Shape Up Study Guide

Study Guide, PHED P106Shape Up

Terms such as exercise and physical fitness are probably spoken in every home in America. Although these and related terms actually have very specific meanings, the way in which they are used in everyday conversations varies widely among individuals and across groups. Let's eliminate any possible confusion by setting operational definitions for a few key terms at the onset.

PHYSICAL ACTIVITY AND EXERCISE

We'll begin by defining two core terms: physical activity and exercise. These have similar but distinct meanings. Then we'll define intensity because it is a common descriptor used to characterize physical activity or exercise.

Physical Activity

Physical activity is bodily movement produced by muscle contraction that increases energy expenditure above a resting level. Simply put, physical activity refers to moving around using our own muscle power. Physical activity includes all movement whether it is getting out of bed in the morning, getting dressed, preparing a meal, or walking to class. In recent years it has been shown that consistently incorporating physical activity into one's lifestyle (e.g., increasing incidental walking, commuting by foot or bike) can yield significant health benefits, particularly for those who have been sedentary. This active lifestyle approach may be more viable and appealing for many Americans who are not interested in playing sports or working out in a gym.

Exercise

Exercise is a subcategory or type of physical activity. Exercise is physical activity that is planned and structured with the primary purpose of improving or maintaining one or more aspects of our physical capacity, commonly known as physical fitness (which will be defined in the next section). Thus, exercise refers to virtually all physical training and sports activities. Sports camps, physical education classes, and basic training in the military are all examples of exercise. Exercising regularly—whether it's training on a sports team or working out at the fitness center—improves health as well as physical capacity and performance.

Intensity

Intensity is arguably the most important characteristic of physical activity or exercise, yet the concept remains fuzzy to many. Intensity refers to the physiological or perceptual effort given during a physical activity. Intensity is assessed with objective measurements (e.g., heart rate, energy expenditure) or subjective ratings (e.g., light, moderate, vigorous, all-out). Although intensity may be measured differently from setting to setting (e.g., health club, sports center, and medical clinic), heart rate and perceived exertion are two widely used methods for gauging intensity. Both are reviewed in the section “Recommendations for Healthy Adults.”

PHYSICAL FITNESS

Physical fitness is a multifaceted concept whose definition has evolved over the past several decades. A generally accepted and enduring definition of physical fitness is “the ability to carry out daily tasks with vigor and alertness, without undue fatigue, and with ample energy to enjoy leisure time pursuits and meet unforeseen emergencies” (Department of Health and Human Services, 2008). A succinct definition from the Surgeon General's Report on Physical Activity and Health is “a set of attributes that people have or achieve that relates to the ability to perform physical activity” (Department of Health and Human Services, 2008). In considering these definitions, three aspects of physical fitness should be noted. First, improved fitness implies an enhanced capacity to perform physical tasks of daily living as well as work-related and leisure-time physical activities. Second, as fitness improves so does physiological well-being, providing protection against numerous inactivity-related diseases. And, third, physical fitness is typically divided into two types or categories—health-related fitness and skill-related fitness—and each type has several components.

Health-related Physical Fitness

Health-related physical fitness includes three fundamental components—cardiorespiratory or aerobic fitness, musculoskeletal or muscular fitness, and body composition (see Table below). The second, musculoskeletal fitness, is in turn composed of three subcomponents—strength, muscular endurance, and flexibility. These terms will be defined and discussed in detail in subsequent sections. For most of you, cardio-respiratory fitness, musculoskeletal fitness, and body composition are familiar concepts because they have long been included in school health and physical education classes.

TABLE 1
PHYSICAL FITNESS: HEALTH-RELATED COMPONENTS

The physical activity or exercise we engage in to develop health-related physical fitness protects against major threats to our health. Cardiorespiratory fitness lowers risk of dying prematurely, especially from heart disease and stroke. Musculoskeletal fitness increases bone density, muscle mass, and joint health and thereby lowers the risk of osteoporosis, low back pain, and degenerative joint conditions. Maintenance of a healthy weight (body composition) through physical activity and good nutrition protects against obesity, diabetes, and related diseases including heart disease, arthritis, and some cancers. How many of these diseases have you witnessed among family members and friends? As we experience these events firsthand, the connection between physical activity and health becomes more real and less abstract.

Body composition, which partitions body weight into lean tissue (muscle and bone) and fat tissue, prescribes a healthy weight from one's level of relative body fatness (% body fat). This health-related fitness component is plainly dependent on both eating patterns and physical activity, much more so than either cardiorespiratory or musculoskeletal fitness. As body composition answers to two masters, it is sometimes discussed under the heading of diet and nutrition and sometimes under exercise and physical fitness. Everyone must eat, but physical activity is largely optional. For that reason, we provide our main discussion of body composition and the importance of healthy weight in Chapter 2, “Choose a Healthy Diet” see the section on weight control to refresh your understanding of this health-related fitness component.

When considering the health-related fitness components, it's natural to think about the high end of fitness performance. For elite endurance athletes (e.g., Olympic marathoners or Tour de France cyclists) and strength athletes (e.g., world-ranked Olympic weight lifters or power lifters), the focus is necessarily narrow and specialized—to be faster or stronger and have the best performance. The sole aim of the high-level endurance or strength athlete is to train relentlessly to improve performance in a single fitness component; any balance with other fitness components is only an afterthought. Such is the nature of national and international sports competition.

Most elite athletes compete at the highest level for only a few years. Once they retire from competition, their training programs are dramatically scaled down and usually reoriented toward general health and fitness goals. For most of us, training and competing in sports and physically challenging activities are enjoyable leisure-time pastimes, not our livelihood or fulltime pursuit. Thus, we are free to take a balanced approach to our fitness programs. Regardless of our athletic abilities or sports interests, each of us can develop an exercise program that includes the three components of health-related fitness—cardiorespiratory fitness, musculoskeletal fitness, and body composition—in a measured and meaningful way.

Skill-related Physical Fitness

The second type of fitness is skill-related physical fitness. While there are many skill-related components of physical fitness, the best known are agility, speed, coordination, and balance. Skill-related components are associated primarily with sport and motor skill performance and only secondarily with improved health. (This view may be changing because clinicians are increasingly citing the importance of balance to health; loss of balance is the major cause of falls for elderly people.) Among young adults, a finely tuned or specialized physical skill can translate into success in a variety of individual and team sports, from golf and tennis to baseball and basketball.

As many of us know from personal experience, components from both skill-related and health-related fitness intertwine in many sports. Soccer players must develop high levels of cardiorespiratory fitness in combination with soccer-specific skills, and gymnasts require high levels of musculoskeletal fitness to execute their skillful performances. Consider your favorite sports for other examples. As recreational athletes, many of us share a common goal—to develop the right combination of fitness components, both health-related and skill-related, to improve performance in our preferred sport(s).

Developing a fitness program involves linking the recommendations to individual goals—such as to improve stamina, gain strength, improve muscle tone, lose body fat, gain muscle mass, control weight, relieve stress, or improve sports performance. What are your goals? Since many exercise goals are related to losing body fat, preventing unwanted weight gain, or gaining muscle mass, we'll review the energy expenditure side of the energy balance equation in this section.

ENERGY BALANCE: ENERGY EXPENDITURE

One of the primary reasons people exercise is for weight control—that is, to help keep a balance between energy intake and energy expenditure. Let's take a closer look at the energy expenditure side. Understanding how physical activity contributes to energy expenditure is critical to preventing unwanted weight gain and to successful long-term weight management.

Estimating Daily Energy Expenditure

Three factors determine a person's daily energy expenditure: (1) resting metabolic rate, (2) the thermic effect of food, and (3) the amount of physical activity. Resting (or basal) metabolic rate is a function of body size and accounts for the majority of one's daily energy expenditure (about 60–75 percent). Plainly, the larger your mass (the more you weigh), the more energy you need to keep all physiological functions operating at the resting level (e.g., sleeping or sitting quietly).

The thermic effect of food refers to the energy associated with digestion. This is that slight but noticeable warm-up many people can feel one or two hours after a large meal. This energy expenditure component is relatively small (5–10 percent) and generally included as part of the resting metabolic rate.

The third factor is the amount of physical activity. This is the only factor which we control. Physical activity is highly variable between people and within an individual. It may account for as little as 25 percent of daily energy expenditure in sedentary folks whose only physical activity is that required by daily living—such as dressing, bathing, and necessary walking. Or physical activity can account for as much as 40 percent or more among highly active people who get an hour or more of exercise every day.

Additional Caloric Expenditure through Physical Activity

The recommended target range for energy expenditure is 150–400 Cal per day. This is above the minimal energy expenditure from activities of daily living. The lower end of the range represents a minimal caloric threshold of about 1,000 Cal per week, which equates to the public health physical activity recommendation of 30 minutes of brisk walking five days per week. Based on the positive dose-response relationship between physical activity and health, gradually moving toward the 300–400 Cal per day level is encouraged. Intentional energy expenditure at 2,000 Cal per week and higher (60–90 minutes per day) has been shown to be successful for both initial weight loss and weight maintenance.

ENERGY BALANCE: ENERGY EXPENDITURE

One of the primary reasons people exercise is for weight control—that is, to help keep a balance between energy intake and energy expenditure. In Chapter 2, the energy intake or food consumption side of the energy balance equation was discussed. Now let's take a closer look at the energy expenditure side. Understanding how physical activity contributes to energy expenditure is critical to preventing unwanted weight gain and to successful long-term weight management.

Estimating Daily Energy Expenditure

Three factors determine a person's daily energy expenditure: (1) resting metabolic rate, (2) the thermic effect of food, and (3) the amount of physical activity. Resting (or basal) metabolic rate is a function of body size and accounts for the majority of one's daily energy expenditure (about 60–75 percent). Plainly, the larger your mass (the more you weigh), the more energy you need to keep all physiological functions operating at the resting level (e.g., sleeping or sitting quietly).

The thermic effect of food refers to the energy associated with digestion. This is that slight but noticeable warm-up many people can feel one or two hours after a large meal. This energy expenditure component is relatively small (5–10 percent) and generally included as part of the resting metabolic rate.

The third factor is the amount of physical activity. This is the only factor which we control. Physical activity is highly variable between people and within an individual. It may account for as little as 25 percent of daily energy expenditure in sedentary folks whose only physical activity is that required by daily living—such as dressing, bathing, and necessary walking. Or physical activity can account for as much as 40 percent or more among highly active people who get an hour or more of exercise every day.

Using Table 2, you can estimate your daily energy expenditure using your weight and activity level. As you complete these calculations, you will see how the reference values (printed on nutrition-facts food labels) for the average woman and man—2,000 and 2,500 Cal/day, respectively—were derived.

TABLE 2
ESTIMATING DAILY ENERGY EXPENDITURE

Additional Caloric Expenditure through Physical Activity

The recommended target range for energy expenditure is 150–400 Cal per day. This is above the minimal energy expenditure from activities of daily living. The lower end of the range represents a minimal caloric threshold of about 1,000 Cal per week, which equates to the public health physical activity recommendation of 30 minutes of brisk walking five days per week. Based on the positive dose-response relationship between physical activity and health, gradually moving toward the 300–400 Cal per day level is encouraged. Intentional energy expenditure at 2,000 Cal per week and higher (60–90 minutes per day) has been shown to be successful for both initial weight loss and weight maintenance.

Average calories burned for different types of moderate and vigorous physical activities are shown in Table 3.6. Note that the energy expended during resistance training is quite a bit lower than what is expended for aerobic activities. This should be clear from the earlier discussion on energy pathways and the different energy demands of aerobic training versus muscular fitness training.

A second point to highlight is the trade-off between intensity and duration among aerobic activities. If your primary aim is to expend calories, you have a choice between exercising at a higher intensity for a shorter period of time or longer at a lower intensity. The total energy required to move a mass—in this case, a person's body weight—over a given distance is not substantially affected by the speed at which it occurs. The energy expended by running 2 miles in 15 minutes (about 200 Cal) is only about 10 percent more than is expended by walking the same distance in 30 minutes (about 180 Cal). For a beginning exerciser or a high-BMI person though, the preferable option is to extend duration and go at a moderate pace.

DEVELOPING AN EXERCISE PLAN

It's time to translate the cardiorespiratory and musculoskeletal guidelines into a specific exercise prescription based on your goals. Alternatively, if you are already a regular exerciser, take the time to assess your program in light of the current recommendations. To help with the translation process, consider the following points.

Physical Activity or Exercise?

Most people in their 20s still have moderate to high functional capacities simply due to their youth. Relying solely on moderate-intensity physical activity, such as brisk walking, may not be sufficiently challenging unless you have been very sedentary—in that case, moderate-intensity activity is the appropriate choice. The majority of college students report that they prefer exercise that is more vigorous whether it's recreational sports, basic fitness training, or a combination.

Since nearly all college students have good-to-excellent sports and exercise facilities available on campus, it's truly a once-in-a-lifetime opportunity to explore different types of classes. The variety of college physical education and campus recreation classes taught by high-caliber instructors is impressive. High school offerings pale in comparison and your choices following college will be more restricted due to fewer choices, higher cost, and less accessibility and time. Develop an exercise program based on activities and sports you enjoy and, at least once a year, take the plunge and try a new exercise/activity class for a semester.

Initial Screening and Risks of Exercise

For nearly all apparently healthy young adults, it is safe to begin or intensify an exercise program. In fact, some experts contend that you put yourself at greater risk for medical problems if you remain physically inactive. Every year a few sudden deaths occur among high school and college athletes during training or competition. There is always immediate speculation as to the cause of death because the official explanation is typically not known for several weeks pending medical tests. At that point, the official cause of death receives only back-page coverage. Nearly all confirmed causes of sudden deaths in young athletes are attributable to cardiac abnormalities, heat stress, drug or supplement use, or a combination of these factors.

One's initial fitness level is the foundation upon which the exercise prescription (frequency, intensity, time, type) is built. If you don't already know your fitness level, consider using standard fitness tests to see where you stand; for example, completing a 1-mile walk or a 2-mile run test to assess cardiorespiratory fitness and using push-up, sit-up, and trunkflexion tests to assess muscular fitness. A variety of fitness tests with instructions and scoring are available online (http://www.topendsports.com/-testing/tests). How you rate against normative scores may be of interest. The true value of assessment though is in using the results as a baseline for tailoring your exercise plan. Be honest in your self-appraisal and set the initial training overload on your current condition. One of the main reasons the exercise recommendations are presented as ranges is to accommodate the wide variability of initial fitness levels.

Equally important is how to gauge the rate of progression for increasing the exercise overload. Rate of progression is dependent on the principle of individual differences in which you learn your dose-response curve through trial and error. Envision an exercise program as having three stages: an initial conditioning stage, which lasts a few weeks; an improvement stage, which can be highly variable in length; and a maintenance stage, which goes on indefinitely. A systematic increase in overload is applied during the second stage. Small, steady increments in the exercise dose (5–15 percent) every week or two is a reasonable rate of progression. Ramping up too quickly is often counterproductive.

Anatomy of the Exercise Session

The format for an exercise session or workout is a warm-up period, the main conditioning phase that involves aerobic and/or muscular training, and a cool-down period. In Figure 3, this format is illustrated for a 30-minute aerobic workout in which heart rate is plotted against time. Warm-up facilitates the transition from rest to exercise, and cool-down allows gradual recovery and transition back to the resting level.

FIGURE 3
FORMAT OF AN AEROBIC TRAINING SESSION WITH WARM-UP, CONDITIONING PHASE, AND COOL-DOWN

Note: 200 is the estimated maximal heart rate for a 20-yr old.

When feasible, resistance training and aerobic training should be performed on alternate days, although both activities can be combined into the same workout. If combined, the training you wish to emphasize (aerobic or resistance) should be done first. Stretching is typically done during the cool-down, as stretching is more effective when the muscles are warm. However, flexibility exercises are also often included as part of a warm-up and sometimes done at a separate time altogether.

Aerobic, resistance, and flexibility training are the components of a complete fitness program. There is wide latitude in how you choose to mix or blend these activities. For example, participation in sports such as basketball, soccer, and tennis can be a core element of your fitness program or simply be included for competitive and social pleasures. Yet, regardless of the specific sport or fitness activity, it's wise to follow the exercise session format and include a warm-up and a cool-down.

The human body transforms the food we eat into our cells and tissues—blood, bone, muscle—along with continuously producing energy to sustain life. To understand how exercise can improve health and fitness, it's necessary to review the basic concept of energy metabolism, or how energy is produced for different types of physical activity.

ENERGY PRODUCTION FOR MUSCLE CONTRACTION

Energy for physical activity is produced in the muscle cells through a complex series of reactions that transforms chemical energy into mechanical energy, resulting in muscle contraction. The most immediate source of energy comes from the high-energy chemical bonds of adenosine triphosphate (ATP). However, ATP—the energy currency of cells—is stored in the muscle in very small amounts and can supply energy for only a few seconds. Only momentary movements such as picking up the phone or turning your head can be accomplished using stored ATP.

As movement is continued or repeated, ATP must be replenished from available macronutrients—primarily carbohydrate (glycogen) and fat. This is when the real work of energy production for muscular activity begins. Energy can be generated to replenish ATP via anaerobic or aerobic pathways. Anaerobic metabolism is the production of energy without oxygen present. This pathway of energy production is also relatively short-lived. It can provide a very high-energy output but only for about 30 seconds. Anaerobic metabolism is the predominant pathway for short-term, high-intensity activities such as weight lifting and sprinting. Lactic acid is a byproduct (see the “Breaking It Down” box about lactic acid).

Breaking It Down
Lactic Acid— Updating the Conventional Wisdom

Among athletes and coaches, it is widely accepted that the buildup of lactic acid in the muscles leads to detrimental effects such as muscle pain, fatigue, and soreness. But scientists have been suspicious of the lactic acid–soreness connection for many years and finally have shown with certainty that it just isn't so. In fact, we now know that a number of the performance limitations and side effects ascribed to lactic acid are simply not true. Let's take a quick look at how the lactic acid story arose.

In the early 1900s, prominent British researchers W. M. Fletcher and F. G. Hopkins found that isolated muscle fibers (from a frog) accumulated large amounts of lactic acid as they fatigued (when stimulated electrically). Following exercise, as the lactic acid dissipated, the muscle fibers recovered and were soon ready to contract again. This finding supported the hypothesis that a buildup of lactic acid causes muscle acidosis, which diminishes force production and in turn leads to fatigue. Good scientists always consider the most obvious or simplest explanation first. And that is what happened in this case. Follow-up studies appeared to support the hypothesis, so this research was readily published and widely disseminated over the next half century.

With better techniques and high-tech tools, our understanding of exercise physiology and energy production has advanced enormously, particularly since the 1970s. George Brooks, Ph.D., a professor at the University of California–Berkeley, led the way in furthering our understanding about lactic acid. For example, his findings show that the body can efficiently use lactic acid as a source of fuel on par with carbohydrates stored in muscle (glycogen) and sugar in blood (glucose). In fact, it appears that lactic acid plays a role in linking the two metabolic cycles—the oxygen-based aerobic pathway and the oxygen-free anaerobic pathway—previously thought to be distinct.

So what is the bottom line on lactic acid? First, lactic acid can serve as an important fuel when athletes work out and compete; lactic acid has an important role in physiological adaptations due to training. Second, although lactic acid may be responsible for some of the discomfort of intense exercise, its role in muscle fatigue remains unclear. The evidence is mixed; some shows lactic acid may prevent fatigue, and some indicates just the opposite. Third, lactic acid does not cause muscle soreness; there is simply no evidence or plausible mechanism. The likely explanation—with substantial evidence to support it—is that the muscle soreness we experience a few days following exercise is due to cellular-level disruption to muscle fibers and surrounding tissue.

Interestingly, these scientific revelations about lactic acid had little impact on how coaches train athletes. Through trial and error, coaches know what works, and to a large degree, this coaching knowledge is independent of understanding the details of human physiology. Since these new scientific insights don't directly affect the actual training of athletes, the misconception persist. As with many aspects of human physiology and performance, the conventional wisdom is often a simplified version of early research and once established in popular thinking becomes difficult to revise or update. As with many questions of human physiology and performance, oversimplifications abound. Now you know the real story about lactic acid, so spread the word and illuminate your friends. Let's set the record straight about lactic acid—it is much more than just a byproduct of intense exercise.

Energy production in the presence of oxygen is called aerobic metabolism. This pathway can provide moderate energy output for many hours. Prolonged activities such as distance running, cycling, swimming, and other endurance sports rely on aerobic metabolism for continued energy production. Aerobic metabolism is dependent on the cardiorespiratory system to transport oxygen from the air we breathe to the working muscles, where energy for muscle contraction is released during the oxidation of carbohydrates and fats. Carbon dioxide is a byproduct. The oxygen transport and uptake processes are complex yet efficient.

Only a few types of exercise are purely anaerobic or aerobic. For example, performance in short-duration events such as a 40-yard sprint, high jump, or bench press is essentially completely dependent on the anaerobic pathway. In contrast, performance in events lasting several hours such as running a marathon (26.2 miles) or cycling 50 miles is almost totally dependent on the aerobic pathway, which in turn reflects the capacity of the cardiorespiratory system to transport oxygen to the active muscles. For competitions that are intermediate in duration, such as events lasting 1–10 minutes, both aerobic and anaerobic pathways contribute proportionately, with the relative significance of each depending on the duration. Table 4 presents the relative proportions of energy produced from aerobic and anaerobic pathways to perform events of different durations.

TABLE 4 CONTRIBUTIONS OF AEROBIC AND ANAEROBIC METABOLISM TO ENERGY OUTPUT DURING MAXIMAL EXERCISE OF DIFFERENT DURATIONS

Source: Adapted from Astrand PO, Rodahl, K. Textbook of Work Physiology. New York: McGraw-Hill, 1977.

Many physical activities and sports are “stop and go,” or intermittent in nature. Tennis is a good example. For these activities, both aerobic and anaerobic systems are called on to varying degrees throughout the game. With the other factors being similar, the more continuous the action in the sport, the greater the dependence on aerobic metabolism (and thus the cardiovascular system). For instance, soccer and basketball, although not pure aerobic activities, are relatively more aerobic than baseball or volleyball.

CARDIORESPIRATORY FUNCTION AND OXYGEN UPTAKE

Cardiorespiratory function refers to the integration of the heart, lungs, and circulation to transport oxygen in the blood to muscles and other tissues and to remove carbon dioxide, a byproduct of oxidation. Oxygen delivery to the working muscles is paramount to sustain physical activities. To understand the circulation of blood through the body, remember that arteries are vessels carrying blood away from the heart, and veins are vessels carrying blood to the heart (refer to Figure 5). As blood circulates through the lungs, hemoglobin in the red blood cells binds oxygen. The freshly oxygenated blood returns to the heart via the pulmonary veins and is pumped to the major parts of the body.

FIGURE 5 FLOW OF BLOOD THROUGH CIRCULATORY SYSTEM, WITH SITES OF O₂/CO₂ EXCHANGE

1. Blood rich in CO₂ is pumped from the heart into the lungs through the pulmonary arteries.
2. In the lungs, CO₂ in the blood is exchanged for O₂.
3. The O₂-rich blood is carried back to the heart through the pulmonary veins.
4. This oxygen-rich blood is then pumped from the heart through the aorta to the many tissues and organs of the body.
5. In the tissues, the arteries narrow to tiny capillaries. Here, O₂ in the blood is exchanged for CO₂.
6. The capillaries widen into veins, which carry the CO₂-rich blood back to the heart.

Source: Figure 7 from online tutorial Hemoglobin and the Heme Group. Used with permission of Kit Mao, Ph.D., Department of Chemistry, Washington University, St. Louis.

Arteries distribute the oxygen-rich blood to the working muscles, where oxygen is removed (or taken up, thus the term oxygen uptake) and used for aerobic metabolism. Carbon dioxide is produced, and the veins return the oxygen-poor, carbon dioxide-rich blood back to the heart and then to the lungs, where carbon dioxide is released in exhaled air and exchanged for oxygen. And the cycle repeats with hemoglobin binding oxygen.

The heart is a special type of muscle (cardiac muscle), and like skeletal muscle, it must have an adequate oxygen supply to continue its work. The coronary arteries branch off the aorta, the main artery from the heart, and back onto the heart's surface. They are responsible for delivering oxygen and nutrients to the heart muscle (myocardium). If these vessels become narrowed with fatty deposits (atherosclerosis), blood flow can become restricted. This can lead to chest pain and heart attack.

Each beat of the heart can be felt as a pulse wherever an artery is close to the surface of the body, such as at the wrist or neck. Heart rate is expressed as beats per minute. The amount of blood ejected with each contraction of the heart is the stroke volume and is expressed in milliliters per beat. The product of heart rate and stroke volume yields cardiac output, which is the rate of blood flow in liters per minute. Cardiac output is directly proportional to oxygen transport: the greater the rate at which blood is being circulated, the greater the delivery of oxygen to the working muscles.

Resistance training relies primarily on the anaerobic pathway, while sports such as running or cycling depend mostly on the aerobic pathway.

For most physical activities, the body responds in specific, predictable ways. As exercise begins—for example, going from sitting at your desk to walking across campus—the leg muscles need additional oxygen, so heart rate and stroke volume ramp up to deliver more blood. The elevation in heart rate is directly proportional to the demand for oxygen. Perhaps you are late for a class, so you walk quickly or jog. Again, your heart rate will increase proportionately as your exercise intensity increases. This is why heart rate is a widely used measure of exercise intensity.

In a training or competition, if the exercise is demanding and prolonged, heart rate will continue to increase until the person's aerobic capacity, or maximal oxygen uptake, is reached. For example, this is the case for runners, cyclists, and triathletes during the final stages of their races. Aerobic capacity refers to one's maximal ability to produce energy aerobically during exhaustive exercise. It can be measured in the lab via a graded exercise test on a treadmill or a stationary bike (cycle ergometer), or it can be estimated from a field test such as a 2-mile run. Aerobic capacity is the single best measure of cardiorespiratory fitness and is the main determinant of endurance performance. This concept plays a central role in individualizing exercise programs and will be discussed in more detail later in this chapter.

For exercise that is focused on musculoskeletal fitness, such as weight training or calisthenics, the demand on the cardiorespiratory system is much less pronounced. As mentioned earlier, the physiological demands of different types of exercise vary widely. At one end of the continuum is cross-country skiing—a total-body form of exercise involving multiple major muscle groups in the legs, trunk, and arms that requires sustained, high-energy output to move one's body weight over a long distance. This type of exercise is clearly aerobic.

At the other end of the continuum is a single-arm bicep curl in which a relatively small muscle is producing force over a distance of only a few feet per repetition. The energy demand on the single muscle can be very high but it is localized. Relative to the entire body, the energy demand to move a dumbbell a short distance is small and short-lived, since multiple repetitions take less than a minute. Consequently, muscular fitness training relies primarily on the anaerobic pathway. The contribution and interaction of aerobic and anaerobic systems vary depending on the duration, continuousness, and type of exercise.

ADAPTATIONS TO PHYSICAL TRAINING

The physiological and performance changes that occur with regular training are collectively termed the training effect. A primary adaptation to aerobic exercise is improved oxygen delivery to the muscles. At the level of the lungs, training enhances the exchange of oxygen and carbon dioxide at higher rates. Concurrently, as the heart strengthens, it can eject more blood with each beat. This increase in stroke volume results in the decrease in resting heart rate commonly observed after a few weeks of training. These cumulative pulmonary and cardiovascular changes account for the improved delivery of oxygen to the muscles.

Another fundamental adaptation that improves oxygen uptake and thus energy production takes place within the muscle cells themselves. Most of the cellular changes occur in the cell structure known as the mitochondrion, which is the actual site of aerobic metabolism—the conversion of food to usable energy in the presence of oxygen. Training causes the mitochondria to increase in size and number. The aerobic enzymes, catalysts in the aerobic pathway of energy production, also increase in quantity. Moreover, endurance is further improved by the enhanced ability of the muscles to use fats as fuel, sparing muscle carbohydrate stores (glycogen).

Resistance or weight training to improve musculoskeletal fitness can bring about significant increases in muscle size (hypertrophy), strength, muscular endurance, and flexibility. Improvements in muscle tone and force production are induced in several ways. A program of resistance training coupled with flexibility exercises improves blood flow to and neural control of muscle fibers, allowing for more efficient energy production, fiber recruitment, and muscle recovery. Over the same period, concentrations of anaerobic and/or aerobic enzymes are increased in the muscle cells, depending on the energy pathways being challenged. In addition, resistance/flexibility training maintains healthy joints and connective tissues such as tendons, ligaments, and cartilage, and it is protective against musculoskeletal injury.

A balanced exercise program develops or maintains both cardiorespiratory and musculoskeletal fitness. Such a program improves performance whether in specific fitness tests, sports competitions, or challenging physical tasks in daily life. A balanced exercise program also yields direct health benefits modulated through the body's multiple biological systems. Primary health benefits include

• improved metabolism (namely, normalized blood lipids and glucose) and lower risk for heart disease and diabetes

• improved body composition and weight control and lower risk for obesity

• maintenance of healthy bones, muscles, and joints and lower risk for osteoporosis, joint diseases, and low back pain

• improved psychological health and reduced risk for anxiety and depression

A dose-response relationship is evident for both cardiorespiratory and musculoskeletal exercise. That is, the more you exercise, the greater the improvements in health and fitness. If a pill could bring about this impressive combination of benefits, it would be hailed as a wonder drug. Moreover, regular physical activity has few unwanted side effects, and costs are easily minimized. A final point to remember about the training effect: improved fitness and associated health benefits are reversible and diminish with inactivity. We can't store up fitness. The key is to establish exercise as a habit.

Recommendations for Healthy Adults

The physical activity recommendations presented focus on the amounts and type of exercise known to improve health. These recommendations are based on the Physical Activity Guidelines for Americans, 2008 and earlier position from leading scientific and public health organizations including the American College of Sports Medicine, the American Heart Association, and the Centers for Disease Control and Prevention.

For those who lead a sedentary lifestyle, these guidelines provide a reasonable goal. For those who are fit and engage in more advanced training programs, these guidelines serve as a benchmark to check on how balanced your program is. You can glean tips to adjust your current program and gain insights on how to assist a friend in starting or maintaining a program.

First, we'll review the basic principles of exercise training as a prelude to presenting the physical activity recommendations for cardiorespiratory fitness followed by those for musculoskeletal fitness. Then we'll present the physical activity continuum to summarize the recommendations and to illustrate how they fit within the broad spectrum of physical activity levels.

PRINCIPLES OF EXERCISE TRAINING

Exercise training is based on four principles: overload, reversibility, specificity, and individual differences. Consider each principle when planning an exercise program.

Principle of Overload

To improve a physiological system, it must be exposed to a stimulus greater than it is normally accustomed to—such as a faster pace or a heavier weight. This is the fundamental principle upon which all exercise training is based. Repeated exposure over time to progressively greater amounts of exercise results in responsive changes by the lungs, heart, muscle, and connective tissue—structural and functional adaptations from the molecular to the system level. These adaptations include enhanced capacity and efficiency in cardiorespiratory and/or musculoskeletal function. This translates into improved fitness.

Principle of Reversibility

This principle is simply the converse of the principle of overload. When one stops exercising and the training overload is removed, the previously developed physiological systems will over time return to pre-training levels. To maintain fitness, one must continue to exercise. Fitness cannot be stored.

Principle of Specificity

Many training effects are specific to the type of exercise, the particular muscles involved, and the intensity. For example, if the goal is to run a 5-kilometer (5K) race at a seven-minute-per-mile pace, then the principle of specificity suggests that the person should focus on running and include training sessions at the goal pace. If all training is done at an eight- to nine-minute-per-mile pace, it will be difficult to shift gears on race day, because the body has not been exposed to the specific demands of running at the faster pace. Moreover, swimming and cycling, although excellent aerobic exercise, will do little to improve running performance because the muscles are used in different ways for each activity. Training specificity is critical for reaching performance goals.

Principle of Individual Differences

There is tremendous variability from one person to the next in both natural fitness level and in the rate of improvement that occurs with exercise training. Due to different genetic endowments, each of us begins at a different point on the fitness continuum. This is no different from variability among individuals in height or hair color. What is not as apparent though is the variability among individuals' responses to a similar training program. A standard exercise dose may be just right for many people but too hard for others and not enough for some. For best results, customize your exercise program as you learn how your body responds. The exercise recommendations that follow should be used as a guide. The fine-tuning is left up to each of us individually.

CARDIORESPIRATORY FITNESS

Cardiorespiratory fitness, also known as aerobic fitness or cardiorespiratory endurance, is the ability of the respiratory and circulatory systems to supply oxygen to the working muscles during sustained physical activity. For most people, being fit or in shape means having good cardiorespiratory fitness. Due to the potent beneficial effect that cardiorespiratory fitness has on lowering chronic disease risk, particularly heart disease, many scientists believe this fitness component is the most important.

The FITT Acronym

Four factors constitute an exercise plan: frequency, intensity, time (duration), and type. Conveniently, the first letters of these four factors form the acronym FITT, which is an easy way to remember them. Recommendations for cardiorespiratory fitness follow.

Frequency: How Often?

The recommended exercise frequency is at least three and up to seven days per week. One should consider frequency relative to both intensity and duration. For example, those who enjoy vigorous exercise should consider training every other day, while those who prefer moderate intensity activity could probably exercise most days of the week (e.g., hard running 3–4 days/week vs. brisk walking 5–6 days/week).

Intensity: How Hard Do I Have to Push Myself?

Exercise intensity should be within 50–85 percent of aerobic capacity, with moderate intensity defined as 50–59 percent and vigorous intensity as 60–85 percent. For most young adults, a moderate-intensity activity is similar to brisk walking (about 4 mi/hr), and a vigorous-intensity activity is like jogging/running. (A lower threshold of 40 percent of aerobic capacity may be appropriate for older, chronically sedentary people.)

Heart Rate (HR) as a Measure of Intensity.

Since oxygen uptake and heart rate are highly correlated, heart rate can be used to gauge intensity. There are several approaches to using heart rate to calculate training zones (ranges). Although it is common to take a straight percentage of a person's estimated maximal heart rate, we prefer the heart rate reserve method because it takes into account resting heart rate and is directly proportional to increases in oxygen uptake (energy expenditure). The heart rate reserve is simply the difference between the resting heart rate (HR_rest) and the maximal heart rate (HR_max). Maximal heart rate is estimated by subtracting age from 220. To calculate the training heart rate range, determine the lower and upper limits as follows:

For example, the heart rate training range (from 50 to 85 percent intensity) for a 20-year-old with a resting heart rate of 70 would be 135–181 beats per minute. As this is a large range, the next step would be to decide whether to train in the moderate or vigorous part of the range. The training heart rate zone for different ages is plotted in Figure 6.

FIGURE 6
TRAINING HEART RATE ZONE BY AGE, USING THE HEART RATE RESERVE METHOD

Source: David C. Nieman. Exercise Testing and Prescription, 5th ed. New York: McGraw-Hill, 2003.

How to Measure Heart Rate.

Measuring heart rate is easy to learn. Take your pulse on the inside of the wrist (at the radial artery) by placing the tips of your first two fingers about an inch below the base of the thumb. When you feel the pulse, count the beats for 10 seconds and then multiply the 10-second count by six to determine your heart rate in beats per minute. To measure your resting heart rate, check your pulse after sitting quietly for at least 10 minutes or upon waking in the morning. To estimate your exercise heart rate, count your pulse immediately after exercise. Take time to practice so you can reliably measure the higher heart rates. An option is to use a heart rate monitor that displays heart rate continuously. Heart rate monitors that consist of a chest strap (transmitter) and watch (receiver) are reliable, durable, and relatively inexpensive.

Perceived Exertion as a Measure of Intensity.

Another approach for monitoring intensity is to use the rating of perceived exertion (RPE) scale developed by Professor Gunnar Borg. This scale links a person's subjective rating of physical exertion to a number between 6 and 20 using verbal descriptors. For example, if the exercise effort feels “very light,” the rating would be a 9; “somewhat hard” would be a 13; and “extremely hard” a 19. The RPE scale correlates well with heart rate and is commonly used during exercise testing and exercise class. The scale is useful in teaching people how hard or easy to work to achieve an appropriate exercise intensity. For most young adults, ratings from 12 to 14 correspond to about 50–60 percent of the heart rate training range (HR reserve method), and ratings from 15 to18 correspond to 60–85 percent.

TABLE 7
RATING OF PERCEIVED EXERTION: BORG RPE SCALE®

© Gunnar Borg, 1970, 1985, 1998© Gunnar Borg, 1985, 1998, 2005

Exercise intensity is arguably the most important factor in determining the rate of improvement. For motivated people, it's tempting to work out very hard (at high intensities) from the onset of an exercise program with the aim of improving quickly. Yet, exercising too hard without appropriate transition time to build up is generally counterproductive. High-intensity starter programs often lead to prolonged fatigue, extreme soreness, and an increased risk of injury. Forget the crash programs and limit the intensity the first few weeks of a new program.

Time: How Long?

Consensus guidelines recommend engaging in a minimum duration of 20 or 30 minutes depending on intensity. Intensity and duration generally balance each other—the higher the intensity, the shorter the duration, and vice versa. For moderate-intensity aerobic activities like brisk walking, the recommended duration is at least 30 minutes. Durations as short as 10 minutes can be accumulated throughout the day to reach the 30-minute minimum. For vigorous-intensity aerobic activities like running, the duration should be at least 20 minutes. Because of the dose-response relationship between physical activity and health, more is better: 45–60 minutes is preferable and 60–90 minutes may be necessary for weight control (refer to Chapter 2).

Type: Which Activities?

The type or mode of activity should stress the cardiorespiratory, or aerobic, system. Such activities use major muscle groups, are rhythmic, and can be done continuously. The most common aerobic activities are walking, running, cycling, and swimming, but many other activities meet the criteria as well. Most important though is to select an activity that you like. If the activity is not fun or enjoyable, it's unlikely that you will continue. A word of caution for beginning exercisers or older people: avoid high-impact activities like running and aerobic dance as initial choices because of their higher injury rates.

MUSCULOSKELETAL FITNESS

Musculoskeletal fitness, or muscular fitness, is composed of three intertwined elements: muscular strength, muscular endurance, and flexibility. Muscular strength is a muscle or muscle group's maximal capacity to exert force against an external resistance—that is, the most weight you can lift one time (e.g., one-repetition maximum, 1RM), such as with the arm curl for the biceps muscle or the leg extension for the quadriceps. Muscular endurance is slightly different. It refers to the muscle's ability to sustain a submaximal force or to persist at some relative level—for example, the number of repetitions you can complete at a resistance equal to 50 percent of 1RM. Flexibility is the functional range of motion in a joint or group of joints. Flexibility varies from joint to joint and depends on the muscles, tendons, and ligaments at the involved joint or group of joints.

Musculoskeletal fitness is demonstrated in nearly all sports. To varying degrees during a competition, muscles are called on to exert peak force, resist fatigue, and perform well through multiple ranges of motion. Musculoskeletal fitness is certainly related to our health status, too. It enables us to perform demanding physical tasks—both routine and unplanned—that arise in everyday situations as well as helping to combat osteoporosis, low back pain, joint disease, loss of mobility, and frailty.

To develop musculoskeletal fitness, we must engage in some form of resistance training. Although it is better known as strength training or weight training, resistance training is the preferred term for several reasons. First, strength and muscular endurance are usually developed in combination. Second, “resistance” can be applied to the muscles using iron weights (dumbbells, barbells, weight stacks), but it can also be accomplished by using muscle and movement-specific equipment with pneumatic, hydraulic, and computer-controlled resistance, as well as simply body weight (e.g., calisthenics—sit-ups, push-ups, pull-ups). Resistance training coupled with appropriate attention to flexibility exercises is the basis for improving musculoskeletal fitness.

Compared to the hundreds of scientific studies on cardiorespiratory fitness, far fewer have been conducted to determine what constitutes a sufficient (health-promoting) exercise dose for developing strength and muscular endurance. Yet, enough solid research is available to provide guidance with confidence. An adequate evidence base for recommending flexibility exercises is another matter though. Although stretching exercises have proven effectiveness in regaining range of motion during rehabilitation from musculoskeletal injury or surgery, there is little quality research on the health benefits of flexibility for people with no physical limitations. Among the many official statements on physical activity recommendations from scientific organizations, only the American College of Sports Medicine provides commentary and suggestions on flexibility. Although of less overall importance than either aerobic or resistance training, flexibility training is certainly important as a complementary component and should be included as part of a balanced program of physical activity.

Using the FITT acronym's categories, we present recommendations for achieving a healthy level of musculoskeletal fitness.

Frequency: How Often?

The recommended training frequency for resistance training is two or more nonconsecutive days per week. Training three days per week will result in greater benefits. The standard practice is to allow two days of rest between resistance training sessions to give muscles time to recover and adapt (e.g., workout on Monday and Wednesday).

Intensity: How Hard Do I Have To Work?

Intensity is a function of the resistance (e.g., amount of weight lifted). The recommendation is to complete at least one set of 8–12 repetitions of 8 to 10 exercises that condition the major muscle groups. Through trial and error, you will learn to select the proper resistance for each exercise—that is, a weight that results in substantial muscle fatigue during the final 1–2 repetitions of the set. In Table 8, we provide a sample program of 10 basic exercises.

TABLE 8
STANDARD RESISTANCE EXERCISES AND ASSOCIATED MUSCLE GROUPS

Time: How Long?

As a practical matter, the time or duration is the length of the entire workout session, which includes the actual time exercising (e.g., lifting weights) and the recovery time between exercises. A duration of 20–30 minutes is a reasonable estimate for the total time needed to complete one set of 8–10 exercises.

Type: Which Activities?

In contrast to training for cardiorespiratory fitness, the options for improving or maintaining muscular fitness are more limited. Such training must involve resistance exercise whether it is traditional weight training, calisthenics, or newer variations of resistance-based exercise. Regardless of the type of equipment or specific program, the overall goal is the same: to condition all the major muscles, not just a couple specific muscles. Web resources at the end of the chapter provide access to information on a variety of resistance exercises and programs.

To maintain range of motion with the involved joints, flexibility exercises should be included as part of a musculoskeletal training program. Although flexibility training per se is not included in the physical activity position statements of all organizations, it is an accepted component for individual exercise plans. Stretching should focus on muscle/tendons groups at major joints (e.g., ankle, knee, hips, back, neck, shoulder, elbow, wrist). Stretches should be slow and steady (static), maintained at a position of mild discomfort for 15–30 seconds, and repeated three or four times. Refer to the Web resources at the end of the chapter for descriptions and illustrations of flexibility exercises.

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