of the acute responses and chronic adaptations to physical activity and exercise. Much of the knowl- edge base in the study of exercise physiology and exercise science in general is the result of research and observation into the acute responses and chronic adaptations of the systems of the body to physical activity and exercise.
performance (34,94). Food restriction, either through dieting or starvation or prolonged exercise can alter substrate utilization by promoting a greater contri- bution of energy supplied from protein (94,99).
The intensity and duration of physical activity and exercise are two primary factors affecting substrate utilization. During low-intensity physical activity and exercise, fat is the primary substrate for energy production. As physical activ- ity or exercise intensity increases, there is a greater reliance on carbohydrate as an energy source. When exercise intensity reaches close to 100% of maximal oxygen consumption (VO2max), almost 100% of the energy is provided by the metabolism of carbohydrates (20,52). There is an exercise intensity that results in a point being reached when there is a shift from fat to carbohydrate as the predominant energy substrate. This point has been labeled the crossover point (20). During exercise, there are several factors that cause the shift in substrate utilization to occur. As exercise intensity increases, more fast glycolytic fi bers are recruited to power muscular contraction and these fi bers rely predominately on carbohydrate as the energy substrate (52). The second factor directly affecting the shift in substrate utilization is an increase in the concentration of the hormone epinephrine. As the exercise intensity increases, the sympathetic nervous system and the adrenal medulla increase the release of epinephrine into the circulation.
Epinephrine increases carbohydrate metabolism by increasing muscle glycogen breakdown and inhibiting the release of fat from adipose tissue (52). Improve- ments in cardiorespiratory fi tness as a result of increased levels of regular physi- cal activity or exercise result in a shift in substrate utilization so that greater fat utilization occurs at higher exercise intensities. This results in less carbohydrate and more fat being used during physical activity or exercise (19). Figure 3.2 pro- vides an illustration of the role of exercise intensity on substrate utilization and the crossover point.
During long-duration physical activity or exercise, there is a gradual increase in the utilization of fat as an energy substrate. This results in a decrease in the use of carbohydrates as a fuel source (53). There are several factors that cause the shift in fuel utilization during prolonged exercise to occur. The use of fat as an energy source in skeletal muscle depends in part on the mobilization of fat from adipose tissue and delivery of fat by the circulatory system to working skeletal muscle (19).
Percent Energy from Fat and Carbohydrates 100
80
60
40
20
0
% VO2 max
0 20 40 60 80 100
Carbohydrate Fat
•
FIGURE 3.2 ▼ Effect of exercise intensity on substrate utiliza- tion and the crossover point (Source: Powers SK, Howley ET.
Exercise Physiology: Theory and Application to Fitness and Performance. Dubuque: Brown
& Benchmark; 2007, p. 56, Chapter 4, Figure 4.11.)
Potteiger_Chap03.indd 85
Potteiger_Chap03.indd 85 11/6/2009 5:23:55 PM11/6/2009 5:23:55 PM
The mobilization of fat from adipose is controlled by enzymes called lipases. The activity level of these enzymes is infl uenced by the hormones epinephrine, nor- epinephrine, and glucagon. During long-duration exercise, the concentration of these hormones in the blood increases resulting in an increase in fatty acid release into the circulation. The delivery of the fats to the working tissues occurs as a result of increased blood fl ow to the muscles that are active during physical activ- ity and exercise (52,53). Fat mobilization can be inhibited by high blood levels of lactic acid and the hormone insulin. If during long-duration exercise there is an increase in the exercise intensity so that blood lactic acid levels increase, this could result in a decrease in fat mobilization and utilization as a fuel substrate (19).
Insulin also inhibits fat mobilization, but generally during long-duration exercise there is a decline in blood insulin levels (19). Table 3.6 identifi es some of the prominent factors affecting fuel utilization during exercise.
Implications for Physical Activity and Exercise
The control of factors affecting substrate metabolism has implications for exercise science professionals designing physical activity and exercise programs. For exam- ple, if the intention is to decrease body weight and body fat then physical activity and exercise that promotes the use of fat as the substrate should be emphasized.
It is often recommended that exercise intensity be kept low to promote the use of fat as a fuel substrate because there is a greater percentage of fat utilized at low intensities. However, this recommendation does not take into account the total amount of energy expended during the exercise session. At higher exercise inten- sities, more total fat could be used as a fuel source even though the percentage of fat being utilized may be less than that which occurs at a lower intensity of exer- cise (89). Another consideration is whether the consumption of a pre-exercise meal or beverage high in carbohydrate is necessary. If a meal or glucose beverage is consumed too close to the start of exercise, there will likely be an increase in the insulin concentration in the blood, which will decrease the mobilization of fat and result in a reduction in the use of fat as a fuel substrate (57).
Table 3.6 Some Factors Affecting Fuel Utilization During Exercise
FACTOR GENERAL RESPONSE
Increased exercise intensity Increased use of carbohydrate Increased exercise duration Increased use of fat
Epinephrine and norepinephrine Increased use of carbohydrate
Glucagon Increased use of fat
Lactic acid Decreased use of fat
Hypoglycemic A condition of abnormally low blood glucose levels.
Potteiger_Chap03.indd 86
Potteiger_Chap03.indd 86 11/6/2009 5:23:56 PM11/6/2009 5:23:56 PM
Implications for Sport and Athletic Performance
Athletes who participate in sports or athletic competitions that are of prolonged duration (e.g., triathlons, long-distance running and cycling) must be careful about maintaining blood glucose and muscle glycogen concentrations during exercise.
During long-duration exercise performed at moderate to high intensity, there is a signifi cant reliance on muscle glycogen as the carbohydrate energy source. During prolonged exercise that lasts 3 to 4 hours or longer, both blood glucose and muscle glycogen contribute to energy production in an equal percentage. As muscle gly- cogen concentration decreases, the use of blood glucose increases so that when an athlete is nearing a level of muscle glycogen depletion blood glucose may be the predominate source of carbohydrate as a fuel (31).
If blood glucose concentration is not maintained during long-duration exer- cise, the athlete could become hypoglycemic which may lead to fatigue, a decrease in performance, and possible serious medical conditions. Consequently, it is important to maintain blood glucose concentrations during exercise (89).
The timing and type of carbohydrate consumption are two factors that are criti- cal for helping to maintain blood glucose concentrations during long-duration exercise. It is suggested that a carbohydrate feeding of between 1 and 5 g of car- bohydrate per kilogram body mass should occur between 1 and 4 hours prior to the start of exercise (94). Although this procedure results in an increased use of carbohydrate as a substrate early in the exercise bout, the additional carbohydrate consumed will help maintain blood glucose for a longer period of time during exercise (94). Carbohydrate should also be consumed during exercise ( Figure 3.3)
FIGURE 3.3 ▼Carbohydrate consumption during exercise. (From ACSM’s Resource for the Personal Trainer, 3rd ed. Baltimore (MD):
Lippincott Williams & Wilkins; 2010.)
Potteiger_Chap03.indd 87
Potteiger_Chap03.indd 87 11/6/2009 5:23:56 PM11/6/2009 5:23:56 PM
and this procedure has been shown to delay the onset of fatigue and improve long-duration exer- cise performance (108). The best type of carbohy- drate to consume during exercise is glucose, sucrose, or glucose polymer solutions at a concentration of approximately 6% to 8% (94). Exercise scientist professionals continue to examine ways to improve athletic performance and decrease the potential for the adverse effects of muscle glycogen depletion and hypoglycemia through the ingestion of glucose and glucose polymer beverages, and energy bars prior to and during exercise.
Muscle Control of Glucose Uptake
The control of blood glucose uptake by tissues of the body at rest and during physical activity and exercise has important implications for overall health and athletic performance, and is of considerable interest to exercise science profes- sionals. The body closely regulates energy utilization so that as skeletal muscle uses energy during physical activity or exercise there is an increase in energy production by the tissues. As the need for energy increases, there are a variety of changes that occur in the body. For example, the use of carbohydrates as an energy source in muscle results in the breakdown and utilization of muscle gly- cogen and an increase in the uptake of glucose from the blood into the individual muscle cells. There is also an increase in liver glycogen breakdown into glucose with the subsequent release of the glucose into the blood in an effort to maintain blood glucose concentration and to provide glucose to the working tissues. Vari- ous hormones, including epinephrine, norepinephrine, and glucagon, stimulate the breakdown (19).
The movement of glucose from the blood into the cell depends on the inter- action of the hormone insulin and a glucose transport protein. Insulin is released from the pancreas to help control blood glucose levels in the body and help glu- cose enter into the cells of the body. Glucose enters the cells by interacting with a glucose transport protein on the cell membrane. There are several glucose transport proteins, but the one specifi c to skeletal muscle is called the glucose transport protein 4 or Glut 4. When a muscle cell needs glucose, it increases the number of the Glut 4 proteins in the cell membrane (32). Several factors stimulate an increase in Glut 4 proteins including insulin, increased blood fl ow to skeletal muscle, increased glucose concentration, and muscle contractions (92).
Figure 3.4 illustrates the process by which insulin facilitates the Glut 4 transport protein to take glucose into the cell.
➤
Thinking CriticallyWhy would consumption of carbohy- drate prior to and during exercise have different effects on an individual exercising to improve health versus an individual competing in a prolonged athletic competition?
Glucose polymer solution A beverage that contains multiple glucose molecules linked together in solution.
Glucose transport protein 4 A type of protein molecule that works with insulin to facilitate glucose uptake by skeletal muscle fibers.
Potteiger_Chap03.indd 88
Potteiger_Chap03.indd 88 11/6/2009 5:23:56 PM11/6/2009 5:23:56 PM
Once glucose enters a muscle fi ber, it can be stored as muscle glycogen or immediately used to produce energy. The stored muscle glycogen can be broken down for energy by chemical compounds, called enzymes, in the muscle fi ber. The muscle glycogen and blood glucose are metabolized through a series of chemical reactions and provide energy to support the functions of the contracting muscle fi bers. As mentioned previously, the exercise intensity is one of the strongest factors infl uencing the use of carbohydrate as a fuel source. As exercise intensity increases, the reliance on carbohydrates as an energy source increases (Figure 3.1).
Implications for Physical Activity and Exercise
The role of physical activity and exercise in regulating glucose metabolism is of importance in promoting health and wellness. Diabetes mellitus is a disease condition whereby insuffi cient insulin is produced by the pancreas (type 1) or the insulin does not promote the uptake of glucose by the cell (type 2). At one time, it was believed that individuals with type 1 diabetes mellitus should not participate in physical activity, exercise, sport, or athletic competition because these individuals have diffi culty controlling their blood glucose concentration.
However, with a better understanding of the role of glucose transport proteins in controlling blood glucose, individuals with type 1 diabetes who are otherwise
Metabolism
Metabolism Glucose
Glucose GLUT4
GLUT4
-P -P
Insulin Insulin receptor Extracellular
Intracellular
Extracellular
Intracellular Basal
Insulin
FIGURE 3.4 ▼ Insulin and Glut 4 facilitate glucose uptake by cells of the body.
Potteiger_Chap03.indd 89
Potteiger_Chap03.indd 89 11/6/2009 5:23:56 PM11/6/2009 5:23:56 PM
healthy are now encouraged to participate in exercise programs and sports to improve their health and wellness (7). A key consideration for the type 1 diabetic is whether the individual is in metabolic control. Physical activity and exercise can have a dramatic impact on the blood glucose concentration and if the individual is not careful about regulating diet and the intensity of the activity or exercise then hypoglycemia and insulin shock can occur. Metabolic control indicates that the individual is on a regular schedule of diet, insulin, and physical activity that allows for the maintenance of blood glucose levels in the normal range with little fl uctuation (70,91).
Type 2 diabetes mellitus manifests itself through a series of processes whereby the body becomes resistant to insulin. Inadequate amounts of physical activity and poor nutritional habits can lead to the development of insulin resistance (12,101).
This condition is characterized by relatively well-maintained insulin secretion and normal to elevated plasma insulin levels. Type 2 diabetes generally occurs later in life and there are usually several other health risks associated with the condi- tion including hypertension, high blood cholesterol levels, abdominal obesity, and physical inactivity (6). Physical activity and exercise are benefi cial to the individ- ual with type 2 diabetes as increased levels of activity help control blood glucose concentration and also have a positive effect on the associated health risks (6).
The combination of a regular program of exercise and modifi cation to the nutri- tional intake may allow the individual with type 2 diabetes to eliminate the need for exogenous insulin or the oral medications used to stimulate insulin secretion by the pancreas (6). Exercise science professionals involved in the prescription of physical activity and exercise must be aware of the process by which glucose is taken up and used for energy so that effective individualized programs can be developed for healthy and diseased individuals.
Implications for Sport and Athletic Performance
Success in certain types of sport and athletic performance is dependent on the ability of the body and muscle fi bers to produce energy very quickly. Those ath- letic events that are characterized by short duration and high-intensity periods of activity (e.g., sprinting, football, volleyball) depend heavily on the ability to rapidly use carbohydrates (primarily muscle glycogen) as an energy source. Exer- cise scientists, strength and conditioning coaches, sports coaches, and athletes have long been interested in improving energy production from carbohydrates and delaying the onset of fatigue that occurs from a buildup of lactic acid in the muscle: a by-product of carbohydrate metabolism.
The formation of lactic acid in skeletal muscle is a complex process. During rest and low-intensity exercise, most of the energy is produced by using oxygen in the aerobic metabolism of carbohydrates and fats. Although there is a small amount of lactic acid being produced in the muscle, it is quickly cleared by a num- ber of processes in the body. As the exercise intensity increases, there is an increase in the production of lactic acid and at the same time an increase in the removal of lactic acid from the tissues. There is, however, an exercise intensity whereby lactic acid accumulates in the muscle and blood. As lactic acid (or more accurately the negatively charged lactate ion and its associated positively charged hydrogen ion) increases in concentration, it contributes to fatigue in the contracting muscle.
Potteiger_Chap03.indd 90
Potteiger_Chap03.indd 90 11/6/2009 5:23:56 PM11/6/2009 5:23:56 PM
The fatigue in the muscle is likely caused by the increased acidity that affects various enzymes involved in energy production or interferes with the muscle’s contractile process (103).
The development of various training programs designed to increase energy production from aerobic sources and decrease the fatiguing effects of lactic acid has long been a goal of coaches and athletes. In aerobic endurance events, the success- ful competitor among athletes with similar VO2max values is usually the person who can maintain aerobic energy production at the highest percentage of his or her VO2max, without accumulating large amounts of lactic acid in the muscle and blood (74). Although many terms are used to describe this phenomenon, the lactate threshold is the one most commonly seen in the literature. The lactate threshold is that exercise intensity at which a specifi c blood lactate concentration is observed or where blood lactate concentration begins to increase above resting levels (102).
Research has shown that an athlete’s lactate threshold appears to be a strong pre- dictor of aerobic endurance performance (35,36). The maximal lactate steady state is another term that also appears in the aerobic training literature. The maximal lactate steady state is the exercise intensity where maximal lactate production equals maximal lactate clearance within the body (13). Many experts consider the maximal lactate steady state exercise intensity to be a better indicator of aerobic endurance performance than either maximal oxygen consumption or the lactate threshold (13,48). What is clear from this information is that aerobic endurance athletes must improve their ability to decrease the production of lactic acid and increase the removal of lactic acid from the muscle and blood. This requires the athlete to train at elevated levels of blood and muscle lactate to maximize training- induced improvements that decrease lactic acid production and increase lactic acid clearance by the body (87). The development of and use of nutritional ergogenic aids, such as sodium bicarbonate and sodium citrate, to help minimize the effects of lactic acid buildup during high-intensity exercise is also of interest to exercise science professionals. Both sodium bicarbonate and sodium citrate help maintain the normal pH levels of the body as lactic acid concentrations increase (88).
Skeletal Muscle Physiology
Skeletal muscle has a number of important functions in the human body (see Chapter 2). As a result, exercise scientists have been studying skeletal muscle to gain a better understanding of how muscle performs various functions during exercise and sport performance. Skeletal muscle fi bers develop from embryonic myotubes into mature muscle fi bers and this development is infl uenced by various growth-promoting factors (55). As the fi bers develop, various regulatory
Insulin shock Acute hypoglycemia usually resulting from an excessive insulin and characterized by sweating, trembling, dizziness, and, if left untreated, convulsions and coma.
Exogenous insulin Insulin administered from outside the body.
Aerobic metabolism The production of energy through the use of oxygen in the cell.
Embryonic myotubes Immature structures that can potentially convert into muscle fibers.
Potteiger_Chap03.indd 91
Potteiger_Chap03.indd 91 11/6/2009 5:23:56 PM11/6/2009 5:23:56 PM
and contractile proteins are arranged in a systematic pattern that allows the fi ber to generate force upon stimulation from a motor neuron (55). Skeletal muscle fi bers are a heterogeneous group of fi bers with distinct contractile and meta- bolic characteristics (69) (see Chapter 2, Table 2.3). Each of these fi ber types has characteristics that allow them to perform distinct functions and respond in differ- ent ways during muscle contraction. Skeletal muscle fi bers are unique to individ- uals and specifi c muscle groups. Generally, muscle fi ber types cannot be changed unless there is a dramatic change in the physical activity or exercise habits of an individual. In this instance, the muscle fi ber types will take on characteristics that help the muscle meet the requirements of the physical activity or exercise.
Implications for Physical Activity and Exercise
Skeletal muscle plays an important role in promoting health and wellness for a wide age range of individuals. Exercise science professionals have long been interested in identifying the most appropriate resistance exercise program to enhance muscular strength development and improve muscular fi tness, balance, coordination, and other measures of motor performance (95). Through years of research, much has been learned about the various components needed for a suc- cessful resistance exercise program. Some of the characteristics affecting strength development are shown in Figure 3.5. Resistance exercise training causes muscle fi ber hypertrophy (40,80) as a result of increased protein synthesis (86). Increases in muscular strength (90) and muscular power (104) are observed following par- ticipation in resistance training programs. The importance of muscular strength
Exercise intensity
Exercise frequency
Duration of exercise
training
Types of exercises selected
Total amount of work performed Age and
gender Fitness
level
Factors affecting strength development
FIGURE 3.5 ▼ Factors affecting strength development (95).
Potteiger_Chap03.indd 92
Potteiger_Chap03.indd 92 11/6/2009 5:23:57 PM11/6/2009 5:23:57 PM