Guyton and Hall Physiology Chapter 85 - Sports Physiology PDF

CHAPTER 85 UNIT XV Sports Physiology There are few stresses to which t...

CHAPTER 85 UNIT XV Sports Physiology There are few stresses to which the body is exposed that Testosterone secreted by the male testes has a power- approach the extreme stresses of heavy exercise. In fact, if ful anabolic effect in causing greatly increased deposition some of the extremes of exercise were continued for even of protein everywhere in the body, but especially in the moderately prolonged periods, they might be lethal. There- muscles. In fact, even a male who participates in very little sports activity but who nevertheless has a normal level of fore, sports physiology is mainly a discussion of the ulti- testosterone will have muscles that grow about 40% larger mate limits to which several of the bodily mechanisms can than those of a comparable female without the testoster- be stressed. To give one simple example: In a person who one. has extremely high fever approaching the level of lethality, The female sex hormone estrogen probably also ac- the body metabolism increases to about 100% above nor- counts for some of the difference between female and male mal. By comparison, the metabolism of the body during a performance, although not nearly so much as testosterone. marathon race may increase to 2000% above normal. Estrogen increases the deposition of fat in the female, espe- cially in the breasts, hips, and subcutaneous tissue. At least Female and Male Athletes partly for this reason, the average young (age 16–19 years) nonathletic female has about 34% body fat composition, in Most of the quantitative data that are given in this chapter contrast to the nonathletic young male (age 16–19 years), are for the young male athlete, not because it is desirable to who has about 23% (Figure 85-1). The average percentages know only these values, but because it is only in young male for body fat are higher in older men and women and have athletes that relatively complete measurements have been increased substantially over the past 20 to 30 years as the made. Measurements in older athletes and in females are prevalence of obesity has risen in populations of most de- much less complete. However, for measurements that have veloped countries. In the United States, for example, the been made in the female athlete, similar basic physiological prevalence of obesity in now approximately 37% of the principles apply, except for quantitative differences caused adult population. Increased body fat composition is a detri- by differences in body size, body composition, and the ment to the highest levels of athletic performance in events presence or absence of the male sex hormone testosterone. in which performance depends on speed or on the ratio of In general, most quantitative values for women—such total body muscle strength to body weight. as muscle strength, pulmonary ventilation, and cardiac out- put, all of which are related mainly to the muscle mass— Muscles in Exercise vary between two-thirds and three-quarters of the values Strength, Power, and Endurance of Muscles recorded in men, although there are many exceptions to this generalization. When measured in terms of strength The final common determinant of success in athletic events per square centimeter of cross-sectional area, the female is what the muscles can do for you—that is, what strength muscle can achieve almost exactly the same maximal force they can give when it is needed, what power they can of contraction as that of the male muscle—between 3 and achieve in the performance of work, and how long they can 4 kg/cm2. Therefore, most of the difference in total muscle continue their activity. performance lies in the extra percentage of the male body The strength of a muscle is determined mainly by its size, that is muscle, which is caused partly by endocrine differ- with a maximal contractile force between 3 and 4 kg/cm2 ences that we will discuss later. of muscle cross-sectional area. Thus, a person who has The performance capabilities of the female versus male enlarged his or her muscles through an exercise train- athlete are illustrated by the relative running speeds for a ing program will have correspondingly increased muscle marathon race. In a comparison, the top female performer strength. had a running speed that was 11% less than that of the top To give an example of muscle strength, a world-class male performer. For other events, however, women have at male weight lifter might have a quadriceps muscle with times held records faster than men—for example, for the a cross-sectional area as great as 150 square centimeters. two-way swim across the English Channel, for which the This measurement would translate into a maximal con- availability of extra fat seems to be an advantage for heat tractile strength of 525 kilograms (or 1155 pounds), with insulation, buoyancy, and extra long-term energy. all this force applied to the patellar tendon. Therefore, one 1073 Unit XV Sports Physiology Male Female kg-m/min 50 First 8 to 10 seconds 7000 Body fat (% of body weight) 40 Next 1 minute 4000 Next 30 minutes 1700 30 Thus, it is clear that a person has the capability of extreme power surges for short periods, such as during a 100-meter 20 dash that is completed entirely within 10 seconds, whereas for long-term endurance events, the power output of the muscles 10 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 0 the next 30 minutes, because the efficiency for translation 8–11 12–15 16–19 20–39 40–59 60–79 ≥80 of muscle power output into athletic performance is often Age group (yrs) much less during rapid activity than during less rapid but Figure 85-1 Average percentage body fat in males and females at sustained activity. Thus, the velocity of the 100-meter dash different ages. (Data from the National Health and Nutrition Examina- is only 1.75 times as great as the velocity of a 30-minute tion Survey, United States, 1999–2004). race, despite the 4-fold difference in short-term versus long-term muscle power capability. Another measure of muscle performance is endurance. can readily understand how it is possible for this tendon at Endurance, to a great extent, depends on the nutritive sup- times to be ruptured or actually to be avulsed from its in- port for the muscle—more than anything else, it depends sertion into the tibia below the knee. Also, when such forc- on the amount of glycogen that has been stored in the mus- es occur in tendons that span a joint, similar forces are ap- cle before the period of exercise. A person who consumes a plied to the surfaces of the joint or sometimes to ligaments high-carbohydrate diet stores far more glycogen in muscles spanning the joints, thus accounting for such happenings as than does a person who consumes either a mixed diet or a displaced cartilages, compression fractures about the joint, high-fat diet. Therefore, endurance is enhanced by a high- and torn ligaments. carbohydrate diet. When athletes run at speeds typical for The holding strength of muscles is about 40% greater the marathon race, their endurance (as measured by the than the contractile strength. That is, if a muscle is already time that they can sustain the race until complete exhaus- contracted and a force then attempts to stretch out the tion) is approximately the following: muscle, as occurs when landing after a jump, this action Minutes requires about 40% more force than can be achieved by a shortening contraction. Therefore, the force of 525 kilo- High-carbohydrate diet 240 grams previously calculated for the patellar tendon dur- Mixed diet 120 ing muscle contraction becomes 735 kilograms (1617 High-fat diet 85 pounds) during holding contractions, which further compounds the problems of the tendons, joints, and liga- The corresponding amounts of glycogen stored in the ments. It can also lead to internal tearing in the muscle. In muscle before the race started explain these differences. fact, forceful stretching of a maximally contracted muscle The amounts stored are approximately the following: is one of the surest ways to create the highest degree of g/kg Muscle muscle soreness. Mechanical work performed by a muscle is the High-carbohydrate diet 40 amount of force applied by the muscle multiplied by the Mixed diet 20 distance over which the force is applied. The power of High-fat diet 6 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 Muscle Metabolic Systems in Exercise is therefore determined not only by the strength of mus- The same basic metabolic systems are present in muscle as cle contraction but also by its distance of contraction in other parts of the body; these systems are discussed in and the number of times that it contracts each minute. detail in Chapters 68 through 74. However, special quanti- Muscle power is generally measured in kilogram me- tative measures of the activities of three metabolic systems ters (kg-m) per minute. That is, a muscle that can lift are exceedingly important in understanding the limits of 1-kilogram weight to a height of 1 meter or that can physical activity. These systems are (1) the phosphocreatine- move some object laterally against a force of 1 kilogram creatine system, (2) the glycogen–lactic acid system, and (3) for a distance of 1 meter in 1 minute is said to have a the aerobic system. power of 1 kg-m/min. The maximal power achievable Adenosine Triphosphate. The source of energy actually by all the muscles in the body of a highly trained athlete used to cause muscle contraction is adenosine triphosphate with all the muscles working together is approximately (ATP), which has the following basic formula: the following: Adenosine − PO3 ∼ PO3 ∼ PO3 − 1074  Chapter 85 Sports Physiology I. Phosphocreatine Creatine + PO3− ATP Energy II. Glycogen Lactic acid ADP for muscle UNIT XV contraction III. Glucose AMP Fatty acids + O2 CO2 + H2O Amino acids + Figure 85-2 Important metabolic systems that sup- Urea ply energy for muscle contraction. The bonds attaching the last two phosphate radicals to Thus, the energy from the phosphagen system is used for the molecule, designated by the symbol ∼, are high-energy maximal short bursts of muscle power. phosphate bonds. Each of these bonds stores 7300 calories Glycogen–Lactic Acid System. The stored glycogen in of energy per mole of ATP under standard conditions (and muscle can be split into glucose, and the glucose can then even slightly more than this under the physical conditions in be used for energy. The initial stage of this process, called the body, which is discussed in detail in Chapter 68). There- glycolysis, occurs without use of oxygen and, therefore, is fore, when one phosphate radical is removed, more than said to be anaerobic metabolism (see Chapter 68). During 7300 calories of energy are released to energize the mus- glycolysis, each glucose molecule is split into two pyruvic cle contractile process. Then, when the second phosphate acid molecules, and energy is released to form four ATP radical is removed, still another 7300 calories become avail- molecules for each original glucose molecule, as explained able. Removal of the first phosphate converts the ATP into in Chapter 68. Ordinarily, the pyruvic acid then enters the adenosine diphosphate (ADP), and removal of the second mitochondria of muscle cells and reacts with oxygen to converts this ADP into adenosine monophosphate (AMP). form still many more ATP molecules. However, when there The amount of ATP present in the muscles, even in a well- is insufficient oxygen for this second stage (the oxidative trained athlete, is sufficient to sustain maximal muscle power stage) of glucose metabolism to occur, most of the pyru- for only about 3 seconds, which might be enough for one half vic acid then is converted into lactic acid, which diffuses of a 50-meter dash. Therefore, except for a few seconds at a out of the muscle cells into the interstitial fluid and blood. time, it is essential that new ATP be formed continuously, Therefore, much of the muscle glycogen is transformed to even during the performance of short athletic events. Figure lactic acid, but in doing so, considerable amounts of ATP 85-2 shows the overall metabolic system, demonstrating the are formed entirely without consumption of oxygen. breakdown of ATP first to ADP and then to AMP, with release Another characteristic of the glycogen–lactic acid system of energy to the muscles for contraction. The left-hand side of is that it can form ATP molecules about 2.5 times as rapidly the figure shows the three metabolic systems that provide a as can the oxidative mechanism of mitochondria. Therefore, continuous supply of ATP in the muscle fibers. when large amounts of ATP are required for short to moder- ate periods of muscle contraction, this anaerobic glycolysis Phosphocreatine-Creatine System mechanism can be used as a rapid source of energy. However, Phosphocreatine (also called creatine phosphate) is anoth- it is only about one-half as rapid as the phosphagen system. er chemical compound that has a high-energy phosphate Under optimal conditions, the glycogen–lactic acid system bond, with the following formula: can provide 1.3 to 1.6 minutes of maximal muscle activity in addition to the 8 to 10 seconds provided by the phosphagen Creatine ∼ PO3 − system, although at somewhat reduced muscle power. Phosphocreatine can decompose to creatine and phos- Aerobic System. The aerobic system is the oxidation phate ion, as shown in Figure 85-2, and in doing so releases of foodstuffs in the mitochondria to provide energy. As large amounts of energy. In fact, the high-energy phosphate shown to the left in Figure 85-2, glucose, fatty acids, and bond of phosphocreatine has more energy than the bond amino acids from the foodstuffs—after some intermediate of ATP: 10,300 calories per mole compared with 7300 for processing—combine with oxygen to release tremendous the ATP bond. Therefore, phosphocreatine can easily pro- amounts of energy that are used to convert AMP and ADP vide enough energy to reconstitute the high-energy bond into ATP, as discussed in Chapter 68. of ATP. Furthermore, most muscle cells have two to four In comparing this aerobic mechanism of energy supply times as much phosphocreatine as ATP. with the glycogen–lactic acid system and the phosphagen sys- A special characteristic of energy transfer from phos- tem, the relative maximal rates of power generation in terms of phocreatine to ATP is that it occurs within a small frac- moles of ATP generation per minute are the following: tion of a second. Therefore, all the energy stored in muscle phosphocreatine is almost instantaneously available for Moles of ATP/min muscle contraction, just as is the energy stored in ATP. Phosphagen system 4 The combined amounts of cell ATP and cell phospho- creatine are called the phosphagen energy system. These Glycogen–lactic acid system 2.5 substances together can provide maximal muscle power Aerobic system 1 for 8 to 10 seconds, almost enough for the 100-meter run. 1075 Unit XV Sports Physiology Table 85-1 Energy Systems Used in Various Sports 5 Rate of oxygen uptake (L/min) Phosphagen System, Almost Entirely 4 100-meter dash Jumping Weight lifting 3 Diving Football dashes 2 Alactacid oxygen debt = 3.5 liters Exercise Baseball triple Phosphagen and Glycogen–Lactic Acid Systems 1 Lactic acid oxygen debt = 8 liters 200-meter dash Basketball 0 Ice hockey dashes 0 4 8 12 16 20 24 28 32 36 40 44 Glycogen–lactic acid system, mainly Minutes 400-meter dash Figure 85-3 Rate of oxygen uptake by the lungs during maximal ex- 100-meter swim ercise for 4 minutes and then for about 40 minutes after the exercise Tennis is over. This figure demonstrates the principle of oxygen debt. Soccer Glycogen–Lactic Acid and Aerobic Systems Reconstitution of the lactic acid system means mainly 800-meter dash the removal of the excess lactic acid that has accumulated 200-meter swim in the body fluids. Removal of the excess lactic acid is espe- 1500-meter skating cially important because buildup of lactic acid contributes Boxing to fatigue and the “burning” sensation in active muscles 2000-meter rowing during intense exercise. When adequate amounts of en- 1500-meter run ergy are available from oxidative metabolism, removal of 1-mile run lactic acid is achieved in two ways: (1) A small portion of it 400-meter swim is converted back into pyruvic acid and then metabolized Aerobic System oxidatively by the body tissues, and (2) the remaining lactic 10,000-meter skating acid is reconverted into glucose mainly in the liver, and the Cross-country skiing glucose in turn is used to replenish the glycogen stores of Marathon run (26.2 miles, 42.2 kilometers) the muscles. Jogging 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 When comparing the same systems for endurance, the rela- from two effects: (1) the so-called oxygen debt and (2) de- tive values are the following: pletion of glycogen stores of the muscles. Oxygen Debt. The body normally contains about 2 lit- Time ers of stored oxygen that can be used for aerobic metabo- Phosphagen system 8-10 seconds lism even without breathing any new oxygen. This stored oxygen consists of the following: (1) 0.5 liter in the air of Glycogen–lactic acid system 1.3-1.6 minutes the lungs, (2) 0.25 liter dissolved in the body fluids, (3) 1 Aerobic system Unlimited time (as long liter combined with the hemoglobin of the blood, and (4) as nutrients last) 0.3 liter stored in the muscle fibers, combined mainly with Thus, one can readily see that the phosphagen system is myoglobin, an oxygen-binding chemical similar to hemo- used by the muscle for power surges of a few seconds, and globin. the aerobic system is required for prolonged athletic activ- In heavy exercise, almost all this stored oxygen is used ity. In between is the glycogen–lactic acid system, which is within a minute or so for aerobic metabolism. Then, after especially important for providing extra power during such the exercise is over, this stored oxygen must be replenished intermediate races as 200- to 800-meter runs. by breathing extra amounts of oxygen over and above the What Types of Sports Use Which Energy Systems? By normal requirements. In addition, about 9 liters more oxy- considering the vigor of a sports activity and its duration, gen must be consumed to reconstitute the phosphagen sys- one can estimate closely which of the energy systems is tem and the lactic acid system. All this extra oxygen that used for each activity. Various approximations are present- must be “repaid,” about 11.5 liters, is called the oxygen debt. ed in Table 85-1. Figure 85-3 shows this principle of oxygen debt. Dur- Recovery of Muscle Metabolic Systems After ing the first 4 minutes, as depicted in the figure, the person exercises heavily, and the rate of oxygen uptake increases Exercise. In the same way that energy from phosphocre- more than 15-fold. Then, even after the exercise is over, atine can be used to reconstitute ATP, energy from the the oxygen uptake still remains above normal; at first it is glycogen–lactic acid system can be used to reconstitute very high while the body is reconstituting the phosphagen phosphocreatine and ATP. Energy from the oxidative me- system and repaying the stored oxygen portion of the tabolism of the aerobic system can then be used to recon- oxygen debt, and then it is still above normal although at stitute all the other systems—the ATP, phosphocreatine, a lower level for another 40 minutes while the lactic acid and glycogen–lactic acid systems. 1076  Chapter 85 Sports Physiology Muscle glycogen content (g/kg muscle) 2 hours of 100 0 exercise High-carbohydrate diet Percent carbohydrate usage 24 75 25 Percent fat usage 20 High-carbohydrate diet UNIT XV 16 50 50 Mixed 12 diet 8 25 Exhaustion 75 High-fat diet No food Fat and protein diet 4 0 100 0 0 10 20 40 2 4 1 2 3 4 0 10 20 30 40 50 5 days Seconds Minutes Hours Hours of recovery Duration of exercise Figure 85-4 The effect of diet on the rate of muscle glycogen re- plenishment after prolonged exercise. (Modified from Fox EL: Sports Figure 85-5 The effect of duration of exercise, as well as type of diet, Physiology. Philadelphia: Saunders College Publishing, 1979.) on relative percentages of carbohydrate or fat used for energy by muscles. (Data from Fox EL: Sports Physiology. Philadelphia: Saunders College Publishing, 1979.) 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 Not all the energy from carbohydrates comes from the amounts to about 8 liters. stored muscle glycogen. In fact, almost as much glycogen is Recovery of Muscle Glycogen. Recovery from exhaus- stored in the liver as in the muscles, and this glycogen can tive muscle glycogen depletion is not a simple matter. This be released into the blood in the form of glucose and then process often requires days, rather than the seconds, min- taken up by the muscles as an energy source. In addition, utes, or hours required for recovery of the phosphagen and glucose solutions given to an athlete to drink during the lactic acid metabolic systems. Figure 85-4 shows this re- course of an athletic event can provide as much as 30% to covery process under three conditions: (1) in people who 40% of the energy required during prolonged events such consume a high-carbohydrate diet; (2) in people who con- as marathon races. sume a high-fat, high-protein diet; and (3) in people who Therefore, if muscle glycogen and blood glucose are consume no food. Note that for persons who consume a available, they are the energy nutrients of choice for intense high-carbohydrate diet, full recovery occurs in about 2 days. muscle activity. Even so, for a long-term endurance event, Conversely, people who consume a high-fat, high-protein one can expect fat to supply more than 50% of the required diet or no food at all show very little recovery, even after as energy after about the first 3 to 4 hours. long as 5 days. The messages of this comparison are (1) it Effect of Athletic Training on Muscles and Muscle is important for athletes to consume a high-carbohydrate Performance diet before a grueling athletic event and (2) athletes should not participate in exhaustive exercise during the 48 hours Maximal Resistance Training Increases Muscle Strength. preceding the event. One of the cardinal principles of muscle development dur- ing athletic training is the following: Muscles that function Nutrients Used During Muscle Activity under no load, even if they are exercised for hours on end, increase little in strength. At the other extreme, muscles In addition to the use of a large amount of carbohydrates that contract at more than 50% maximal force of contrac- by the muscles during exercise, especially during the ear- tion will develop strength rapidly even if the contractions ly stages of exercise, muscles use large amounts of fat for are performed only a few times each day. Using this prin- energy in the form of fatty acids and acetoacetic acid (see ciple, experiments on muscle building have shown that six Chapter 69), as well as (to a much less extent) proteins in nearly maximal muscle contractions performed in three sets the form of amino acids. In fact, even under the best condi- 3 days a week give approximately optimal increase in mus- tions, in endurance athletic events that last longer than 4 to cle strength without producing chronic muscle fatigue. 5 hours, the glycogen stores of the muscle become almost The upper curve in Figure 85-6 shows the approximate totally depleted and are of little further use for energizing percentage increase in strength that can be achieved in a muscle contraction. Instead, the muscle now depends on previously untrained young person by this resistive training energy from other sources, mainly from fats. program, demonstrating that the muscle strength increases Figure 85-5 shows the approximate relative usage of about 30% during the first 6 to 8 weeks but almost plateaus carbohydrates and fat for energy during prolonged ex- after that time. Along with this increase in strength is an haustive exercise under three dietary conditions: a high- approximately equal percentage increase in muscle mass, carbohydrate diet, a mixed diet, and a high-fat diet. Note which is called muscle hypertrophy. that most of the energy is derived from carbohydrates In old age, many people become so sedentary that their during the first few seconds or minutes of the exercise, muscles atrophy tremendously. In these cases, however, but at the time of exhaustion, as much as 60% to 85% of muscle training may increase muscle strength more than the energy is being derived from fats rather than carbo- 100%. hydrates. 1077 Unit XV Sports Physiology far more mitochondria than do the fast-twitch fibers. Resistive training Percent increase in strength 30 In addition, they contain considerably more myoglobin, a hemoglobin-like protein that combines with oxygen 25 within the muscle fiber; the extra myoglobin increases 20 the rate of diffusion of oxygen throughout the fiber by 15 shuttling oxygen from one molecule of myoglobin to the next. In addition, the enzymes of the aerobic metabolic 10 system are considerably more active in slow-twitch fib- 5 ers than in fast-twitch fibers. No-load training 4. The number of capillaries is greater in the vicinity of 0 slow-twitch fibers than in the vicinity of fast-twitch 0 2 4 6 8 10 fibers. Weeks of training In summary, fast-twitch fibers can deliver extreme amounts of power for a few seconds to a minute or so. Figure 85-6 Approximate effect of optimal resistive exercise training on increase in muscle strength over a training period of 10 weeks. Conversely, slow-twitch fibers provide endurance, deliver- ing prolonged strength of contraction over many minutes Muscle Hypertrophy. The average size of a person’s to hours. muscles is determined to a great extent by heredity plus Hereditary Differences Among Athletes for Fast-Twitch the level of testosterone secretion, which, in men, causes Versus Slow-Twitch Muscle Fibers. Some people have con- considerably larger muscles than in women. With training, siderably more fast-twitch than slow-twitch fibers, and however, the muscles can become hypertrophied perhaps others have more slow-twitch fibers; this factor could de- an additional 30% to 60%. Most of this hypertrophy results termine to some extent the athletic capabilities of differ- from increased diameter of the muscle fibers rather than ent individuals. Athletic training may change the relative increased numbers of fibers. However, a very few greatly proportions of fast-twitch and slow-twitch fibers as much enlarged muscle fibers are believed to split down the mid- as 10%. However, the relative proportions of fast-twitch dle along their entire length to form entirely new fibers, and slow-twitch fibers seem to be determined to a great thus increasing the number of fibers slightly. extent by genetic inheritance, which in turn helps deter- The changes that occur inside the hypertrophied muscle mine which area of athletics is most suited to each person: fibers include (1) increased numbers of myofibrils, propor- some people appear to be born to be marathoners, whereas tionate to the degree of hypertrophy; (2) up to 120% in- others are born to be sprinters and jumpers. For example, crease in mitochondrial enzymes; (3) as much as 60% to the following values are recorded percentages of fast-twitch 80% increase in the components of the phosphagen meta- versus slow-twitch fiber in the quadriceps muscles of dif- bolic system, including ATP and phosphocreatine; (4) as ferent types of athletes: much as 50% increase in stored glycogen; and (5) as much as 75% to 100% increase in stored triglyceride (fat). Because Fast-Twitch Fiber Slow-Twitch Fiber of all these changes, the capabilities of both the anaerobic Marathoners 18 82 and the aerobic metabolic systems are increased, especially increasing the maximum oxidation rate and efficiency of Swimmers 26 74 the oxidative metabolic system as much as 45%. Average male 55 45 Fast-Twitch and Slow-Twitch Muscle Fibers. In the hu- Weight lifters 55 45 man being, all muscles have varying percentages of fast- Sprinters 63 37 twitch and slow-twitch muscle fibers. For example, the gastrocnemius muscle has a higher preponderance of fast- Jumpers 63 37 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- Respiration in Exercise twitch muscle fibers, and therefore is used to a greater ex- Although one’s respiratory ability is of relatively little con- tent for prolonged lower leg muscle activity. cern in the performance of sprint types of athletics, it is The basic differences between the fast-twitch and the critical for maximal performance in endurance athletics. slow-twitch fibers are the following: Oxygen Consumption and Pulmonary Ventilation in 1. Fast-twitch fibers are about twice as large in diameter Exercise. Normal oxygen consumption for a young man at compared with slow-twitch fibers. rest is about 250 ml/min. However, under maximal condi- 2. The enzymes that promote rapid release of energy from tions, this consumption can be increased to approximately the phosphagen and glycogen–lactic acid energy sys- the following average levels: tems are two to three times as active in fast-twitch fib- ers as in slow-twitch fibers, thus making the maximal power that can be achieved for very short periods by ml/min fast-twitch fibers about twice as great as that of slow- Untrained average male 3600 twitch fibers. Athletically trained average male 4000 3. Slow-twitch fibers are mainly organized for endurance, especially for generation of aerobic energy. They have Male marathon runner 5100 1078  Chapter 85 Sports Physiology 120 3.8 Total ventilation (L/min) 110 3.6 Vo2max (L/min) 100 3.4 80 UNIT XV Training frequency 60 3.2 = 5 days/wk = 4 days/wk
40 3.0 = 2 days/wk 20 Moderate Severe exercise exercise 2.8 0 0 2 4 6 8 10 12 14 0 1.0 2.0 3.0 4.0 Weeks of training O2 consumption (L/min) Figure 85-8 Increase in V̇o2 max over a period of 7 to 13 weeks of Figure 85-7 Effect of exercise on oxygen consumption and ventila- athletic training. (Modified from Fox EL: Sports Physiology. Philadel- tory rate. (Modified from Gray JS: Pulmonary Ventilation and Its Physi- phia: Saunders College Publishing, 1979.) ological Regulation. Springfield, IL: Charles C Thomas, 1950.) Figure 85-7 shows the relation between oxygen consump- marathoners. However, it is also likely. that many years of tion and total pulmonary ventilation at different levels of training increase the marathoner’s Vo2max by values con- exercise. As would be expected, there is a linear relation. siderably greater than the 10% that has been recorded in Both oxygen consumption and total pulmonary ventilation short-term experiments such as that in Figure 85-8. increase about 20-fold between the resting state and maxi- Oxygen-Diffusing Capacity of Athletes. The oxygen- mal intensity of exercise in the well-trained athlete. diffusing capacity is a measure of the rate at which oxygen Limits of Pulmonary Ventilation. How severely do we can diffuse from the pulmonary alveoli into the blood. This stress our respiratory systems during exercise? This ques- capacity is expressed in terms of milliliters of oxygen that tion can be answered by the following comparison for a will diffuse each minute for each millimeter of mercury dif- normal young man: ference between alveolar partial pressure of oxygen and pul- monary blood oxygen pressure. That is, if the partial pres- L/min sure of oxygen in the alveoli is 91 mm Hg and the oxygen Pulmonary ventilation at 100–110 pressure in the blood is 90 mm Hg, the amount of oxygen maximal exercise that diffuses through the respiratory membrane each min- Maximal breathing capacity 150–170 ute is equal to the diffusing capacity. The following values are measured values for different diffusing capacities: Thus, the maximal breathing capacity is about 50% greater than the actual pulmonary ventilation during maximal exer- ml/min cise. This difference provides an element of safety for athletes, Nonathlete at rest 23 giving them extra ventilation that can be called on in such con- Nonathlete during maximal exercise 48 ditions as (1) exercise at high altitudes, (2) exercise under very hot conditions, and (3) abnormalities in the respiratory system. Speed skater during maximal exercise 64 The important point is that the respiratory system is not Swimmer during maximal exercise 71 normally the most limiting factor in the delivery of oxygen Oarsman during maximal exercise 80 to the muscles during maximal muscle aerobic metabolism. We shall see shortly that the ability of the heart to pump The most startling fact about these results is the severalfold in- blood to the muscles is usually a greater limiting factor. crease in diffusing capacity between the resting state and the. Effect of Training on Vo2max. The abbreviation for the state of maximal exercise. This finding results mainly from the rate of oxygen. usage (in L/min) under maximal aerobic fact that blood flow through many of the pulmonary capillar- metabolism is Vo2max. Figure. 85-8 shows the progressive ies is sluggish or even dormant in the resting state, whereas effect of athletic training on Vo2max recorded in a group in maximal exercise, increased blood flow through the lungs of subjects beginning at the level of no training and then causes all the pulmonary capillaries to be perfused at their while pursuing the training program. for 7 to 13 weeks. In maximal rates, thus providing a far greater surface area through this study, it is surprising that the Vo2max increased only which oxygen can diffuse into the pulmonary capillary blood. about 10%. Furthermore, the frequency of training, wheth- It is also clear from these values that athletes who re- er two times. or five times per week, had little effect. on the quire greater amounts of oxygen per minute have higher increase in Vo2max. Yet, as pointed out earlier, the Vo2max diffusing capacities. Is this the case because people with of a marathoner is about 45% greater. than that of an un- naturally greater diffusing capacities choose these types of trained person. Part of this greater Vo2max of the mara- sports, or is it because something about the training pro- thoner may be genetically determined; that is, people who cedures increases the diffusing capacity? The answer is un- have greater chest sizes in relation to body size and stronger certain, but it is very likely that training, particularly endur- respiratory muscles may select themselves to become ance training, does play an important role. 1079 Unit XV Sports Physiology Blood Gases During Exercise. Because of the great oxy- Rhythmic exercise gen usage by the muscles in exercise, one might expect the 40 Calf blood flow (100 ml/min) oxygen pressure of the arterial blood to decrease markedly during strenuous athletics and the carbon dioxide pressure of the venous blood to increase far above normal. How- ever, this normally is not the case. Both of these values re- main nearly normal, demonstrating the extreme ability of 20 the respiratory system to provide adequate aeration of the blood, even during heavy exercise. This demonstrates another important point: The blood gases do not always have to become abnormal for respiration to be stimulated in exercise. Instead, respiration is stimu- lated mainly by neurogenic mechanisms during exercise, as discussed in Chapter 42. Part of this stimulation results 0 10 16 18 from direct stimulation of the respiratory center by the Minutes same nervous signals that are transmitted from the brain Figure 85-9 Effects of muscle exercise on blood flow in the calf of to the muscles to cause the exercise. An additional part is a leg during strong rhythmic contractions. The blood flow was much believed to result from sensory signals transmitted into the less during contraction than between contractions. (Modified from respiratory center from the contracting muscles and mov- Barcroft J, Dornhorst AC: The blood flow through the human calf ing joints. All this extra nervous stimulation of respiration during rhythmic exercise, J Physiol 109:402, 1949.) is normally sufficient to provide the necessary increase in pulmonary ventilation required to keep the blood oxygen great increase in flow—about 13-fold—but also the flow and carbon dioxide very near to normal. decrease during each muscle contraction. Two points can Effect of Smoking on Pulmonary Ventilation in Exercise. be made from this study: It is widely known that smoking can decrease an athlete’s 1. The actual contractile process itself temporarily de- “wind.” This is true for many reasons: creases muscle blood flow because the contracting 1. One effect of nicotine is constriction of terminal bron- skeletal muscle compresses the intramuscular blood chioles of the lungs, which increases the resistance of vessels; therefore, strong tonic muscle contractions can airflow into and out of the lungs. cause rapid muscle fatigue because of lack of delivery of 2. The irritating effects of smoke cause increased fluid se- enough oxygen and other nutrients during the continu- cretion into the bronchial tree, as well as some swelling ous contraction. of the epithelial linings. 2. The blood flow to muscles during exercise increases mark- 3. Nicotine paralyzes the cilia on the surfaces of respira- edly. The following comparison shows the maximal in- tory epithelial cells that normally beat continuously crease in blood flow that can occur in a well-trained athlete: to remove excess fluids and foreign particles from the respiratory passageways. As a result, much debris ac- ml/100 g Muscle/min cumulates in the passageways and adds further to the Resting blood flow 3.6 difficulty of breathing. Blood flow during maximal 90 After putting all these factors together, even a light exercise smoker often feels respiratory strain during maximal exer- cise, and the level of performance may be reduced. Thus, muscle blood flow can increase a maximum of about Much more severe are the effects of chronic smoking. 25-fold during the most strenuous exercise. Almost one- There are few chronic smokers in whom some degree of half this increase in flow results from intramuscular vaso- emphysema does not develop. In this disease, the following dilation caused by the direct effects of increased muscle mechanisms occur: (1) chronic bronchitis, (2) obstruction metabolism, as explained in Chapter 21. The remaining of many of the terminal bronchioles, and (3) destruction of increase results from multiple factors, the most important many alveolar walls. In persons with severe emphysema, of which is probably the moderate increase in arterial blood as much as four-fifths of the respiratory membrane can be pressure that occurs in exercise, which is usually about a destroyed; then even the slightest exercise can cause res- 30% increase. The increase in pressure not only forces more piratory distress. In fact, many such patients cannot even blood through the blood vessels but also stretches the walls perform the simple feat of walking across the floor of a sin- of the arterioles and further reduces vascular resistance. gle room without gasping for breath. Therefore, a 30% increase in blood pressure can often more than double the blood flow, which multiplies the great in- Cardiovascular System in Exercise crease in flow already caused by the metabolic vasodilation Muscle Blood Flow. A key requirement of cardiovascular at least another 2-fold. function in exercise is to deliver the required oxygen and Work Output, Oxygen Consumption, and Cardiac Out- other nutrients to the exercising muscles. For this purpose, put During Exercise. Figure 85-10 shows the interrelations the muscle blood flow increases drastically during exercise. among work output, oxygen consumption, and cardiac out- Figure 85-9 shows a recording of muscle blood flow in the put during exercise. It is not surprising that all these factors calf of a person for a period of 6 minutes during moder- are directly related to one another, as shown by the linear ately strong intermittent contractions. Note not only the functions, because the muscle work output increases oxy- 1080  Chapter 85 Sports Physiology 35 Table 85-2 Comparison of Cardiac Function Between Oxygen consumption (L/min) Marathoner and Nonathlete Cardiac output (L/min) 30 Cardiac index (L/min/m2) ex 15 c ind 4 Stroke Volume Heart Rate 25 cardia d Type of Athlete (ml) (beats/min) 20 u t an 3 10 utp tion Resting UNIT XV 15 iaco mp ard su 2 C n con Nonathlete 75 75 10 e 5 O xyg 1 Marathoner 105 50 5 Maximum 0 0 0 Nonathlete 110 195 0 200 400 600 800 1000120014001600 Marathoner 162 185 Work output during exercise (kg-m/min) Figure 85-10 Relation between cardiac output and work output (sol- id line) and between oxygen consumption and work output (dashed 190 line) during different levels of exercise. The different colored dots and squares show data derived from different studies in humans. (Modi- 170 Stroke volume (ml/beat) Stroke volume Heart rate (beats/min) fied from Guyton AC, Jones CE, Coleman TB: Circulatory Physiology: 165 150 Cardiac Output and Its Regulation. Philadelphia: WB Saunders, 1973.) 150 130 110 gen consumption, and increased oxygen consumption in 135 turn dilates the muscle blood vessels, thus increasing venous Heart rate 90 return and cardiac output. Typical cardiac outputs at several 120 70 levels of exercise are as follows: 105 50 L/min Cardiac output in a young man at rest 5.5 5 10 15 20 25 30 Cardiac output (L/min) Maximal cardiac output during exercise in a 23 young untrained man Figure 85-11 Approximate stroke volume output and heart rate at different levels of cardiac output in a marathon athlete. Maximal cardiac output during exercise in an 30 average male marathoner Thus, the normal untrained person can increase cardiac Role of Stroke Volume and Heart Rate in Increas- output a little over 4-fold, and the well- trained athlete ing Cardiac Output. Figure 85-11 shows the approxi- can increase output about 6-fold. Cardiac outputs as mate changes in stroke volume and heart rate as the cardiac great as 35 to 40 L/min, or seven to eight times normal output increases from its resting level of about 5.5 L/min resting output, have been measured in individual mara- to 30 L/min in the marathon runner. The stroke volume thoners. increases from 105 to 162 ml, an increase of about 50%, Effect of Training on Heart Hypertrophy and on Cardiac whereas the heart rate increases from 50 to 185 beats/min, Output. From the foregoing data, it is clear that maratho- an increase of 270%. Therefore, the heart rate increase by ners can achieve maximal cardiac outputs that are about far accounts for a greater proportion of the increase in car- 40% greater than those achieved by untrained persons. diac output than does the increase in stroke volume during This results mainly from the fact that the heart chambers sustained strenuous exercise. The stroke volume normally of marathoners enlarge about 40%; along with this enlarge- reaches its maximum by the time the cardiac output has in- ment of the chambers, the heart mass also increases 40% or creased only halfway to its maximum. Any further increase more. Therefore, not only do the skeletal muscles hypertro- in cardiac output must occur by increasing the heart rate. phy during athletic training, but so does the heart. Howev- Relation of Cardiovascular Performance to V̇o2 max. er, heart enlargement and increased pumping capacity oc- During maximal exercise, both the heart rate and stroke cur mainly in the endurance types, not in the sprint types, volume are increased to about 95% of their maximal levels. of athletic training. Because the cardiac output is equal to stroke volume times Even though the heart of the marathoner is considerably heart rate, the cardiac output is about 90% of the maximum larger than that of the normal person, resting cardiac out- that the person can achieve, which is in contrast to about put is almost exactly the same as that in a normal person. 65% of maximum for pulmonary ventilation. Therefore, one However, this normal cardiac output is achieved by a large can readily see that the cardiovascular system is normally stroke volume at a reduced heart rate. Table 85-2 compares much more limiting on V̇o2 max than is the respiratory sys- stroke volume and heart rate in the untrained person and tem because oxygen utilization by the body can never be the marathoner. more than the rate at which the cardiovascular system can Thus, the heart-pumping effectiveness of each heartbeat transport oxygen to the tissues. is 40% to 50% greater in the highly trained athlete than in For this reason, it is frequently stated that the level of the untrained person, but there is a corresponding decrease athletic performance that can be achieved by the maratho- in the heart rate at rest. ner mainly depends on the performance capability of his or 1081 Unit XV Sports Physiology her heart, because this is the most limiting link in the de- This entire complex is called heatstroke, and failure to livery of adequate oxygen to the exercising muscles. There- treat it immediately can lead to death. In fact, even though fore, the 40% greater cardiac output that the marathoner the person has stopped exercising, the temperature does can achieve over the average untrained male is probably the not easily decrease, partly because at these high tempera- single most important physiological benefit of the mara- tures, the temperature-regulating mechanism often fails thoner’s training program. (see Chapter 74). A second reason is that in heatstroke, Effect of Heart Disease and Old Age on Athletic the very high body temperature approximately doubles the Performance. Because of the critical limitation that the rates of all intracellular chemical reactions, thus liberating cardiovascular system places on maximal performance in still more heat. endurance athletics, one can readily understand that any The treatment of heatstroke is to reduce the body tem- type of heart disease that reduces maximal cardiac output perature as rapidly as possible. The most practical way to will cause an almost corresponding decrease in achievable reduce the body temperature is to remove all clothing, total body muscle power. Therefore, a person with conges- maintain a spray of cool water on all surfaces of the body tive heart failure frequently has difficulty achieving even or continually sponge the body, and blow air over the body the muscle power required to climb out of bed, much less with a fan. Experiments have shown that this treatment to walk across the floor. can reduce the temperature either as rapidly or almost as The maximal cardiac output of older people also de- rapidly as any other procedure, although some physicians creases considerably; there is as much as a 50% decrease be- prefer total immersion of the body in water containing a tween ages 18 and 80 years. Also, there is even more of a de- mush of crushed ice, if available. crease in maximal breathing capacity. For these reasons, as well as because of reduced skeletal muscle mass, the maxi- Body Fluids and Salt in Exercise mal achievable muscle power is greatly reduced in old age. As much as a 5- to 10-pound weight loss has been recorded in athletes in a period of 1 hour during endurance athletic Body Heat in Exercise events under hot and humid conditions. Essentially all this Almost all the energy released by the body’s metabolism of weight loss results from loss of sweat. Loss of enough sweat nutrients is eventually converted into body heat. This ap- to decrease body weight only 3% can significantly dimin- plies even to the energy that causes muscle contraction for ish a person’s performance, and a 5% to 10% rapid decrease the following reasons: First, the maximal efficiency for con- in weight can often be serious, leading to muscle cramps, version of nutrient energy into muscle work, even under nausea, and other adverse effects. Therefore, it is essential the best of conditions, is only 20% to 25%; the remainder of to replace fluid as it is lost. the nutrient energy is converted into heat during the course Replacement of Sodium Chloride and Potassium. Be- of the intracellular chemical reactions. Second, almost all cause sweat contains a large amount of sodium chloride, it the energy that does go into creating muscle work still be- has long been stated that all athletes should take salt (so- comes body heat because all but a small portion of this en- dium chloride) tablets when performing exercise on hot ergy is used for (1) overcoming viscous resistance to the and humid days. However, overuse of salt tablets has of- movement of the muscles and joints, (2) overcoming the ten done as much harm as good. Furthermore, if an athlete friction of the blood flowing through the blood vessels, and becomes acclimatized to the heat by progressive increase (3) other, similar effects, all of which convert the muscle in athletic exposure over a period of 1 to 2 weeks rather contractile energy into heat. than performing maximal athletic feats on the first day, the Now, recognizing that the oxygen consumption by the sweat glands also become acclimatized, so the amount of body can increase as much as 20-fold in the well-trained salt lost in the sweat becomes only a small fraction of that athlete and that the amount of heat liberated in the body is lost before acclimatization. This sweat gland acclimatiza- almost exactly proportional to the oxygen consumption (as tion results mainly from increased aldosterone secretion by discussed in Chapter 73), one quickly realizes that tremen- the adrenal cortex. The aldosterone in turn has a direct ef- dous amounts of heat are injected into the internal body fect on the sweat glands, increasing reabsorption of sodium tissues when performing endurance athletic events. Next, chloride from the sweat before the sweat issues forth from with a vast rate of heat flow into the body, on a very hot the sweat gland tubules onto the surface of the skin. Once and humid day that prevents the sweating mechanism from the athlete is acclimatized, only rarely do salt supplements eliminating the heat, an intolerable and even lethal condi- need to be considered during athletic events. tion called heatstroke can develop in the athlete. Exercise-associated hyponatremia (low plasma sodium Heatstroke. During endurance athletics, even under concentration) can sometimes occur after sustained physi- normal environmental conditions, the body temperature cal exertion. In fact, severe hyponatremia can be an impor- often rises from its normal level of 98.6°F to 102°F or 103°F tant cause of fatalities in endurance athletes. As noted in (37°C to 40°C). With very hot and humid conditions or Chapter 25, severe hyponatremia can cause tissue edema, excess clothing, the body temperature can rise to 106°F to especially in the brain, which can be lethal. In persons who 108°F (41°C to 42°C). At this level, the elevated tempera- experience life-threatening hyponatremia after heavy exer- ture becomes destructive to tissue cells, especially the brain cise, the main cause is not simply the loss of sodium due to cells. When this phenomenon occurs, multiple symptoms sweating; instead, the hyponatremia is often due to inges- begin to appear, including extreme weakness, exhaustion, tion of hypotonic fluid (water or sports drinks that usually headache, dizziness, nausea, profuse sweating, confusion, have a sodium concentration of less than 18 mmol/L) in staggering gait, collapse, and unconsciousness. excess of sweat, urine, and insensible (mainly respiratory) 1082  Chapter 85 Sports Physiology fluid losses. This excess fluid consumption can be driven by studies have shown mortality to be three times less in the thirst but also may be due to conditioned behavior that is most fit people than in the least fit people. based on recommendations to drink fluid during exercise Why does body fitness prolong life? The following rea- to avoid dehydration. Copious supplies of water are also sons are some of the most important. generally available in marathons, triathlons, and other en- Body fitness and weight control greatly reduce cardiovas- UNIT XV durance athletic events. cular disease. This results from (1) maintenance of moderately Experience by military units exposed to heavy exercise lower blood pressure and (2) reduced blood cholesterol and in the desert has demonstrated still another electrolyte low-density lipoprotein along with increased high-density problem—the loss of potassium. Potassium loss results lipoprotein. As pointed out earlier, these changes all work partly from the increased secretion of aldosterone during together to reduce the number of heart attacks, brain strokes, heat acclimatization, which increases the loss of potassium and kidney disease. in the urine, as well as in the sweat. As a consequence of The athletically fit person has more bodily reserves to these findings, some of the supplemental fluids for athlet- call on when he or she does become sick. For example, an ics contain properly proportioned amounts of potassium 80-year-old, nonfit person may have a respiratory system that along with sodium, usually in the form of fruit juices. limits oxygen delivery to the tissues to no more than 1 L/min; this means a respiratory reserve of no more than 3-fold to 4- Drugs and Athletes fold. However, an athletically fit old person may have twice Without belaboring this issue, let us list some of the effects as much reserve. This extra reserve is especially important in of drugs in athletics. preserving life when the older person experiences conditions First, some persons believe that caffeine increases ath- such as pneumonia that can rapidly require all available respir- letic performance. In one experiment performed by a atory reserve. In addition, the ability to increase cardiac output marathon runner, running time for the marathon was im- in times of need (the “cardiac reserve”) is often 50% greater proved by 7% through judicious use of caffeine in amounts in the athletically fit old person than in the nonfit old person. similar to those found in one to three cups of coffee. Yet Exercise and overall body fitness also reduce the risk for experiments by other investigators have failed to confirm several chronic metabolic disorders associated with obesi- any advantage, thus leaving this issue in doubt. ty, such as insulin resistance and type 2 diabetes. Moderate Second, use of male sex hormones (androgens) or other exercise, even in the absence of significant weight loss, has anabolic steroids to increase muscle strength undoubt- been shown to improve insulin sensitivity and reduce, or edly can increase athletic performance under some con- in some cases eliminate, the need for insulin treatment in ditions, especially in women and even in men. However, patients with type 2 diabetes. anabolic steroids also greatly increase the risk of cardio- Improved body fitness also reduces the risk for several vascular disease because they often cause hypertension, types of cancers, including breast, prostate, and colon can- decreased high-density blood lipoproteins, and increased cer. Much of the beneficial effects of exercise may be related low-density lipoproteins, all of which promote heart at- to a reduction in obesity. However, studies in animals used tacks and strokes. in experiments and in humans have also shown that regular In men, any type of male sex hormone preparation also exercise reduces the risk for many chronic diseases through leads to decreased testicular function, including both de- mechanisms that are, at least to some extent, independent creased formation of sperm and decreased secretion of of weight loss or decreased adiposity. the person’s own natural testosterone, with residual effects sometimes lasting at least for many months and perhaps in- definitely. In a woman, even more significant effects such as Bibliography facial hair, a bass voice, ruddy skin, and cessation of menses Blaauw B, Schiaffino S, Reggiani C: Mechanisms modulating skeletal can occur because she is not normally adapted to the male muscle phenotype. Compr Physiol 3:1645, 2013. sex hormone. Booth FW, Roberts CK, Thyfault JP, et al: Role of inactivity in chronic Other drugs, such as amphetamines and cocaine, have diseases: evolutionary insight and pathophysiological mechanisms. been reputed to increase athletic performance. It is equally Physiol Rev 97:1351, 2017. true that overuse of these drugs can lead to deterioration Del Buono MG, Arena R, Borlaug BA, et al: Exercise intolerance in of performance. Furthermore, experiments have failed to patients with heart failure: JACC state-of-the-art review. J Am Coll prove the value of these drugs except as a psychic stimu- Cardiol 73:2209, 2019. lant. Some athletes have been known to die during athletic Diaz-Canestro C, Montero D: Sex dimorphism of VO2max trainability: events because of interaction between such drugs and the a systematic review and meta-analysis. Sports Med 49:1949, 2019. norepinephrine and epinephrine released by the sympa- Grgic J, Mcllvenna LC, Fyfe JJ, et al: Does aerobic training promote the same skeletal muscle hypertrophy as resistance training? A sys- thetic nervous system during exercise. One of the possible tematic review and meta-analysis. Sports Med 49:233, 2019. causes of death under these conditions is overexcitability of Handelsman DJ, Hirschberg AL, Bermon S: Circulating testosterone the heart, leading to ventricular fibrillation, which is lethal as the hormonal basis of sex differences in athletic performance. within seconds. Endocr Rev 39:803, 2018. Jones AM, Burnley M, Black MI, et al: The maximal metabolic Body Fitness Prolongs Life steady state: redefining the ‘gold standard’. Physiol Rep 2019 Multiple studies have shown that people who maintain ap- May;7(10):e14098. doi: 10.14814/phy2.14098 propriate body fitness, using judicious regimens of exercise Joyner MJ, Casey DP: Regulation of increased blood flow (hyperemia) and weight control, have the additional benefit of pro- to muscles during exercise: a hierarchy of competing physiological longed life. Especially between the ages of 50 and 70 years, needs. Physiol Rev 95:549, 2015. 1083 Unit XV Sports Physiology Joyner MJ, Lundby C: Concepts about V˙O2max and trainability are Rosner MH: Exercise-associated hyponatremia. Semin Nephrol context dependent. Exerc Sport Sci Rev 46:138, 2018. 29:271, 2009. Joyner MJ: Physiological limits to endurance exercise performance: Schiaffino S: Muscle fiber type diversity revealed by anti-myosin heavy influence of sex. J Physiol 595:2949, 2017. chain antibodies. FEBS J 285:3688, 2018. Kent-Braun JA, Fitts RH, Christie A: Skeletal muscle fatigue. Compr Seals DR, Edward F: Adolph Distinguished Lecture: the remarkable Physiol 2:997, 2012. anti-aging effects of aerobic exercise on systemic arteries. J Appl Montero D, Lundby C: Regulation of red blood cell volume with exer- Physiol 117:425, 2014. cise training. Compr Physiol 9:149, 2018. Trangmar SJ, González-Alonso J: Heat, hydration and the human Powers SK, Jackson MJ: Exercise-induced oxidative stress: cellular brain, heart and skeletal muscles. Sports Med 49(Suppl 1):69, mechanisms and impact on muscle force production. Physiol Rev 2019. 88:1243, 2008. 1084