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Sports Physiology
There are few stresses to which the body is exposed that approach the extreme stresses of heavy exercise. In fact, if some of the extremes of exercise were continued for even moderately prolonged periods, they might be lethal. Therefore, sports physiology is mainly a discussion of the ultimate limits to which several of the bodily mechanisms can be stressed. To give one simple example: In a person who has extremely high fever approaching the level of lethality, the body metabolism increases to about 100% above normal. By comparison, the metabolism of the body during a marathon race may increase to 2000% above normal.
Female and Male Athletes
Most of the quantitative data that are given in this chapter are for the young male athlete, not because it is desirable to know only these values, but because it is only in young male athletes that relatively complete measurements have been made. Measurements in older athletes and in females are much less complete. However, for measurements that have been made in the female athlete, similar basic physiological principles apply, except for quantitative differences caused by differences in body size, body composition, and the presence or absence of the male sex hormone testosterone.
In general, most quantitative values for women—such as muscle strength, pulmonary ventilation, and cardiac output, all of which are related mainly to the muscle mass—vary between two-thirds and three-quarters of the values recorded in men, although there are many exceptions to this generalization. When measured in terms of strength per square centimeter of cross-sectional area, the female muscle can achieve almost exactly the same maximal force of contraction as that of the male muscle—between 3 and 4 kg/cm 2 . Therefore, most of the difference in total muscle performance lies in the extra percentage of the male body that is muscle, which is caused partly by endocrine differences that we will discuss later.
The performance capabilities of the female versus male athlete are illustrated by the relative running speeds for a marathon race. In a comparison, the top female performer had a running speed that was 11% less than that of the top male performer. For other events, however, women have at times held records faster than men—for example, for the two-way swim across the English Channel, for which the availability of extra fat seems to be an advantage for heat insulation, buoyancy, and extra long-term energy.
Testosterone secreted by the male testes has a powerful anabolic effect in causing greatly increased deposition of protein everywhere in the body, but especially in the muscles. In fact, even a male who participates in very little sports activity but who nevertheless has a normal level of testosterone will have muscles that grow about 40% larger than those of a comparable female without the testosterone.
The female sex hormone estrogen probably also accounts for some of the difference between female and male performance, although not nearly so much as testosterone. Estrogen increases the deposition of fat in the female, especially in the breasts, hips, and subcutaneous tissue. At least partly for this reason, the average young (age 16–19 years) nonathletic female has about 34% body fat composition, in contrast to the nonathletic young male (age 16–19 years), who has about 23% ( Figure 85-1 ). The average percentages for body fat are higher in older men and women and have increased substantially over the past 20 to 30 years as the prevalence of obesity has risen in populations of most developed countries. In the United States, for example, the prevalence of obesity in now approximately 37% of the adult population. Increased body fat composition is a detriment to the highest levels of athletic performance in events in which performance depends on speed or on the ratio of total body muscle strength to body weight.
Muscles in Exercise
Strength, Power, and Endurance of Muscles
The final common determinant of success in athletic events is what the muscles can do for you—that is, what strength they can give when it is needed, what power they can achieve in the performance of work, and how long they can continue their activity.
The strength of a muscle is determined mainly by its size, with a maximal contractile force between 3 and 4 kg/cm 2 of muscle cross-sectional area. Thus, a person who has enlarged his or her muscles through an exercise training program will have correspondingly increased muscle strength.
To give an example of muscle strength, a world-class male weight lifter might have a quadriceps muscle with a cross-sectional area as great as 150 square centimeters. This measurement would translate into a maximal contractile strength of 525 kilograms (or 1155 pounds), with all this force applied to the patellar tendon. Therefore, one can readily understand how it is possible for this tendon at times to be ruptured or actually to be avulsed from its insertion into the tibia below the knee. Also, when such forces occur in tendons that span a joint, similar forces are applied to the surfaces of the joint or sometimes to ligaments spanning the joints, thus accounting for such happenings as displaced cartilages, compression fractures about the joint, and torn ligaments.
The holding strength of muscles is about 40% greater than the contractile strength. That is, if a muscle is already contracted and a force then attempts to stretch out the muscle, as occurs when landing after a jump, this action requires about 40% more force than can be achieved by a shortening contraction. Therefore, the force of 525 kilograms previously calculated for the patellar tendon during muscle contraction becomes 735 kilograms (1617 pounds) during holding contractions, which further compounds the problems of the tendons, joints, and ligaments. It can also lead to internal tearing in the muscle. In fact, forceful stretching of a maximally contracted muscle is one of the surest ways to create the highest degree of muscle soreness.
Mechanical work performed by a muscle is the amount of force applied by the muscle multiplied by the distance over which the force is applied. The power of muscle contraction is different from muscle strength because power is a measure of the total amount of work that the muscle performs in a unit period of time. Power is therefore determined not only by the strength of muscle contraction but also by its distance of contraction and the number of times that it contracts each minute. Muscle power is generally measured in kilogram meters (kg-m) per minute. That is, a muscle that can lift 1-kilogram weight to a height of 1 meter or that can move some object laterally against a force of 1 kilogram for a distance of 1 meter in 1 minute is said to have a power of 1 kg-m/min. The maximal power achievable by all the muscles in the body of a highly trained athlete with all the muscles working together is approximately the following:
| kg-m/min | |
|---|---|
| First 8 to 10 seconds | 7000 |
| Next 1 minute | 4000 |
| Next 30 minutes | 1700 |
Thus, it is clear that a person has the capability of extreme power surges for short periods, such as during a 100-meter dash that is completed entirely within 10 seconds, whereas for long-term endurance events, the power output of the muscles is only one-fourth as great as during the initial power surge.
This does not mean that one’s athletic performance is four times as great during the initial power surge as it is for the next 30 minutes, because the efficiency for translation of muscle power output into athletic performance is often much less during rapid activity than during less rapid but sustained activity. Thus, the velocity of the 100-meter dash is only 1.75 times as great as the velocity of a 30-minute race, despite the 4-fold difference in short-term versus long-term muscle power capability.
Another measure of muscle performance is endurance. Endurance, to a great extent, depends on the nutritive support for the muscle—more than anything else, it depends on the amount of glycogen that has been stored in the muscle before the period of exercise. A person who consumes a high-carbohydrate diet stores far more glycogen in muscles than does a person who consumes either a mixed diet or a high-fat diet. Therefore, endurance is enhanced by a high-carbohydrate diet. When athletes run at speeds typical for the marathon race, their endurance (as measured by the time that they can sustain the race until complete exhaustion) is approximately the following:
| Minutes | |
|---|---|
| High-carbohydrate diet | 240 |
| Mixed diet | 120 |
| High-fat diet | 85 |
The corresponding amounts of glycogen stored in the muscle before the race started explain these differences. The amounts stored are approximately the following:
| g/kg Muscle | |
|---|---|
| High-carbohydrate diet | 40 |
| Mixed diet | 20 |
| High-fat diet | 6 |
Muscle Metabolic Systems in Exercise
The same basic metabolic systems are present in muscle as in other parts of the body; these systems are discussed in detail in Chapter 68, Chapter 69, Chapter 70, Chapter 71, Chapter 72, Chapter 73, Chapter 74 . However, special quantitative measures of the activities of three metabolic systems are exceedingly important in understanding the limits of physical activity. These systems are (1) the phosphocreatine-creatine system, (2) the glycogen–lactic acid system, and (3) the aerobic system.
Adenosine Triphosphate
The source of energy actually used to cause muscle contraction is adenosine triphosphate (ATP), which has the following basic formula:
The bonds attaching the last two phosphate radicals to the molecule, designated by the symbol ∼, are high-energy phosphate bonds. Each of these bonds stores 7300 calories of energy per mole of ATP under standard conditions (and even slightly more than this under the physical conditions in the body, which is discussed in detail in Chapter 68 ). Therefore, when one phosphate radical is removed, more than 7300 calories of energy are released to energize the muscle contractile process. Then, when the second phosphate radical is removed, still another 7300 calories become available. Removal of the first phosphate converts the ATP into adenosine diphosphate (ADP), and removal of the second converts this ADP into adenosine monophosphate (AMP).
The amount of ATP present in the muscles, even in a well-trained athlete, is sufficient to sustain maximal muscle power for only about 3 seconds, which might be enough for one half of a 50-meter dash. Therefore, except for a few seconds at a time, it is essential that new ATP be formed continuously, even during the performance of short athletic events. Figure 85-2 shows the overall metabolic system, demonstrating the breakdown of ATP first to ADP and then to AMP, with release of energy to the muscles for contraction. The left-hand side of the figure shows the three metabolic systems that provide a continuous supply of ATP in the muscle fibers.
Phosphocreatine-Creatine System
Phosphocreatine (also called creatine phosphate ) is another chemical compound that has a high-energy phosphate bond, with the following formula:
Phosphocreatine can decompose to creatine and phosphate ion, as shown in Figure 85-2 , and in doing so releases large amounts of energy. In fact, the high-energy phosphate bond of phosphocreatine has more energy than the bond of ATP: 10,300 calories per mole compared with 7300 for the ATP bond. Therefore, phosphocreatine can easily provide enough energy to reconstitute the high-energy bond of ATP. Furthermore, most muscle cells have two to four times as much phosphocreatine as ATP.
A special characteristic of energy transfer from phosphocreatine to ATP is that it occurs within a small fraction of a second. Therefore, all the energy stored in muscle phosphocreatine is almost instantaneously available for muscle contraction, just as is the energy stored in ATP.
The combined amounts of cell ATP and cell phosphocreatine are called the phosphagen energy system. These substances together can provide maximal muscle power for 8 to 10 seconds, almost enough for the 100-meter run. Thus, the energy from the phosphagen system is used for maximal short bursts of muscle power .
Glycogen–Lactic Acid System
The stored glycogen in muscle can be split into glucose, and the glucose can then be used for energy. The initial stage of this process, called glycolysis, occurs without use of oxygen and, therefore, is said to be anaerobic metabolism (see Chapter 68 ). During glycolysis, each glucose molecule is split into two pyruvic acid molecules, and energy is released to form four ATP molecules for each original glucose molecule, as explained in Chapter 68 . Ordinarily, the pyruvic acid then enters the mitochondria of muscle cells and reacts with oxygen to form still many more ATP molecules. However, when there is insufficient oxygen for this second stage (the oxidative stage) of glucose metabolism to occur, most of the pyruvic acid then is converted into lactic acid, which diffuses out of the muscle cells into the interstitial fluid and blood. Therefore, much of the muscle glycogen is transformed to lactic acid, but in doing so, considerable amounts of ATP are formed entirely without consumption of oxygen.
Another characteristic of the glycogen–lactic acid system is that it can form ATP molecules about 2.5 times as rapidly as can the oxidative mechanism of mitochondria. Therefore, when large amounts of ATP are required for short to moderate periods of muscle contraction, this anaerobic glycolysis mechanism can be used as a rapid source of energy. However, it is only about one-half as rapid as the phosphagen system. Under optimal conditions, the glycogen–lactic acid system can provide 1.3 to 1.6 minutes of maximal muscle activity in addition to the 8 to 10 seconds provided by the phosphagen system, although at somewhat reduced muscle power.
Aerobic System
The aerobic system is the oxidation of foodstuffs in the mitochondria to provide energy. As shown to the left in Figure 85-2 , glucose, fatty acids, and amino acids from the foodstuffs—after some intermediate processing—combine with oxygen to release tremendous amounts of energy that are used to convert AMP and ADP into ATP, as discussed in Chapter 68 .
In comparing this aerobic mechanism of energy supply with the glycogen–lactic acid system and the phosphagen system, the relative maximal rates of power generation in terms of moles of ATP generation per minute are the following:
| Moles of ATP/min | |
|---|---|
| Phosphagen system | 4 |
| Glycogen–lactic acid system | 2.5 |
| Aerobic system | 1 |
When comparing the same systems for endurance, the relative values are the following:
| Time | |
|---|---|
| Phosphagen system | 8-10 seconds |
| Glycogen–lactic acid system | 1.3-1.6 minutes |
| Aerobic system | Unlimited time (as long as nutrients last) |
Thus, one can readily see that the phosphagen system is 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 providing extra power during such intermediate races as 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 85-1 .
Table 85-1
Energy Systems Used in Various Sports
| Phosphagen System, Almost Entirely |
| 100-meter dash Jumping Weight lifting Diving Football dashes Baseball triple |
| Phosphagen and Glycogen–Lactic Acid Systems |
| 200-meter dash Basketball Ice hockey dashes 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 |
| Aerobic System |
| 10,000-meter skating Cross-country skiing Marathon run (26.2 miles, 42.2 kilometers) Jogging |
Recovery of Muscle Metabolic Systems After Exercise
In the same way that energy from phosphocreatine can be used to reconstitute ATP, energy from the glycogen–lactic acid system can be used to reconstitute phosphocreatine and ATP. Energy from the oxidative metabolism of the aerobic system can then be used to reconstitute all the other systems—the ATP, phosphocreatine, and glycogen–lactic acid systems.
Reconstitution of the lactic acid system means mainly the removal of the excess lactic acid that has accumulated in the body fluids. Removal of the excess lactic acid is especially important because buildup of lactic acid contributes to fatigue and the “burning” sensation in active muscles during intense exercise . 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 the body tissues, and (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 depletion results from two effects: (1) the so-called oxygen debt and (2) depletion of 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, 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 reconstitute 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 85-3 shows this principle of oxygen debt. During the first 4 minutes, as depicted in the figure, the person exercises heavily, and the rate of oxygen uptake increases more than 15-fold. Then, even after the exercise is over, the oxygen uptake still remains above normal; at first it is very high while the body is reconstituting the phosphagen system and repaying the stored oxygen portion of the oxygen debt, and then it is still above normal although at a lower level for another 40 minutes 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 process often requires days, rather than the seconds, minutes, or hours required for recovery of the phosphagen and lactic acid metabolic systems. Figure 85-4 shows this recovery process under three conditions: (1) in people who consume a high-carbohydrate diet; (2) in people who consume a high-fat, high-protein diet; and (3) in people who consume no food. Note that for persons who consume a high-carbohydrate diet, full recovery occurs in about 2 days. Conversely, people who consume a high-fat, high-protein diet or no food at all show very little recovery, even after as long as 5 days. The messages of this comparison are (1) it is important for athletes to consume a high-carbohydrate diet before a grueling athletic event and (2) athletes should not participate in exhaustive exercise during the 48 hours preceding the event.
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