Physiological Responses And Long-Term Adaptations To Exercise PDF

CHAPTER 3 PHYSIOLOGIC RESPONSES AND LONG-TERM ADAPTATIONS TO EXERCISE Contents Introduction............................................................... 61 Physiologic Responses to Episodes of Exercise..........................

CHAPTER 3 PHYSIOLOGIC RESPONSES AND LONG-TERM ADAPTATIONS TO EXERCISE Contents Introduction............................................................... 61 Physiologic Responses to Episodes of Exercise.................................... 61 Cardiovascular and Respiratory Systems...................................... 61 Cardiovascular Responses to Exercise...................................... 62 Cardiac Output..................................................... 62 Blood Flow........................................................ 63 Blood Pressure..................................................... 63 Oxygen Extraction.................................................. 63 Coronary Circulation................................................ 63 Respiratory Responses to Exercise......................................... 64 Resistance Exercise.................................................... 65 Skeletal Muscle.......................................................... 65 Skeletal Muscle Energy Metabolism.......................................... 65 Energy Systems....................................................... 65 Metabolic Rate........................................................ 66 Maximal Oxygen Uptake................................................ 66 Lactate Threshold...................................................... 66 Hormonal Responses to Exercise............................................ 66 Immune Responses to Exercise.............................................. 67 Long-Term Adaptations to Exercise Training..................................... 67 Adaptations of Skeletal Muscle and Bone...................................... 67 Metabolic Adaptations..................................................... 69 Cardiovascular and Respiratory Adaptations................................... 71 Long-Term Cardiovascular Adaptations.................................... 71 Respiratory Adaptations................................................. 71 Contents, continued Maintenance, Detraining, and Prolonged Inactivity................................ 71 Maintaining Fitness and Muscular Strength.................................... 72 Detraining.............................................................. 72 Prolonged Inactivity...................................................... 72 Special Considerations....................................................... 73 Disability............................................................... 73 Environmental Conditions................................................. 73 Effects of Age............................................................ 75 Differences by Sex........................................................ 76 Conclusions............................................................... 77 Research Needs.......................................................... 77 References................................................................. 77  CHAPTER 3 PHYSIOLOGIC RESPONSES AND LONG-TERM ADAPTATIONS TO EXERCISE Introduction Selected Textbooks on Exercise Physiology W hen challenged with any physical task, the human body responds through a series of integrated changes in function that involve most, if Åstrand PO, Rodahl K. Textbook of work physiology. 3rd edition. New York: McGraw-Hill Book Company, 1986. not all, of its physiologic systems. Movement re- quires activation and control of the musculoskeletal Brooks GA, Fahey TD, White TP. Exercise physiology: system; the cardiovascular and respiratory systems human bioenergetics and its applications. 2nd edition. provide the ability to sustain this movement over Mountain View, CA: Mayfield Publishing Company, 1996. extended periods. When the body engages in exer- cise training several times a week or more frequently, Fox E, Bowers R, Foss M. The physiological basis for each of these physiologic systems undergoes specific exercise and sport. 5th edition. Madison, WI: Brown adaptations that increase the body’s efficiency and and Benchmark, 1993. capacity. The magnitude of these changes depends McArdle WD, Katch FI, Katch VL. Essentials of largely on the intensity and duration of the training exercise physiology. Philadelphia, PA: Lea and sessions, the force or load used in training, and the Febiger, 1994. body’s initial level of fitness. Removal of the train- Powers SK, Howley ET. Exercise physiology: theory ing stimulus, however, will result in loss of the and application to fitness and performance. Dubuque, efficiency and capacity that was gained through IA: William C. Brown, 1990. these training-induced adaptations; this loss is a Wilmore JH, Costill DL. Physiology of sport and process called detraining. exercise. Champaign, IL: Human Kinetics, 1994. This chapter provides an overview of how the body responds to an episode of exercise and adapts to exercise training and detraining. The discussion Physiologic Responses to Episodes focuses on aerobic or cardiorespiratory endurance of Exercise exercise (e.g., walking, jogging, running, cycling, The body’s physiologic responses to episodes of swimming, dancing, and in-line skating) and resis- aerobic and resistance exercise occur in the muscu- tance exercise (e.g., strength-developing exercises). loskeletal, cardiovascular, respiratory, endocrine, It does not address training for speed, agility, and and immune systems. These responses have been flexibility. In discussing the multiple effects of studied in controlled laboratory settings, where ex- exercise, this overview will orient the reader to the ercise stress can be precisely regulated and physi- physiologic basis for the relationship of physical ologic responses carefully observed. activity and health. Physiologic information perti- nent to specific diseases is presented in the next chapter. For additional information, the reader is Cardiovascular and Respiratory Systems referred to the selected textbooks shown in the The primary functions of the cardiovascular and sidebar. respiratory systems are to provide the body with Physical Activity and Health oxygen (O2) and nutrients, to rid the body of carbon dioxide (CO2) and metabolic waste products, to Figure 3-1. Changes in cardiac output (A), heart rate maintain body temperature and acid-base balance, (B), and stroke volume (C) with increasing rates of work on the cycle ergometer and to transport hormones from the endocrine glands to their target organs (Wilmore and Costill (A) 1994). To be effective and efficient, the cardiovascu- 22 lar system should be able to respond to increased Cardiac output (liters/min) 20 skeletal muscle activity. Low rates of work, such as walking at 4 kilometers per hour (2.5 miles per 18 hour), place relatively small demands on the cardio- 16 vascular and respiratory systems. However, as the 14 rate of muscular work increases, these two systems 12 will eventually reach their maximum capacities and 10 will no longer be able to meet the body’s demands. 8 Cardiovascular Responses to Exercise 6 25 50 75 100 125 150 175 200 The cardiovascular system, composed of the heart, Power (watts) blood vessels, and blood, responds predictably to the increased demands of exercise. With few excep- (B) tions, the cardiovascular response to exercise is 200 directly proportional to the skeletal muscle oxygen demands for any given rate of work, and oxygen 180 Heart rate (beats/min) uptake (V̇O2) increases linearly with increasing rates 160 of work. 140 Cardiac Output Cardiac output (Q̇) is the total volume of blood 120 pumped by the left ventricle of the heart per minute. 100 It is the product of heart rate (HR, number of beats per minute) and stroke volume (SV, volume of blood 80 pumped per beat). The arterial-mixed venous oxygen 25 50 75 100 125 150 175 200 - ) difference is the difference between the oxy- Power (watts) (A-vO 2 gen content of the arterial and mixed venous blood. A (C) person’s maximum oxygen uptake (V̇O2 max) is a 120 function of cardiac output (Q̇) multiplied by the Stroke volume (ml/beat) - difference. Cardiac output thus plays an im- A-vO 110 2 portant role in meeting the oxygen demands for 100 work. As the rate of work increases, the cardiac output increases in a nearly linear manner to meet 90 the increasing oxygen demand, but only up to the point where it reaches its maximal capacity (Q̇ max). 80 To visualize how cardiac output, heart rate, and 70 stroke volume change with increasing rates of work, consider a person exercising on a cycle ergometer, 60 starting at 50 watts and increasing 50 watts every 2 25 50 75 100 125 150 175 200 Power (watts) minutes up to a maximal rate of work (Figure 3-1 A, B, and C). In this scenario, cardiac output and heart rate increase over the entire range of work, whereas stroke volume only increases up to approximately 40 62  Physiologic Responses and Long-Term Adaptations to Exercise to 60 percent of the person’s maximal oxygen uptake is generally much higher in these patients, likely owing ( V̇O2 max), after which it reaches a plateau. Recent to a lesser reduction in total peripheral resistance. studies have suggested that stroke volume in highly For the first 2 to 3 hours following exercise, trained persons can continue to increase up to near blood pressure drops below preexercise resting lev- maximal rates of work (Scruggs et al. 1991; Gledhill, els, a phenomenon referred to as postexercise hy- Cox, Jamnik 1994). potension (Isea et al. 1994). The specific mechanisms underlying this response have not been established. Blood Flow The acute changes in blood pressure after an episode The pattern of blood flow changes dramatically when of exercise may be an important aspect of the role of a person goes from resting to exercising. At rest, the physical activity in helping control blood pressure in skin and skeletal muscles receive about 20 percent of hypertensive patients. the cardiac output. During exercise, more blood is sent to the active skeletal muscles, and, as body Oxygen Extraction temperature increases, more blood is sent to the skin. The A-vO- difference increases with increasing rates 2 This process is accomplished both by the increase in of work (Figure 3-2) and results from increased cardiac output and by the redistribution of blood flow oxygen extraction from arterial blood as it passes away from areas of low demand, such as the splanch- through exercising muscle. At rest, the A-vO- differ- 2 nic organs. This process allows about 80 percent of the ence is approximately 4 to 5 ml of O2 for every 100 ml cardiac output to go to active skeletal muscles and of blood (ml/100 ml); as the rate of work approaches skin at maximal rates of work (Rowell 1986). With - difference reaches 15 to 16 maximal levels, the A-vO 2 exercise of longer duration, particularly in a hot and ml/100 ml of blood. humid environment, progressively more of the car- diac output will be redistributed to the skin to counter Coronary Circulation the increasing body temperature, thus limiting both The coronary arteries supply the myocardium with the amount going to skeletal muscle and the exercise blood and nutrients. The right and left coronary endurance (Rowell 1986). arteries curve around the external surface of the heart, then branch and penetrate the myocardial muscle Blood Pressure bed, dividing and subdividing like branches of a tree Mean arterial blood pressure increases in response to to form a dense vascular and capillary network to dynamic exercise, largely owing to an increase in supply each myocardial muscle fiber. Generally one systolic blood pressure, because diastolic blood pres- capillary supplies each myocardial fiber in adult hu- sure remains at near-resting levels. Systolic blood mans and animals; however, evidence suggests that pressure increases linearly with increasing rates of the capillary density of the ventricular myocardium work, reaching peak values of between 200 and 240 can be increased by endurance exercise training. millimeters of mercury in normotensive persons. Be- At rest and during exercise, myocardial oxygen cause mean arterial pressure is equal to cardiac output demand and coronary blood flow are closely linked. times total peripheral resistance, the observed increase This coupling is necessary because the myocardium in mean arterial pressure results from an increase in depends almost completely on aerobic metabolism cardiac output that outweighs a concomitant decrease and therefore requires a constant oxygen supply. in total peripheral resistance. This increase in mean Even at rest, the myocardium’s oxygen use is high arterial pressure is a normal and desirable response, the relative to the blood flow. About 70 to 80 percent of result of a resetting of the arterial baroreflex to a higher the oxygen is extracted from each unit of blood pressure. Without such a resetting, the body would crossing the myocardial capillaries; by comparison, experience severe arterial hypotension during intense only about 25 percent is extracted from each unit activity (Rowell 1993). Hypertensive patients typically crossing skeletal muscle at rest. In the healthy heart, reach much higher systolic blood pressures for a given a linear relationship exists between myocardial oxy- rate of work, and they can also experience increases in gen demands, consumption, and coronary blood diastolic blood pressure. Thus, mean arterial pressure flow, and adjustments are made on a beat-to-beat 63 Physical Activity and Health Figure 3-2. Changes in arterial and mixed venous oxygen content with increasing rates of work on the cycle ergometer 20 18 arterial oxygen content Oxygen content (ml/100 ml of blood) 16 14 - A-vO 12 2 difference 10 8 6 4 2 mixed venous oxygen content 0 25 50 75 100 125 150 175 200 225 250 275 Power (watts) basis. The three major determinants of myocardial coronary artery and coronary vasodilation to meet the oxygen consumption are heart rate, myocardial increased need for blood flow required by the increase contractility, and wall stress (Marcus 1983; in myocardial oxygen use. Jorgensen et al. 1977). Acute increases in arterial pressure increase left ventricular pressure and wall Respiratory Responses to Exercise stress. As a result, the rate of myocardial metabolism The respiratory system also responds when chal- increases, necessitating an increased coronary blood lenged with the stress of exercise. Pulmonary ven- flow. A very high correlation exists between both tilation increases almost immediately, largely myocardial oxygen consumption and coronary blood through stimulation of the respiratory centers in flow and the product of heart rate and systolic blood the brain stem from the motor cortex and through pressure (SBP) (Jorgensen et al. 1977). This so- feedback from the proprioceptors in the muscles called double product (HR SBP) is generally used and joints of the active limbs. During prolonged to estimate myocardial oxygen and coronary blood exercise, or at higher rates of work, increases in CO2 flow requirements. During vigorous exercise, all production, hydrogen ions (H+ ), and body and three major determinants of myocardial oxygen re- blood temperatures stimulate further increases in quirements increase above their resting levels. pulmonary ventilation. At low work intensities, the The increase in coronary blood flow during exer- increase in ventilation is mostly the result of in- cise results from an increase in perfusion pressure of creases in tidal volume. At higher intensities, the the coronary artery and from coronary vasodilation. respiratory rate also increases. In normal-sized, Most important, an increase in sympathetic nervous untrained adults, pulmonary ventilation rates can system stimulation leads to an increase in circulating vary from about 10 liters per minute at rest to more catecholamines. This response triggers metabolic pro- than 100 liters per minute at maximal rates of work; cesses that increase both perfusion pressure of the in large, highly trained male athletes, pulmonary 64  Physiologic Responses and Long-Term Adaptations to Exercise ventilation rates can reach more than 200 liters per density, and high mitochondrial content (Terjung minute at maximal rates of work. 1995). FTb fibers have low oxidative capacity, low fatigue resistance, high glycolytic capacity, and fast Resistance Exercise contractile speed. Further, they have relatively low The cardiovascular and respiratory responses to blood flow capacity, capillary density, and mito- episodes of resistance exercise are mostly similar to chondrial content (Terjung 1995). those associated with endurance exercise. One no- There is a direct relationship between predomi- table exception is the exaggerated blood pressure nant fiber type and performance in certain sports. response that occurs during resistance exercise. Part For example, in most marathon runners, slow-twitch of this response can be explained by the fact that fibers account for up to or more than 90 percent of resistance exercise usually involves muscle mass the total fibers in the leg muscles. On the other hand, that develops considerable force. Such high, isolated the leg muscles in sprinters are often more than 80 force leads to compression of the smaller arteries and percent composed of fast-twitch fibers. Although the results in substantial increases in total peripheral issue is not totally resolved, muscle fiber type ap- resistance (Coyle 1991). Although high-intensity pears to be genetically determined; researchers have resistance training poses a potential risk to hyperten- shown that several years of either high-intensity sive patients and to those with cardiovascular dis- sprint training or high-intensity endurance training ease, research data suggest that the risk is relatively do not significantly alter the percentage of the two low (Gordon et al. 1995) and that hypertensive major types of fibers (Jolesz and Sreter 1981). persons may benefit from resistance training (Tipton 1991; American College of Sports Medicine 1993). Skeletal Muscle Energy Metabolism Metabolic processes are responsible for generating Skeletal Muscle adenosine triphosphate (ATP), the body’s energy The primary purpose of the musculoskeletal system is source for all muscle action. ATP is generated by three to define and move the body. To provide efficient and basic energy systems: the ATP-phosphocreatine effective force, muscle adapts to demands. In response (ATP-PCr) system, the glycolytic system, and the to demand, it changes its ability to extract oxygen, oxidative system. Each system contributes to energy choose energy sources, and rid itself of waste prod- production in nearly every type of exercise. The ucts. The body contains three types of muscle tissue: relative contribution of each will depend on factors skeletal (voluntary) muscle, cardiac muscle or myo- such as the intensity of work rate at the onset of cardium, and smooth (autonomic) muscle. This sec- exercise and the availability of oxygen in the muscle. tion focuses solely on skeletal muscle. Skeletal muscle is composed of two basic types of Energy Systems muscle fibers distinguished by their speed of con- The ATP-PCr system provides energy from the ATP traction—slow-twitch and fast-twitch—a character- stored in all of the body’s cells. PCr, also found in all istic that is largely dictated by different forms of the cells, is a high-energy phosphate molecule that stores enzyme myosin adenosinetriphosphatase (ATPase). energy. As ATP concentrations in the cell are reduced Slow-twitch fibers, which have relatively slow con- by the breakdown of ATP to adenosine diphosphate tractile speed, have high oxidative capacity and fa- (ADP) to release energy for muscle contraction, PCr is tigue resistance, low glycolytic capacity, relatively broken down to release both energy and a phosphate high blood flow capacity, high capillary density, and to allow reconstitution of ATP from ADP. This process high mitochondrial content (Terjung 1995). Fast- describes the primary energy system for short, high- twitch muscle fibers have fast contractile speed and intensity exercise, such as a 40- to 200-meter sprint; are classified into two subtypes, fast-twitch type “a” during such exercise, the system can produce energy (FTa) and fast-twitch type “b” (FTb). FTa fibers have at very high rates, and ATP and PCr stores, which are moderately high oxidative capacity, are relatively depleted in 10–20 seconds, will last just long enough fatigue resistant, and have high glycolytic capacity, to complete the exercise. relatively high blood flow capacity, high capillary 65 Physical Activity and Health At high rates of work, the active muscle cell’s oxygen uptake ( V̇O2max) (Figure 3-3). A person’s oxygen demand exceeds its supply. The cell must V̇O2max is in part genetically determined; it can be then rely on the glycolytic energy system to produce increased through training until the point that the ATP in the absence of oxygen (i.e., anaerobically). genetically possible maximum is reached. V̇O2max is This system can only use glucose, available in the considered the best estimate of a person’s cardio- blood plasma and stored in both muscle and the liver as respiratory fitness or aerobic power (Jorgensen et al. glycogen. The glycolytic energy system is the primary 1977). energy system for all-out bouts of exercise lasting from 30 seconds to 2 minutes, such as an 800-meter Lactate Threshold run. The major limitation of this energy system is Lactate is the primary by-product of the anaerobic that it produces lactate, which lowers the pH of both glycolytic energy system. At lower exercise intensi- the muscle and blood. Once the pH drops below a ties, when the cardiorespiratory system can meet the value of 6.4 to 6.6, enzymes critical for producing oxygen demands of active muscles, blood lactate energy are no longer able to function, and ATP levels remain close to those observed at rest, because production stops (Wilmore and Costill 1994). some lactate is used aerobically by muscle and is The oxidative energy system uses oxygen to removed as fast as it enters the blood from the produce ATP within the mitochondria, which are muscle. As the intensity of exercise is increased, special cell organelles within muscle. This process however, the rate of lactate entry into the blood from cannot generate ATP at a high enough rate to sustain muscle eventually exceeds its rate of removal from an all-out sprint, but it is highly effective at lower the blood, and blood lactate concentrations increase rates of work (e.g., long distance running). ATP can above resting levels. From this point on, lactate also be produced from fat and protein metabolism levels continue to increase as the rate of work in- through the oxidative energy system. Typically, car- creases, until the point of exhaustion. The point at bohydrate and fat provide most of the ATP; under which the concentration of lactate in the blood most conditions, protein contributes only 5 to 10 begins to increase above resting levels is referred to percent at rest and during exercise. as the lactate threshold (Figure 3-3). Lactate threshold is an important marker for endur- Metabolic Rate ance performance, because distance runners set their The rate at which the body uses energy is known as race pace at or slightly above the lactate threshold the metabolic rate. When measured while a person is (Farrell et al. 1979). Further, the lactate thresholds at rest, the resulting value represents the lowest (i.e., of highly trained endurance athletes occur at a much basal) rate of energy expenditure necessary to main- higher percentage of their V̇O2max, and thus at higher tain basic body functions. Resting metabolic rate is relative workloads, than do the thresholds of un- measured under highly controlled resting condi- trained persons. This key difference is what allows tions following a 12-hour fast and a good night’s endurance athletes to perform at a faster pace. sleep (Turley, McBride, Wilmore 1993). To quantify the rate of energy expenditure during exercise, the Hormonal Responses to Exercise metabolic rate at rest is defined as 1 metabolic The endocrine system, like the nervous system, equivalent (MET); a 4 MET activity thus represents integrates physiologic responses and plays an im- an activity that requires four times the resting meta- portant role in maintaining homeostatic conditions bolic rate. The use of METs to quantify physical at rest and during exercise. This system controls the activity intensity is the basis of the absolute intensity release of hormones from specialized glands through- scale. (See Chapter 2 for further information.) out the body, and these hormones exert their actions on targeted organs and cells. In response to an Maximal Oxygen Uptake episode of exercise, many hormones, such as cat- During exercise,V̇O2 increases in direct proportion to echolamines, are secreted at an increased rate, though the rate of work. The point at which a person’s V̇O2 is insulin is secreted at a decreased rate (Table 3-1). no longer able to increase is defined as the maximal The actions of some of these hormones, as well as 66  Physiologic Responses and Long-Term Adaptations to Exercise Figure 3-3. Changes in oxygen uptake and blood lactate concentrations with increasing rates of work on the cycle ergometer*. 55 VO2max 12 50 10 Oxygen uptake (ml/kg/min) Blood lactate (mmol/liter) 45 8 40 6 35 4 Lactate threshold 30 2 *Lactate threshold (LT) and maximum oxygen uptake ( V̇O 2 max) are indicated. 25 0 25 50 75 100 125 150 175 200 225 250 275 Power (watts) their specific responses to exercise, are discussed in Exercise of high intensity and long duration or more detail in Chapter 4. exercise that involves excessive training may have adverse effects on immune function. In general, a Immune Responses to Exercise high-intensity, single episode of exercise results in a marked decline in the functioning of all major cells of The immune system is a complex adaptive system the immune system (Newsholme and Parry-Billings that provides surveillance against foreign proteins, 1994; Shephard and Shek 1995). In addition, over- viruses, and bacteria by using the unique functions of training may reduce the response of T-lymphocytes to cells produced by the bone marrow and the thymus mutagenic stimulation, decrease antibody synthesis gland. By interacting with neural and endocrine and plasma level of immunoglobins and complement, factors, the immune system influences the body’s and impair macrophage phagocytosis. The reduced overall response to exercise (Reichlin 1992). A grow- plasma glutamine levels that occur with high-intensity ing body of literature indicates that the incidence of exercise or excessive training are postulated to con- some infections may be influenced by the exercise tribute to these adverse effects on the immune system history of the individual (Nieman 1994; Hoffman- (Newsholme and Parry-Billings 1994). Goetz and Pedersen 1994). Moderate exercise has been shown to bolster the function of certain components of the human immune system—such as natural killer cells, circulating T- and Long-Term Adaptations to B-lymphocytes, and cells of the monocyte-macroph- Exercise Training age system—thereby possibly decreasing the inci- Adaptations of Skeletal Muscle and Bone dence of some infections (Keast, Cameron, Morton 1988; Pedersen and Ullum 1994; Woods and Davis Skeletal muscle adapts to endurance training chiefly 1994) and perhaps of certain types of cancer (Shephard through a small increase in the cross-sectional area and Shek 1995). of slow-twitch fibers, because low- to moderate- 67 Physical Activity and Health Table 3-1. A summary of hormonal changes during an episode of exercise Exercise Hormone response Special relationships Probable importance Catecholamines Increases Greater increase with intense Increased blood glucose; exercise; norepinephrine > increased skeletal muscle and epinephrine; increases less after liver glycogenolysis; increased training lipolysis Growth hormone (GH) Increases Increases more in untrained persons; declines faster in trained Unknown persons Adrenocorticotropic Increases Greater increase with intense Increased gluconeogenesis in hormone (ACTH)-cortisol exercise; increases less liver; increased mobilization of after training with fatty acids submaximal exercise Thyroid-stimulating Increases Increased thyroxine turnover hormone (TSH)-thyroxine with training but no toxic effects Unknown are evident Luteinizing hormone (LH) No change None None Testosterone Increases None Unknown Estradiol-progesterone Increases Increases during luteal phase Unknown of the menstrual cycle Insulin Decreases Decreases less after training Decreased stimulus to use blood glucose Glucagon Increases Increases less after training Increased blood glucose via glycogenolysis and gluconeogenesis Renin-angiotensin- Increases Same increase after training Sodium retention to maintain aldosterone in rats plasma volume Antidiuretic hormone Expected None Water retention to maintain (ADH) increase plasma volume Parathormone Unknown None Needed to establish proper (PTH)-calcitonin bone development Erythropoietin Unknown None Would be important to increase erythropoiesis Prostaglandins May May increase in response to May be local vasodilators increase sustained isometric contractions; may need ischemic stress Adapted from Wilmore JH, Costill DL. Physiology of sport and exercise. Champaign, IL: Human Kinetics, 1994, p. 136. 68  Physiologic Responses and Long-Term Adaptations to Exercise intensity aerobic activity primarily recruits these mass that result from either endurance or resistance fibers (Abernethy, Thayer, Taylor 1990). Prolonged training are relatively small (Chesnut 1993). The endurance training (i.e., months to years) can lead to role of resistance training in increasing or maintain- a transition of FTb fibers to FTa fibers, which have a ing bone mass is not well characterized. Endurance higher oxidative capacity (Abernethy, Thayer, Taylor training has little demonstrated positive effect on 1990). No substantive evidence indicates that fast- bone mineral and mass. Nonetheless, even small twitch fibers will convert to slow-twitch fibers under increases in bone mass gained from endurance or normal training conditions (Jolesz and Sreter 1981). resistance training can help prevent or delay the Endurance training also increases the number of process of osteoporosis (Drinkwater 1994). (See capillaries in trained skeletal muscle, thereby allow- Chapter 4 for further information on the effects of ing a greater capacity for blood flow in the active exercise on bone.) muscle (Terjung 1995). The musculoskeletal system cannot function with- Resistance-trained skeletal muscle exerts con- out connective tissue linking bones to bones (liga- siderably more force because of both increased muscle ments) and muscles to bones (tendons). Extensive size (hypertrophy) and increased muscle fiber re- animal studies indicate that ligaments and tendons cruitment. Fiber hypertrophy is the result of in- become stronger with prolonged and high-intensity creases in both the size and number of myofibrils in exercise. This effect is the result of an increase in the both fast-twitch and slow-twitch muscle fibers strength of insertion sites between ligaments, ten- (Kannus et al. 1992). Hyperplasia, or increased fiber dons, and bones, as well as an increase in the cross- number, has been reported in animal studies, where sectional areas of ligaments and tendons. These the number of individual muscle fibers can be counted structures also become weaker and smaller with sev- (Gonyea et al. 1986), and has been indirectly demon- eral weeks of immobilization (Tipton and Vailas 1990), strated during autopsies on humans by using direct which can have important implications for muscu- fiber counts to compare dominant and nondominant loskeletal performance and risk of injury. paired muscles (Sjöström et al. 1991). During both aerobic and resistance exercise, Metabolic Adaptations active muscles can undergo changes that lead to Significant metabolic adaptations occur in skeletal muscle soreness. Some soreness is felt immediately muscle in response to endurance training. First, both after exercise, and some can even occur during exer- the size and number of mitochondria increase sub- cise. This muscle soreness is generally not physically stantially, as does the activity of oxidative enzymes. limiting and dissipates rapidly. A more limiting sore- Myoglobin content in the muscle can also be aug- ness, however, may occur 24 to 48 hours following mented, increasing the amount of oxygen stored in exercise. This delayed-onset muscle soreness is pri- individual muscle fibers (Hickson 1981), but this marily associated with eccentric-type muscle action, effect is variable (Svedenhag, Henriksson, Sylvén during which the muscle exerts force while lengthen- 1983). Such adaptations, combined with the increase ing, as can happen when a person runs down a steep in capillaries and muscle blood flow in the trained hill or lowers a weight from a fully flexed to a fully muscles (noted in a previous section), greatly enhance extended position (e.g., the two-arm curl). Delayed- the oxidative capacity of the endurance-trained muscle. onset muscle soreness is the result of structural dam- Endurance training also increases the capacity of age to the muscle; in its most severe form, this damage skeletal muscle to store glycogen (Kiens et al. 1993). may include rupture of the cell membrane and disrup- The ability of trained muscles to use fat as an energy tion of the contractile elements of individual muscle source is also improved, and this greater reliance on fibers (Armstrong, Warren, Warren 1991). Such dam- fat spares glycogen stores (Kiens et al. 1993). The age appears to result in an inflammatory response increased capacity to use fat following endurance (MacIntyre, Reid, McKenzie 1995). training results from an enhanced ability to mobilize Total inactivity results in muscle atrophy and free-fatty acids from fat depots and an improved loss of bone mineral and mass. Persons who are capacity to oxidize fat consequent to the increase in sedentary generally have less bone mass than those the muscle enzymes responsible for fat oxidation who exercise, but the increases in bone mineral and (Wilmore and Costill 1994). 69 Physical Activity and Health These changes in muscle and in cardiorespi- (Kohrt et al. 1991). This variation in response may ratory function are responsible for increases in be due in part to genetic factors and to initial levels both V̇O 2max and lactate threshold. The endurance- of fitness. To illustrate the changes that can be trained person can thus perform at considerably higher expected with endurance training, a hypothetical rates of work than the untrained person. Increases in sedentary man’s pretraining values have been com- V̇O2max generally range from 15 to 20 percent follow- pared with his values after a 6-month period of ing a 6-month training period (Wilmore and Costill endurance training and with the values of a typical 1994). However, individual variations in this response elite endurance runner (Table 3-2). are considerable. In one study of 60- to 71-year-old Responses to endurance training are similar for men and women who endurance trained for 9 to 12 men and women. At all ages, women and men show months, the improvement in V̇O2max varied from 0 similar gains in strength from resistance training to 43 percent; the mean increase was 24 percent (Rogers and Evans 1993; Holloway and Baechle 1990) Table 3-2. A hypothetical example of alterations in selected physiological variables consequent to a 6-month endurance training program in a previously sedentary man compared with those of a typical elite endurance runner Sedentary man Variable Pretraining Posttraining Runner Cardiovascular HR at rest (beats min-1) 71 59 36 HR max (beats min-1) 185 183 174 SV rest (ml) 65 80 125 SV max (ml) 120 140 200 Q̇ rest (L min-1 ) 4.6 4.7 4.5 Q̇ max (L min-1) 22.2 25.6 32.5 Heart volume (ml) 750 820 1,200 Blood volume (L) 4.7 5.1 6.0 Systolic BP rest (mmHg) 135 130 120 Systolic BP max (mmHg) 210 205 210 Diastolic BP rest (mmHg) 78 76 65 Diastolic BP max (mmHg) 82 80 65 Respiratory V̇E rest (L min-1) 7 6 6 V̇E rest (L min-1) 110 135 195 TV rest (L) 0.5 0.5 0.5 TV max (L) 2.75 3.0 3.9 RR rest (breaths min-1) 14 12 12 RR max (breaths min-1 ) 40 45 50 Metabolic - A-vO diff rest (ml 100 ml-1 ) 6.0 6.0 6.0 2 - A-vO2 diff max (ml 100 ml-1) 14.5 15.0 16.0 V̇O2 rest (ml kg-1 min-1 ) 3.5 3.5 3.5 V̇O2 max (ml kg -1 min-1) 40.5 49.8 76.5 Blood lactate rest (mmol L-1) 1.0 1.0 1.0 Blood lactate max (mmol L-1 ) 7.5 8.5 9.0 Adapted from Wilmore JH, Costill DL. Physiology of sport and exercise. Champaign, IL: Human Kinetics, 1994, p. 230. HR = heart rate; max = maximal; SV = stroke volume; Q̇ = cardiac output; BP = blood pressure; V̇E = ventilatory volume; TV = tidal volume; - RR = respiration rate; A-vO 2 diff = arterial-mixed venous oxygen difference; V̇O2 = oxygen consumption. 70  Physiologic Responses and Long-Term Adaptations to Exercise and similar gains in V̇O2max from aerobic endurance increase without excessive stress on the ventricular training (Kohrt et al. 1991; Mitchell et al. 1992). walls. Long-term adaptive responses include hyper- trophy of the cardiac muscle fibers (i.e., increases in Cardiovascular and Respiratory Adaptations the size of each fiber). This hypertrophy increases Endurance training leads to significant cardiovascu- the muscle mass of the ventricles, permitting greater lar and respiratory changes at rest and during steady- force to be exerted with each beat of the heart. state exercise at both submaximal and maximal rates Increases in the thickness of the posterior and septal of work. The magnitude of these adaptations largely walls of the left ventricle can lead to a more forceful depends on the person’s initial fitness level; on mode, contraction of the left ventricle, thus emptying more intensity, duration, and frequency of exercise; and of the blood from the left ventricle (George, Wolfe, on the length of training (e.g., weeks, months, years). Burggraf 1991). Endurance training increases the number of cap- Long-Term Cardiovascular Adaptations illaries in trained skeletal muscle, thereby allowing a greater capacity for blood flow in the active muscle Cardiac output at rest and during submaximal exer- (Terjung 1995). This enhanced capacity for blood cise is essentially unchanged following an endur- flow is associated with a reduction in total peripheral ance training program. At or near maximal rates of resistance; thus, the left ventricle can exert a more work, however, cardiac output is increased sub- forceful contraction against a lower resistance to stantially, up to 30 percent or more (Saltin and flow out of the ventricle (Blomqvist and Saltin 1983). Rowell 1980). There are important differences in Arterial blood pressure at rest, blood pressure the responses of stroke volume and heart rate to during submaximal exercise, and peak blood pres- training. After training, stroke volume is increased sure all show a slight decline as a result of endurance at rest, during submaximal exercise, and during training in normotensive individuals (Fagard and maximal exercise; conversely, posttraining heart Tipton 1994). However, decreases are greater in rate is decreased at rest and during submaximal persons with high blood pressure. After endurance exercise and is usually unchanged at maximal rates training, resting blood pressure (systolic/diastolic) of work. The increase in stroke volume appears to will decrease on average -3/-3 mmHg in persons with be the dominant change and explains most of the normal blood pressure; in borderline hypertensive changes observed in cardiac output. persons, the decrease will be -6/-7 mmHg; and in Several factors contribute to the increase in hypertensive persons, the decrease will be -10/-8 stroke volume from endurance training. Endurance mmHg (Fagard and Tipton 1994). (See Chapter 4 for training increases plasma volume by approximately further information.) the same percentage that it increases stroke volume (Green, Jones, Painter 1990). An increased plasma volume increases the volume of blood available to Respiratory Adaptations return to the right heart and, subsequently, to the The major changes in the respiratory system from en- left ventricle. There is also an increase in the end- durance training are an increase in the maximal rate of diastolic volume (the volume of blood in the heart pulmonary ventilation, which is the result of increases at the end of the diastolic filling period) because of in both tidal volume and respiration rate, and an increased amount of blood and increased return of increase in pulmonary diffusion at maximal rates of blood to the ventricle during exercise (Seals et al. work, primarily due to increases in pulmonary blood 1994). This acute increase in the left ventricle’s flow, particularly to the upper regions of the lung. end-diastolic volume stretches its walls, resulting in a more elastic recoil. Endurance training also results in long-term Maintenance, Detraining, and changes in the structure of the heart that augment Prolonged Inactivity stroke volume. Short-term adaptive responses in- Most adaptations that result from both endurance clude ventricular dilatation; this increase in the vol- and resistance training will be reversed if a person ume of the ventricles allows end-diastolic volume to stops or reduces training. The greatest deterioration 71 Physical Activity and Health in physiologic function occurs during prolonged bed made. The resulting detrimental changes in physi- rest and immobilization by casts. A basic mainte- ologic function and performance are similar to those nance training program is necessary to prevent these resulting from reduced gravitational forces during losses in function. space flight and are more dramatic than those result- ing from detraining studies in which routine daily Maintaining Fitness and Muscular Strength activities in the upright position (e.g., walking, stair Muscle strength and cardiorespiratory capacity are climbing, lifting, and carrying) are not restricted. dependent on separate aspects of exercise. After a per- Results of bed rest studies show numerous physi- son has obtained gains in V̇O 2max by performing ologic changes, such as profound decrements in cardiorespiratory exercise six times per week, two cardiorespiratory function proportional to the dura- to four times per week is the optimal frequency of tion of bed rest (Shephard 1994; Saltin et al. 1968). training to maintain those gains (Hickson and Metabolic disturbances evident within a few days of Rosenkoetter 1981). Further, a substantial part of bed rest include reversible glucose intolerance and the gain can be retained when the duration of each hyperinsulinemia in response to a standard glucose session is reduced by as much as two-thirds, but load, reflecting cell insulin resistance (Lipman et al. only if the intensity during these abbreviated ses- 1972); reduced total energy expenditure; negative sions is maintained at ≥70 percent of V̇O2max nitrogen balance, reflecting loss of muscle protein; (Hickson et al. 1985). If training intensity is reduced and negative calcium balance, reflecting loss of bone by as little as one-third, however, a substantial mass (Bloomfield and Coyle 1993). There is also a reduction in V̇O2max can be expected over the next substantial decrease in plasma volume, which affects 15 weeks (Hickson et al. 1985). aerobic power. In previously untrained persons, gains in mus- From one study, a decline in V̇O2max of 15 per- cular strength can be sustained by as little as a single cent was evident within 10 days of bed rest and session per week of resistance training, but only if progressed to 27 percent in 3 weeks; the rate of loss was the intensity is not reduced (Graves et al. 1988). approximately 0.8 percent per day of bed rest (Bloomfield and Coyle 1993). The decrement inV̇O2max from bed rest and reduced activity results from a Detraining decrease in maximal cardiac output, consequent to a With complete cessation of exercise training, a sig- reduced stroke volume. This, in turn, reflects the nificant reduction in V̇O2max and a decrease in decrease in end-diastolic volume resulting from a plasma volume occur within 2 weeks; all prior func- reduction in total blood and plasma volume, and tional gains are dissipated within 2 to 8 months, even probably also from a decrease in myocardial contrac- if routine low- to moderate-intensity physical activ- tility (Bloomfield and Coyle 1993). Maximal heart ity has taken the place of training (Shephard 1994). - rate and A-vO 2 difference remain unchanged Muscular strength and power are reduced at a much (Bloomfield and Coyle 1993). Resting heart rate slower rate than V̇O2max, particularly during the remains essentially unchanged or is slightly in- first few months after an athlete discontinues resis- creased, whereas resting stroke volume and cardiac tance training (Fleck and Kraemer 1987). In fact, no output remain unchanged or are slightly decreased. decrement in either strength or power may occur for However, the heart rate for submaximal exertion is the first 4 to 6 weeks after training ends (Neufer et al. generally increased to compensate for the sizable 1987). After 12 months, almost half of the strength reduction in stroke volume. gained might still be retained if the athlete remains Physical inactivity associated with bed rest or moderately active (Wilmore and Costill 1994). prolonged weightlessness also results in a progres- sive decrement in skeletal muscle mass (disuse Prolonged Inactivity atrophy) and strength, as well as an associated The effects of prolonged inactivity have been studied reduction in bone mineral density that is approxi- by placing healthy young male athletes and sedentary mately proportional to the duration of immobiliza- volunteers in bed for up to 3 weeks after a control tion or weightlessness (Bloomfield and Coyle 1993). period during which baseline measurements were The loss of muscle mass is not as great as that which 72  Physiologic Responses and Long-Term Adaptations to Exercise occurs with immobilization of a limb by a plaster effects of physical activity on the cardiovascular, cast, but it exceeds that associated with cessation of respiratory, endocrine, and musculoskeletal systems resistance exercise training. The muscle groups have been demonstrated to be similar among persons most affected by prolonged immobilization are the with disabilities, depending on the specific nature of antigravity postural muscles of the lower extremi- the disability. For example, physiologic responses to ties (Bloomfield and Coyle 1993). The loss of nor- exercise have been studied among persons with mal mechanical strain patterns from contraction of paraplegia (Davis 1993), quadriplegia (Figoni 1993), these muscles results in a corresponding loss of mental retardation (Fernhall 1993), multiple sclero- density in the bones of the lower extremity, particu- sis (Ponichtera-Mulcare 1993), and postpolio syn- larly the heel and the spine (Bloomfield and Coyle drome (Birk 1993). 1993). Muscles atrophy faster than bones lose their density. For example, 1 month of bed rest by healthy Environmental Conditions young men resulted in a 10 to 20 percent decrease The basic physiologic responses to an episode of in muscle fiber cross-sectional area and a 21 percent exercise vary considerably with changes in environ- reduction in peak isokinetic torque of knee exten- mental conditions. As environmental temperature sors (Bloomfield and Coyle 1993), whereas a simi- and humidity increase, the body is challenged to lar period of bed rest resulted in a reduction in bone maintain its core temperature. Generally, as the mineral density of only 0.3 to 3 percent for the body’s core temperature increases during exercise, lumbar spine and 1.5 percent for the heel. blood vessels in the skin begin to dilate, diverting Quantitative histologic examination of muscle more blood to the body’s surface, where body heat biopsies of the vastus lateralis of the leg following can be passed on to the environment (unless envi- immobilization shows reduced cross-sectional area ronmental temperature exceeds body temperature). for both slow-twitch and fast-twitch fibers, actual Evaporation of water from the skin’s surface signifi- necrotic changes in affected fibers, loss of capillary cantly aids in heat loss; however, as humidity in- density, and a decline in aerobic enzyme activity, creases, evaporation becomes limited. creatinine phosphate, and glycogen stores (Bloomfield These methods for cooling can compromise car- and Coyle 1993). On resuming normal activity, diovascular function during exercise. Increasing reversibility of these decrements in cardiorespiratory, blood flow to the skin creates competition with the metabolic, and muscle function is fairly rapid (within active muscles for a large percentage of the cardiac days to weeks) (Bloomfield and Coyle 1993). By output. When a person is exercising for prolonged contrast, the reversal of the decrement of bone min- periods in the heat, stroke volume will generally eral density requires weeks to months. decline over time consequent to dehydration and increased blood flow in the skin (Rowell 1993; Montain and Coyle 1992). Heart rate increases sub- Special Considerations stantially to try to maintain cardiac output to com- The physiologic responses to exercise and physi- pensate for the reduced stroke volume. ologic adaptations to training and detraining, re- High air temperature is not the only factor that viewed in the preceding sections, can be influenced stresses the body’s ability to cool itself in the heat. by a number of factors, including physical disability, High humidity, low air velocity, and the radiant heat environmental conditions, age, and sex. from the sun and reflective surfaces also contribute to the total effect. For example, exercising under Disability conditions of 32˚C (90˚F) air temperature, 20 per- Although there is a paucity of information about cent relative humidity, 3.0 kilometers per hour (4.8 physiologic responses to exercise among persons miles per hour) air velocity, and cloud cover is much with disabilities, existing information supports the more comfortable and less stressful to the body than notion that the capacity of these persons to adapt to the same exercise under conditions of 24˚C (75˚F) increased levels of physical activity is similar to that air temperature, 90 percent relative humidity, no air of persons without disabilities. Many of the acute movement, and direct exposure to the sun. 73 Physical Activity and Health Children respond differently to heat than adults Vallerand 1994; Shephard 1993). These include the do. Children have a higher body surface area to body increased generation of body heat by vigorous activ- mass ratio (surface area/mass), which facilitates heat ity and shivering, increased production of catechola- loss when environmental temperatures are below mines, vasoconstriction in both the cutaneous and skin temperature. When environmental tempera- nonactive skeletal muscle beds to provide insulation ture exceeds skin temperature, children are at an for the body’s core, increased lactate production, and even greater disadvantage because these mecha- a higher oxygen uptake for the same work (Doubt nisms then become avenues of heat gain. Children 1991). For the same absolute temperature, exposure also have a lower rate of sweat production; even to cold water is substantially more stressful than though they have more heat-activated sweat glands, exposure to cold air because the heat transfer in each gland produces considerably less sweat than water is about 25 times greater than in air (Toner and that of an adult (Bar-Or 1983). McArdle 1988). Because the ratio of surface area to The inability to maintain core temperature can mass is higher in children than in adults, children lead to heat-related injuries. Heat cramps, character- lose heat at a faster rate when exposed to the same ized by severe cramping of the active skeletal muscles, cold stress. The elderly tend to have a reduced is the least severe of three primary heat disorders. response of generating body heat and are thus more Heat exhaustion results when the demand for blood susceptible to cold stress. exceeds what is available, leading to competition for Altitude also affects the body’s physiologic re- the body’s limited blood supply. Heat exhaustion is sponses to exercise. As altitude increases, barometric accompanied by symptoms including extreme fa- pressure decreases, and the partial pressure of inhaled tigue, breathlessness, dizziness, vomiting, fainting, oxygen is decreased proportionally. A decreased par- cold and clammy or hot and dry skin, hypotension, tial pressure of oxygen reduces the driving force to and a weak, rapid pulse (Wilmore and Costill 1994). unload oxygen from the air to the blood and from the Heatstroke, the most extreme of the three heat disor- blood to the muscle, thereby compromising oxygen ders, is characterized by a core temperature of 40˚C delivery (Fulco and Cymerman 1988). V̇O2max is (104˚F) or higher, cessation of sweating, hot and dry significantly reduced at altitudes greater than 1,500 skin, rapid pulse and respiration, hypertension, and meters. This effect impairs the performance of endur- confusion or unconsciousness. If left untreated, heat- ance activities. The body makes both short-term and stroke can lead to coma, then death. People experi- long-term adaptations to altitude exposure that en- encing symptoms of heat-related injury should be able it to at least partially adapt to this imposed stress. taken to a shady area, cooled with by whatever means Because oxygen delivery is the primary concern, the available, and if conscious given nonalcoholic bever- initial adaptation that occurs within the first 24 hours ages to drink. Medical assistance should be sought. of exposure to altitude is an increased cardiac output To reduce the risk of developing heat disorders, a both at rest and during submaximal exercise. Ventila- person should drink enough fluid to try to match tory volumes are also increased. An ensuing reduction that which is lost through sweating, avoid extreme in plasma volume increases the concentration of red heat, and reduce the intensity of activity in hot blood cells (hemoconcentration), thus providing more weather. Because children are less resistant to the oxygen molecules per unit of blood (Grover, Weil, adverse effects of heat during exercise, special atten- Reeves 1986). Over several weeks, the red blood cell tion should be given to protect them when they mass is increased through stimulation of the bone exercise in the heat and to provide them with extra marrow by the hormone erythropoietin. fluids to drink. Exercising vigorously outdoors when air qual- Stresses associated with exercising in the ex- ity is poor can also produce adverse physiologic treme cold are generally less severe. For most situa- responses. In addition to decreased tolerance for tions, the problems associated with cold stress can be exercise, direct respiratory effects include increased eliminated by adequate clothing. Still, cold stress can airway reactivity and potential exposure to harmful induce a number of changes in the physiologic re- vapors and airborne dusts, toxins, and pollens sponse to exercise (Doubt 1991; Jacobs, Martineau, (Wilmore and Costill 1994). 74  Physiologic Responses and Long-Term Adaptations to Exercise Effects of Age steadily for girls during those years (Figure 3-4) When absolute values are scaled to account for (Krahenbuhl, Skinner, Kohrt 1985). Most likely, differences in body size, most differences in physi- different patterns of physical activity contribute to ologic function between children and adults dis- this variation because the difference in aerobic appear. The exceptions are notable. For the same capacity between elite female endurance athletes absolute rate of work on a cycle ergometer, chil- and elite male endurance athletes is substantially dren will have approximately the same metabolic less than the difference between boys and girls in cost, or V̇O2 demands, but they meet those demands general (e.g., 10 percent vs. 25 percent) (Wilmore differently. Because children have smaller hearts, and Costill 1994). their stroke volume is lower than that for adults for The deterioration of physiologic function with the same rate of work. Heart rate is increased to aging is almost identical to the change in function compensate for the lower stroke volume; but be- that generally accompanies inactivity. Maximal heart cause this increase is generally inadequate, cardiac rate and maximal stroke volume are decreased in output is slightly lower (Bar-Or 1983). The A-vO - older adults; maximal cardiac output is thus de- 2 difference is therefore increased to compensate for creased, which results in a V̇O2max lower than that the lower cardiac output to achieve the same V̇O2. of a young adult (Raven and Mitchell 1980). The The V̇O2max, expressed in liters per minute, in- decline inV̇O2max approximates 0.40 to 0.50 milli- creases during the ages of 6–18 years for boys and liters per kilogram per minute per year in men, 6–14 years for girls (Figure 3-4) before it reaches a according to data from cross-sectional studies; this plateau (Krahenbuhl, Skinner, Kohrt 1985). When rate of decline is less in women (Buskirk and expressed relative to body weight (milliliters per Hodgson 1987). Through training, both older men kilogram per minute),V̇O2max remains fairly stable and women can increase their V̇O 2max values by for boys from 6–18 years of age but decreases approximately the same percentage as those seen.. Figure 3-4. Changes in VO2 max with increasing age from 6 to 18 years of age in boys and girls* 4.0 60 Boys, ml/kg 3.5 Maximal oxygen uptake (ml/kg/min) Maximal oxygen uptake (liters/min) 50 3.0 Girls, ml/kg 40 2.5 Boys, L/min 2.0 30 1.5 Girls, L/min 20 1.0 *Values are expressed in both liters per minute and 10 0.5 relative to body weight (milliliters per kilogram per minute). 0.0 0 6 7 8 9 10 11 12 13 14 15 16 17 18 Age (years) Data were taken from Krahenbuhl GS, Skinner JS, Kohrt WM 1985 and Bar-Or O 1983. 75  Physical Activity and Health in younger adults (Kohrt et al. 1991). The inter- aging process itself. By maintaining an active relationships of age, V̇O2max, and training status lifestyle, or by increasing levels of physical activ- are evident when the loss in V̇O 2max with age is ity if previously sedentary, older persons can compared for active and sedentary individuals maintain relatively high levels of cardiovascular (Figure 3-5). and metabolic function, including V̇O2max (Kohrt When the cardiorespiratory responses of an older et al. 1991), and of skeletal muscle function (Rogers adult are compared with those of a young or middle- and Evans 1993). For example, Fiatarone and col- aged adult at the same absolute submaximal rate of leagues (1994) found an increase of 113 percent in work, stroke volume for an older person is generally the strength of elderly men and women (mean age of lower and heart rate is higher from the attempt to 87.1 years) following a 10-week training program of maintain cardiac output. Because this attempt is progressive resistance exercise. Cross-sectional thigh generally insufficient, the A-vO- difference must muscle area was increased, as was stair-climbing 2 increase to provide the same submaximal oxygen power, gait velocity, and level of spontaneous activ- uptake (Raven and Mitchell 1980; Thompson and ity. Increasing endurance and strength in the elderly Dorsey 1986). Some researchers have shown, how- contributes to their ability to live independently. ever, that cardiac output can be maintained at both submaximal and maximal rates of work through a Differences by Sex higher stroke volume in older adults (Rodeheffer et For the most part, women and men who participate al. 1984). in exercise training have similar responses in car- The deterioration in physiological function nor- diovascular, respiratory, and metabolic function mally associated with aging is, in fact, caused by a (providing that size and activity level are normal- combination of reduced physical activity and the ized). Relative increases in V̇O2max are equivalent. Figure 3-5. Changes in VO2 max with aging, comparing an.active population and sedentary population (the figure also illustrates the expected increase in VO2 max when a previously sedentary person begins an exercise program) 70 60 Active adults Reduction in activity plus “aging” VO2 max (ml kg-1 min-1) 50 40. Reduction in Expected increase in VO2 max activity plus resulting from an exercise intervention 30 weight gain Sedentary adults.. 20 10 0 20 30 40 50 60 70 80 Age (years) Adapted, by permission, from Buskirk ER, Hodgson JL. Federation Proceedings 1987. 76  Physiologic Responses and Long-Term Adaptations to Exercise for women and men (Kohrt et al. 1991; Mitchell et 2. Better characterize mechanisms through which al. 1992). Some evidence suggests that older women the musculoskeletal system responds differen- accomplish this increase in V̇O2max mainly through tially to endurance and resistance exercise. - difference, whereas younger an increase in the A-vO 2 3. Better characterize the mechanisms by which women and men have substantial increases in stroke physical activity reduces the risk of cardiovascular volume, which increases maximal cardiac output disease, hypertension, and non–insulin- (Spina et al. 1993). With resistance training, women dependent diabetes mellitus. experience equivalent increases in strength (Rogers and Evans 1993; Holloway and Baechle 1990), 4. Determine the minimal and optimal amount of although they gain less fat-free mass due to less exercise for disease prevention. muscle hypertrophy. 5. Better characterize beneficial activity profiles for Several sex differences have been noted in the people with disabilities. acute response to exercise. 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