt and amounts to about 3.5 liters. The latter portion is called the lactic acid oxygen debt and amounts to about 8 liters.
Recovery of Muscle Glycogen. Recovery from exhaustive muscle glycogen depletion is not a simple matter. This often requires days, rather than the seconds, minutes, or hours required for recovery of the phosphagen and lactic acid metabolic systems. Figure 84-3 shows this recovery process under three conditions: first, in people on a high-carbohydrate diet; second, in people on a high-fat, high-protein diet; and third, in people with no food. Note that on a high-carbohydrate diet, full recovery occurs in about 2 days. Conversely, people on a high-fat, high-protein diet or on no food at all show very little recovery even after as long as 5 days. The messages of this comparison are (1) that it is important for an athlete to have a high-carbohydrate diet before a grueling athletic event and (2) not to participate in exhaustive exercise during the 48 hours preceding the event.
In addition to the large usage of carbohydrates by the muscles during exercise, especially during the early stages of exercise, muscles use large amounts of fat for energy in the form of fatty acids and acetoacetic acid (see Chapter 68), and they use to a much less extent proteins in the form of amino acids. In fact, even under the best conditions, in those endurance athletic events that last
Seconds Minutes Hours
Duration of exercise
Effect of duration of exercise as well as type of diet on relative percentages of carbohydrate or fat used for energy by muscles. (Based partly on data in Fox EL: Sports Physiology. Philadelphia: Saunders College Publishing, 1979.)
longer than 4 to 5 hours, the glycogen stores of the muscle become almost totally depleted and are of little further use for energizing muscle contraction. Instead, the muscle now depends on energy from other sources, mainly from fats.
Figure 84-4 shows the approximate relative usage of carbohydrates and fat for energy during prolonged exhaustive exercise under three dietary conditions:
high-carbohydrate diet, mixed diet, and high-fat diet. Note that most of the energy is derived from carbohydrates during the first few seconds or minutes of the exercise, but at the time of exhaustion, as much as 60 to 85 per cent of the energy is being derived from fats, rather than carbohydrates.
Not all the energy from carbohydrates comes from the stored muscle glycogen. In fact, almost as much glycogen is stored in the liver as in the muscles, and this can be released into the blood in the form of glucose and then taken up by the muscles as an energy source. In addition, glucose solutions given to an athlete to drink during the course of an athletic event can provide as much as 30 to 40 per cent of the energy required during prolonged events such as marathon races.
Therefore, if muscle glycogen and blood glucose are available, they are the energy nutrients of choice for intense muscle activity. Even so, for a long-term endurance event, one can expect fat to supply more than 50 per cent of the required energy after about the first 3 to 4 hours.
Effect of Athletic Training on Muscles and Muscle Performance
Importance of Maximal Resistance Training. One of the cardinal 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 approximately optimal increase in muscle strength, without producing chronic muscle fatigue.
The upper curve in Figure 84-5 shows the approximate percentage increase in strength that can be achieved in a previously untrained young person by this resistive training program, demonstrating that the
Weeks of training
Weeks of training
Approximate effect of optimal resistive exercise training on increase in muscle strength over a training period of 10 weeks.
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's muscles 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 training, 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 hypertrophy; (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 triglyceride (fat). Because of all these changes, the capabilities of both the anaerobic and the aerobic metabolic systems are increased, increasing especially the maximum oxidation rate and efficiency of the oxidative metabolic system as much as 45 per cent.
Fast-Twitch and Slow-Twitch Muscle Fibers. In the human being, all muscles have varying percentages of fast-twitch and slow-twitch muscle fibers. For instance, the gastrocnemius muscle has a higher preponderance of fast-twitch fibers, which gives it the capability of forceful and rapid contraction of the type used in jumping. In contrast, the soleus muscle has a higher preponderance 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 considerably more 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 relative 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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