Chapter 18 Therapeutic Exercise
Regular physical activity is an important component of a healthy lifestyle. Increases in physical activity and cardiorespiratory fitness have been shown to reduce the risk for death from coronary heart disease as well as from all causes. The primary focus on achieving these health-related goals in the past has been on prescribing exercise to improve cardiorespiratory fitness, body composition, and strength. More recently the Centers for Disease Control and Prevention (CDC) and the American College of Sports Medicine (ACSM) suggested that the focus be broadened to address the needs of more sedentary individuals, especially those who cannot or will not engage in structured exercise programs. There is increasing evidence showing that regular participation in moderate-intensity physical activity is associated with health benefits, even when aerobic fitness remains unchanged. To reflect this evidence, the CDC and ACSM are now recommending that every adult in the United States accumulate 30 minutes or more of moderate-intensity physical activity on most, and preferably all, days of the week. Those who follow these recommendations can experience many of the health-related benefits of physical activity, and if they are interested are ready to achieve higher levels of fitness.44,45,108,121
Important in prescribing exercise is an understanding of the principles of specificity and periodization. The principle of specificity states that metabolic responses to exercise occur most specifically in those muscle groups being used. Furthermore, the types of adaptation will be reflective of the mode and intensity of exercise. The principle of periodization reflects the importance of incorporating adequate rest to accompany harder training bouts. Overall training programs (macrocycles) are divided into phases (microcycles), each with specific desired effects (i.e., enhancing a particular energy system or sport-specific goal).
This chapter provides a brief overview of the basic fundamentals of exercise physiology, including the metabolic energy systems, and the basic muscle and cardiorespiratory physiology associated with exercise. It will then provide an overview of the exercise prescription according to the current ACSM guidelines, and the fundamentals of exercise programming, including preexercise screening.
A 70-kg human has an energy expenditure at rest of about 1.2 kcal/min, with less than 20% of the resting energy expenditure attributed to skeletal muscle. During intense exercise, however, total energy expenditure can increase 15 to 25 times above resting values, resulting in a caloric expenditure between 18 and 30 kcal/min. Most of this increase is used to provide energy to the exercising muscles that can increase energy requirements by a factor of 200.26,103
The energy used to fuel biologic processes comes from the breakdown of adenosine triphosphate (ATP), specifically from the chemical energy stored in the bonds of the last two phosphates of the ATP molecules. When work is performed, the bond between the last two phosphates is broken, producing energy and heat:
The limited stores of ATP in skeletal muscles can fuel approximately 5 to 10 seconds of high-intensity work (Figure 18-1). ATP must be continuously resynthesized from adenosine diphosphate (ADP) to allow exercise to continue.70,114 Muscle fibers contain three metabolic pathways for producing ATP: the creatine phosphate system, rapid glycolysis, and aerobic oxidation.26,103,108
FIGURE 18-1 Energy sources in relation to duration of contraction. Muscular metabolism available from the various substrates participating in supplying energy during the first 2 minutes of an attempted maximal contraction. The relative contribution of each substrate at any moment is indicated. The intensity of metabolic activity over the 2-minute period is adjusted to the change of the isometric tension produced during a sustained voluntary maximal contraction.
(Redrawn from DeLateur BJ: Therapeutic exercise to develop strength and endurance. In: Kottke FJ, Stillwell GK, Lehmann JF, editors: Krusen’s handbook of physical medicine and rehabilitation, Philadelphia, 1982, Saunders, with permission.)
When limited stores of ATP are nearly depleted during high-intensity exercise (5 to 10 seconds), the creatine phosphate system transfers a high-energy phosphate from creatine phosphate to rephosphorylate ATP from ADP:
Because it involves a single reaction, this system can provide ATP at a very rapid rate. Because there is a limited supply of creatine phosphate in the muscle, however, the amount of ATP that can be produced is also limited.
There is enough creatine phosphate stored in skeletal muscle for approximately 25 seconds of high-intensity work (see Figure 18-1). The ATP–creatine phosphate system lasts for about 30 seconds (5 seconds for the stored ATP and 25 seconds for creatine phosphate). This provides energy for activities such as sprinting and weightlifting. The creatine phosphate system is considered an anaerobic system, because oxygen is not required.26,103,108
Glycolysis uses carbohydrates primarily in the form of muscle glycogen as a fuel source. When glycolysis is rapid, the pathways that normally use oxygen to make energy are circumvented in favor of other, faster yet less efficient paths that do not require oxygen. As a result, only a small amount of ATP is produced anaerobically, and lactic acid is produced as a by-product of the reaction.
For many years, lactic acid was considered to be the waste product caused by inadequate oxygen supply. Lactic acid limited physical activity by building up in muscles and leading to fatigue and diminished performance. Since the early 1980s, there has been a fundamental change in thought, and evidence now shows that a limited oxygen supply is not required for lactic acid production. Lactate is produced and used continuously under fully aerobic conditions. This is referred to as the cell-to-cell lactate shuttle in which lactate serves as a metabolic intermediate tying together glycolysis (as an end product) and oxidative metabolism.
Once lactic acid is formed, there are two possible venues it can take. The first involves conversion into pyruvic acid and subsequently into energy (ATP) under aerobic conditions (see “Aerobic Oxidation System” section below). The second involves hepatic gluconeogenesis using lactate to produce glucose, which is known as the Cori cycle.
Anaerobic oxidation starts as soon as high-intensity exercise begins and dominates for approximately 1½ to 2 minutes (see Figure 18-1). It would fuel activities such as middle-distance sprints (400-, 600-, and 800-m runs) or events requiring sudden bursts of energy such as weightlifting.
Although glycolysis is considered an anaerobic pathway, it can readily participate in the aerobic metabolism when oxygen is available and is considered the first step in the aerobic metabolism of carbohydrates.26,103,108
The final metabolic pathway for ATP production combines two complex metabolic processes: the Krebs cycle and the electron transport chain. The aerobic oxydation system resides in the mitochondria. It is capable of using carbohydrates, fat, and small amounts of protein to produce energy (ATP) during exercise, through a process called oxidative phosphorylation. During exercise, this pathway uses oxygen to completely metabolize the carbohydrates to produce energy (ATP), leaving only carbon dioxide and water as byproducts. The aerobic oxidation system is complex and requires 2 to 3 minutes to adjust to a change in exercise intensity (see Figure 18-1). It has an almost unlimited ability to regenerate ATP, however, limited only by the amount of fuel and oxygen that is available to the cell. Maximal oxygen consumption, also known as O2max, is a measure of the power of the aerobic energy system, and is generally regarded as the best indicator of aerobic fitness.26,103,108
All the energy-producing pathways are active during most types of exercise, but different exercise types place greater demands on different pathways. The contribution of the anaerobic pathways (creatine phosphate system and glycolysis) to exercise energy metabolism is inversely related to the duration and intensity of the activity. The shorter and more intense the activity, the greater the contribution of anaerobic energy production, whereas the longer the activity and the lower the intensity, the greater the contribution of aerobic energy production. In general, carbohydrates are used as the primary fuel at the onset of exercise and during high-intensity work. But during prolonged exercise of low to moderate intensity (longer than 30 minutes), a gradual shift from carbohydrate toward an increasing reliance on fat as a substrate occurs. The greatest amount of fat use occurs at about 60% of maximal aerobic capacity (O2max).26,103,108
The cardiorespiratory system consists of the heart, lungs, and blood vessels. The purpose of this system is the delivery of oxygen and nutrients to the cells, as well as the removal of metabolic waste products to maintain the internal equilibrium.70,103,108
Normal resting heart rate (HR) is approximately 60 to 80 beats/min. HR increases in a linear fashion with the work rate and oxygen uptake during exercise. The magnitude of HR response is related to age, body position, fitness, type of activity, the presence of heart disease, medications, blood volume, and environmental factors such as temperature and humidity. HR during maximal exercise can exceed 200 beats/min, depending on the person’s age and training state. With the onset of dynamic exercise, HR increases in proportion to the relative workload. The maximal HR (HRmax) decreases with age, and can be estimated in healthy men and women by using the following formula: HRmax = 220 – age. There is considerable variability in this estimation for any fixed age, with a standard deviation of ±10 beats/min.70,103,108
Stroke volume (SV) is the amount of blood ejected from the left ventricle in a single beat. SV is equal to the difference between end-diastolic volume and end-systolic volume. Greater diastolic filling (preload) will increase SV. Factors that resist ventricular outflow (afterload) will result in a reduced SV.
SV is greater in males than in females. At rest in the upright position, it generally ranges from 60 to 100 mL/beat, while maximum SV approximates 100 to 120 mL/beat. During exercise, SV increases curvilinearly with the work rate until it reaches near maximum at a level equivalent to approximately 50% of aerobic capacity. SV starts to plateau, and further increases in workload do not result in increased SV, primarily because of reduced filling time during diastole.
SV is also affected by body position, with SV being greater in the supine or prone position and lower in the upright position. Static exercise (weight training) can also cause a slight decrease in SV because of increased intrathoracic pressure.70,103,108
Resting cardiac output in both trained and sedentary individuals is approximately 4 to 5 L/min, but during exercise the maximal cardiac output can reach 20 L/min. Maximal cardiac output in an individual depends on many factors, including age, posture, body size, presence of cardiac disease, and physical conditioning. During dynamic exercise, cardiac output initially increases with increasing exercise intensity by increases in SV and HR. Increases in cardiac output initially beyond 40% to 50% of O2max, however, are accounted for only by increases in HR.70,103,108
At rest, 15% to 20% of the cardiac output is distributed to the skeletal muscles, with the remainder going to visceral organs, the brain, and the heart. During exercise, 85% to 90% of the cardiac output is selectively delivered to working muscles and shunted away from the skin and splanchnic vasculature. Myocardial blood flow can increase four to five times with exercise, whereas blood supply to the brain is maintained at resting levels. The difference between the oxygen content of arterial blood and the oxygen content of venous blood is termed the arteriovenous oxygen difference. It reflects the oxygen extracted from arterial blood by the tissues. The oxygen extraction at rest is approximately 25%, but at maximal exercise the oxygen extraction can reach 75%.70,103,108
Blood pressure is the driving force behind blood flow. Systolic blood pressure (SBP) is the maximal force of the blood against the walls of the arteries when cardiac muscle is contracting (systole). Normal resting SBP is less than 130 mm Hg. Diastolic blood pressure (DBP) is the force of the blood against the walls of the arteries when the heart is relaxing (diastole). Normal resting DBP is less than 85 mm Hg.103
SBP increases linearly with increasing work intensity, at 8 to 12 mm Hg per metabolic equivalent (MET) (where 1 MET = 3.5 mL of O2 per kilogram per minute). Maximal values typically reach 190 to 220 mm Hg. Maximal SBP should not be greater than 260 mm Hg. DBP remains unchanged or only slightly increases with exercise.70,103
Because blood pressure is directly related to cardiac output and peripheral vascular resistance, it provides a noninvasive way to monitor the inotropic performance (pumping capacity) of the heart. Failure of SBP to rise, decreased SBP with increasing work rates, or a significant increase in DBP are all abnormal responses to exercise, and indicate either severe exercise intolerance or underlying cardiovascular disease.70,103,108
In the supine position, gravity has less effect on return of blood to the heart, so the SBP is lower. When the body is upright, gravity works against the return of blood to the heart, so SBP increases. DBP does not change significantly with body position in healthy individuals.48,101,108
At similar oxygen consumptions, HR, SBP, and DBP are higher during arm work than during leg work. This is primarily because the total muscle mass in the arms is smaller, and consequently a greater percentage of the available mass is recruited to perform the work. In addition, arm work is less mechanically efficient than leg work.70,103,108
Pulmonary ventilation (e) is the volume of air exchanged per minute, and generally is approximately 6 L/min at rest in an average sedentary adult man. However, at maximal exercise, e increases 15- to 25-fold over resting values. During mild to moderate exercise, e increases primarily by increasing tidal volume, but during vigorous activity it increases by increasing the respiratory rate.48,101
Increases in e are generally directly proportional to an increase in oxygen consumption (O2) and carbon dioxide that is produced (CO2). At a critical exercise intensity (usually 47% to 64% of the O2max in healthy untrained individuals and 70% to 90% O2max in highly trained individuals), however, e increases disproportionately relative to the O2 (paralleling an abrupt increase in serum lactate and CO2). This is called the anaerobic (ventilatory) threshold.48,101,108
The anaerobic threshold signifies the onset of metabolic acidosis during exercise, and traditionally has been determined by serial measurements of blood lactate. It can be noninvasively determined by assessment of expired gases during exercise testing, specifically e and carbon dioxide production (CO2). The anaerobic threshold signifies the peak work rate or oxygen consumption at which the energy demands exceed the circulatory ability to sustain aerobic metabolism.48,101,108
The most widely recognized measure of cardiopulmonary fitness is the aerobic capacity, or O2max. This variable is defined physiologically as the highest rate of oxygen transport and use that can be achieved at maximal physical exertion.
The resting oxygen consumption of an individual (250 mL/min) divided by body weight (70 kg) gives the resting energy requirement, 1 MET (about 3.5 mL/kg per minute). Multiples of this value are used to quantify levels of energy expenditure. For example, running a 6-mph pace requires 10 times the resting energy expenditure, giving an aerobic cost of 10 METs, or 35 mL/kg per minute. Because there is little variation in HRmax and maximal systemic arteriovenous oxygen difference with physical training, O2max virtually defines the pumping capacity of the heart. When expressed as milliliters of oxygen per kilogram of body weight per minute (mL/kg per minute) or in METs, it is considered the best index of physical work capacity or cardiorespiratory fitness.48,101,108
The oxygen pulse (mL/beat) is the ratio of O2 (mL/min) to HR (beats/min), when both measures are obtained simultaneously. Oxygen pulse increases with increasing work effort. A low value during exercise indicates an excessive HR for workload and can be an indicator of heart disease.29
The respiratory quotient (RQ) is the ratio of CO2 produced by cellular metabolism to O2 used by tissues. It quantifies the relative amounts of carbohydrate and fatty acids being oxidized for energy. An RQ of 0.7 implies dependence on free fatty acids. An RQ of 1.0 indicates dependence on carbohydrate. The RQ does not exceed 1.0.
The respiratory exchange ratio (RER) reflects pulmonary exchange of CO2 and O2 at rest and during exercise. The RER also ranges between 0.7 and 1.0 during rest, and can also reflect substrate preference. During strenuous exercise, however, the RER can exceed 1.0 because of increasing metabolic activity not matched by O2 and additional CO2 derived from bicarbonate buffering of lactic acid. The terms RQ and RER are often used interchangeably, but their distinction is important.29
The effects of regular exercise on cardiovascular activity can be grouped into changes that occur at rest, during submaximal exercise, and during maximal work (Box 18-1).103,108 Regular exercise can also affect a number of physiologic parameters (Box 18-2).
The changes induced by regular exercise training generally are lost after 4 to 8 weeks of detraining. If training is re-established, the rate at which the training effects occur does not appear to be faster.103,108
Overtraining fatigue syndrome presents as a prolonged decreased sport-specific performance, usually lasting greater than 2 weeks. It is characterized by premature fatigability, emotional and mood changes, lack of motivation, infections, and overuse injuries. Recovery is markedly longer and variable among affected athletes, sometimes taking months before the athlete returns to baseline performance (Box 18-3).103,108
Exercise prescriptions are designed to enhance physical fitness, promote health by reducing risk factors for chronic disease, and ensure safety during exercise participation. The fundamental objective of the prescription is to bring about a change in personal health behavior to include habitual physical activity. The optimal exercise prescription for an individual is determined from an objective evaluation of that individual’s response to exercise, including observations of HR, blood pressure, rating of perceived exertion (RPE) to exercise, electrocardiogram when appropriate, and O2max measured directly or estimated during a graded exercise test. The exercise prescription should be developed with careful consideration of the individual’s health status, medications, risk factor profile, behavioral characteristics, personal goals, and exercise preferences.45,108,121
These five essential components apply when developing an exercise prescription for persons of all ages and fitness levels. Each component of fitness (e.g., cardiorespiratory endurance, muscular strength and endurance, and flexibility) has its own specific exercise prescription associated with it. The following section reviews the ACSM guidelines for each component of fitness.
Cardiorespiratory endurance is the ability to take in, deliver, and use oxygen. It is dependent on the function of the cardiorespiratory system (heart and lungs) and the cellular metabolic capacities. The degree of improvement that can be expected in cardiorespiratory fitness is directly related to the frequency, intensity, duration, and mode of exercise. O2max can increase between 5% and 30% with training. It has become apparent recently, however, that the level of physical activity necessary to achieve the majority of health benefits is less than that needed to attain a high level of cardiorespiratory fitness.45,108,121
Because of limitations in using O2 calculations for prescribing intensity, the most common methods of setting the intensity of exercise to improve or maintain cardiorespiratory fitness use HR and RPE.45,100,108
Heart rate is used as a guide to set exercise intensity, because of the relatively linear relationship between HR and percentage of O2max. It is best to measure HRmax during a progressive exercise test whenever possible, because HRmax declines with age. HRmax can be estimated by using the following equation: HRmax = 220 – age. This estimation has significant variance, with a standard deviation of 10 beats/min.45,100,108,121
One of the oldest methods of setting the target HR range uses a straight percentage of the HRmax. Using 70% to 85% of an individual’s HRmax approximates 55% to 75% of O2max and provides the stimulus needed to improve or maintain cardiorespiratory fitness.45,100,108,121 For example, if the HRmax is 180 beats/min, then the target HR (70% to 85% of HRmax) would range from 126 to 153 beats/min. (See also Chapter 33.)
The HR reserve (HRR) method is also known as the Karvonen method. In this method the target range is calculated as follows: subtract standing resting HR (HRrest) from HRmax to obtain HRR. Calculate 50% and 85% of the HRR. Add each of these values to the HRrest to obtain the target range. Therefore the target range is as follows:
The small but systematic differences between the two HR methods occur because the percentage HRmax is 55% to 75% of O2max, whereas the percentage HRRmax is 60% to 80% of O2max. Either method can be used to approximate the range of exercise intensities known to increase or maintain cardiorespiratory fitness or O2max.45,108,121
The RPE is a subjective grading of how hard individuals feel they are exercising. Use of RPE is considered an adjunct to monitoring HR. It has proven to be a valuable aid in prescribing exercise for individuals who have difficulty with HR palpation, and in cases where the HR response to exercise may have been altered because of a change in medication. The most commonly used scale of perceived exertion is the Borg Scale (see Table 18-1). The average RPE range associated with physiologic adaptation to exercise is 13 to 16 (“somewhat hard” to “hard”) on the Borg Scale category. One should suit the RPE to the individual on a specific mode of exercise, and not expect an exact matching of the RPE to a percentage HRmax or percentage HRR. It should be used only as a guideline in setting the exercise intensity.45,94,100,108,121
The appropriate exercise intensity is one that is safe and compatible with a long-term active lifestyle for that individual and achieves the desired outcome given the time constraints of the exercise session. The ACSM recommends an intensity that will elicit an RPE within a range of 12 to 16 on the original 6 to 20 Borg Scale (Table 18-1).
|7||Very, very light|
|19||Very, very hard|
Exercise training might not be appropriate for everyone. Patients whose adaptive reserves are severely limited by disease processes might not be able to adapt to or benefit from exercise. In this small subpopulation of people with severe or unstable cardiac, respiratory, metabolic, systemic, or musculoskeletal disease, exercise programming can be fatal, injurious, or simply not beneficial, depending on the clinical status and condition of the individual.46,121
The recommended level of screening before beginning or increasing an exercise program depends on the risk for the individual and the intensity of the planned physical activity. For individuals planning to engage in low- to moderate-intensity activities, the Physical Activities Readiness Questionnaire (PAR-Q) (Box 18-4) should be considered the minimal level of screening. The PAR-Q was designed to identify the small number of adults for whom physical activity might be inappropriate or those who should receive medical advice concerning the most suitable type of activity.46,108,121
From Kenny W, Humphrey R, Bryant C: ACSM’s guidelines for exercise testing and prescription, ed 5, Philadelphia, 1995, Williams & Wilkins, 1995, with permission of Williams & Wilkins.
|Risk Factors||Defining Criteria|
|Family history||Myocardial infarction, coronary revascularization, or sudden death before 55 years of age in father or other male first-degree relative (i.e., brother or son) or before 65 years of age in mother or other female first-degree relative(i.e., sister or daughter)|
|Cigarette smoking||Current cigarette smoker or those who quit within the previous 6 months|
|Hypertension||Systolic blood pressure of ≥140 mm Hg or diastolic ≥90 mm Hg, confirmed by measurements on at least two separate occasions, or on antihypertensive medication|
|Hypercholesterolemia||Total serum cholesterol of >200 mg dL (5.2 mmoL/L) or high-density lipoprotein cholesterol of <35 mg/dL (0.9 mmoL/L) or on lipid-lowering medication. If low-density lipoprotein cholesterol is available, use >130 mg/dL (3.4 mmoL/L) rather than total cholesterol of >200 mg/dL|
|Impaired fasting glucose||Fasting blood glucose of ≥110 mg/dL (6.1 mmoL/L) confirmed by measurements on at least two separate occasions|
|Obesity∗||Body mass index of ≥30 kg · m–2, or waist girth of >100cm|
|Sedentary lifestyle||Persons not participating in a regular exercise program or meeting the minimal physical activity recommendations in the U.S. Suregeon General’s report|
|High serum HDL cholesterol†||>60 mg/dL (1.6 mmoL/L)|