Therapeutic Exercise

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Chapter 18 Therapeutic Exercise

General Principles

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.

Energy Systems

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:

image

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

Rapid Glycolysis (Lactic Acid System)

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

Aerobic Oxidation System

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 imageO2max, 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 (imageO2max).26,103,108

Cardiovascular Exercise

Cardiac Function

Effects of Exercise Training

Cardiovascular System

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).

BOX 18-1 Effects of Regular Exercise on Cardiovascular Activity

BOX 18-2 Physiologic Changes After a Regular Exercise Program

ACSM Recommendations for Cardiorespiratory Endurance Training

Intensity

Calculating Intensity

Because of limitations in using imageO2 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 Methods

Heart rate is used as a guide to set exercise intensity, because of the relatively linear relationship between HR and percentage of imageO2max. 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

Medical Clearance

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

Preexercise Evaluation

A preexercise evaluation by a physician is more comprehensive and should include a patient history and a determination of whether the patient needs an exercise stress test.

Identification of Those Who Need an Exercise Stress Test

Indications for an exercise stress test according to the American College of Cardiology and American Heart Association are as follows114:

The ACSM guidelines are summarized in Box 18-5 and Tables 18-2 and 18-3.78,121 Contraindications to exercise testing are listed in Box 18-6.

Table 18.3 Coronary Artery Disease Risk Factor Thresholds for Use With ACSM Risk Stratification

Risk Factors Defining Criteria
Positive
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
Negative
High serum HDL cholesterol >60 mg/dL (1.6 mmoL/L)

Professional opinions vary regarding the most appropriate markers and thresholds for obesity; therefore exercise professionals should use clinical judgment when evaluating this risk factor.