Chapter 15. Improving Cardiovascular Endurance

At the completion of this chapter, the reader will be able to:

1. Explain the importance of cardiovascular endurance in the overall health of an individual.

2. Describe the physiology of the cardiovascular system.

3. Describe the various types of energy and how each is generated.

4. Explain how the various muscle types are involved with endurance and aerobic activities.

5. Outline the precautions with aerobic conditioning.

6. Discuss the various methods by which aerobic conditioning can be enhanced through a rehabilitation program.

Physical activity has been defined as “any bodily movement produced by skeletal muscles that results in energy expenditure.”1 When a person undertakes work or exercise, a number of body systems adapt to the demands of the required tasks, particularly the cardiorespiratory and neuromuscular systems.2 The maximum work capacity of the cardiorespiratory system is a factor of the maximal amount of oxygen that can be taken in and used by the body, or VO2max, whereas the capacity of the neuromuscular system is a factor of the maximum tension that can be developed by the working muscle, or muscles—the maximal voluntary contraction. Assessment of the cardiovascular system provides the clinician with the justification for monitoring or not monitoring activities during a patient's rehabilitation or providing modifications in the exercise prescription.3

Whenever possible, the clinician should address the impact on the patient resulting from the loss of physical activity. This loss of activity affects both the cardiovascular and the musculoskeletal systems and can occur very rapidly. Thus, it is important that the rehabilitation program includes exercises that maintain, or improve, the patient's cardiovascular endurance, while monitoring safety concerns. If our sedentary society is to change to one that is more physically active, clinicians must play their role in communicating to their patients the amounts and types of physical activity that are needed to prevent disease and promote health, because patients respect their advice.4,5 Patients should be routinely counseled to adopt and maintain regular physical activity. Inadequate reimbursement, limited knowledge of the benefits of physical activity, and a lack of training in community physical activity counseling are barriers to achieving these goals. While policy makers work to improve reimbursement for preventive services, clinicians should develop effective ways to teach physical activity counseling.5 The personal physical activity practices of health professionals should not be overlooked. Health professionals should be physically active, not only to benefit their own health but also to make more credible their endorsement of an active lifestyle.5

According to the Department of Health and Human Services, physical fitness is a set of attributes a person has in regards to his/her ability to perform physical activities that require aerobic fitness, endurance, strength, or flexibility and is determined by a combination of regular activity and genetically inherited ability.6 Although commonly associated with the state of the cardiorespiratory system, which includes the ability to perform work or participate in activity over time using the body's oxygen uptake, delivery, and energy release mechanisms,7 physical fitness also encompasses a number of attributes including:8

• Muscle strength, which is the ability of muscles to exert or resist a force.
• Muscle endurance, which is the ability of the muscle to perform work.
• Muscle power, which is the ability of a muscle to exert high force at high speed.
• Balance, which is the ability to maintain equilibrium when the body is static or moving.
• Agility, which is the ability to perform functional or powerful movements in opposite directions.
• Flexibility, which is the ability to stretch, easily bend, or be pliable.

Physical activity is closely related to, but distinct from, the subsets of exercise and physical fitness. Exercise is defined as “planned, structured, and repetitive bodily movement done to improve or maintain one or more components of physical fitness.”1 This differs from the definition of physical fitness, which is “a set of attributes that people have or achieve that relates to the ability to perform physical activity.”1 Regular physical activity has long been regarded as an important component of a healthy lifestyle, and it is well established from controlled experimental trials that active individuals have high levels of cardiorespiratory fitness. Intermittent activity, provided it is continued, also confers substantial benefits.9–11 Studies have demonstrated that within a few weeks of discontinuing an endurance training program, the positive effects of exercise are almost completely lost, with approximately half of that loss occurring within the first 2 weeks.12,13

Clinical experience and limited studies suggest that people who maintain or improve their levels of physical activity may be better able to perform daily activities, may be less likely to develop pain, and may be better able to avoid disability, especially as they advance into older age.5 Regular physical activity may also contribute to better balance, coordination, and agility, which in turn may help prevent falls in the elderly.14 Epidemiologic research has demonstrated protective effects of physical activity and risk for several chronic diseases, including coronary heart disease,9,15,16 hypertension,17,18 non–insulin-dependent diabetes mellitus,19,20 osteoporosis,21,22 colon cancer,23 and anxiety and depression.24 Patterns of physical activity appear to vary with demographic characteristics. Men are more likely than women to engage in regular activity,25 vigorous exercise, and sports.26 The total amount of time spent engaging in physical activity normally declines with age.27,28 Adults at retirement age (65 years) show some increased participation in activities of light to moderate intensity, but, overall, physical activity declines continuously as age increases.27,29 Elderly African Americans, and other ethnic minority populations, are less active than white Americans,25,29,30 and this disparity is more pronounced for women.30 People with higher levels of education participate in more leisure-time physical activity than do people with less education.25 Differences in education and socioeconomic status account for most, if not all, of the differences in leisure-time physical activity associated with race and ethnicity.31

Oxygen is a vital component for life and the cardiovascular system provides a means by which oxygen is supplied to the various tissues of the body via the heart, blood vessels, blood, and lungs.

By definition, cardiorespiratory endurance is the ability to perform whole body activities (walking, jogging, swimming, etc.) for extended periods of time without unwarranted fatigue. The maximal amount of oxygen that can be used during exercise is referred to as maximal aerobic capacity (VO2max). It is also common to see aerobic capacity expressed in METs.

A number of adaptations occur within the circulatory system in response to exercise:

• Heart rate. As the body begins to exercise, the working tissues require an increased supply of oxygen to meet increased demand. Monitoring heart rate (HR) is an indirect method of estimating oxygen consumption as, normally, these two factors have a linear relationship (this relationship is consistent at very low and very high intensity exercise). If a physical therapy intervention requires an increase in systemic oxygen consumption expressed as either an increase in MET levels, kcal, or VO2, then HR should also be seen to increase.32 Increases in HR produced by exercise are met by a decrease in diastolic filling time. The extent at which the HR increases with escalating workloads is influenced by many factors including age, fitness level, type of activity being performed, body position, presence of disease, medications, blood volume, and environmental factors such as temperature, humidity, and altitude. Failure of the HR to increase with increasing workloads, referred to as chronotropic incompetence, should be of concern, even if the patient is taking beta blockers—beta blockers slow the HR, which can prevent the increase in HR that typically occurs with exercise.32
• Stroke volume. Stroke Volume (SV) is the amount of blood pumped out by the left ventricle of the heart with each beat (the difference between end-diastolic volume and end-systolic volume). The volume of blood being pumped out with each beat increases with exercise, but only to the point when there is enough time between beats for the heart to fill up (approximately 110–120 beats per minute). In the normal heart, as workload increases, SV increases linearly up to 40–50% of aerobic capacity, after which it increases only slightly. Factors that influence the magnitude of change in SV include exercise intensity, body position, and ventricular function.
• Cardiac output. Cardiac output (CO) is the amount of blood (approximately 5L) discharged by each ventricle (not both ventricles combined) per minute, usually expressed as liters per minute. CO, the product of HR and SV, increases linearly with workload because of the increases in HR and SV in response to increasing exercise intensity. During exercise, CO increases to approximately 4 times than that experienced during rest. Factors that influence the magnitude of change in CO include age, posture, body size, presence of disease, and level of physical conditioning. A long-term beneficial training effect that occurs with regard to HR is a reduced resting HR and reduced HR at a standard exercise load. This occurs because the heart becomes more efficient—the SV increases, brought about by increased venous return, and increased contractile conditions in the myocardium.
• Blood flow. The amount of blood flowing to the various organs increases during exercise, but there is a change in overall distribution of the CO—it is increased to active skeletal muscle, but decreased to nonessential organs. Total peripheral resistance, the sum of all forces that resist blood flow within the circulatory system, decreases during exercise primarily because of the vessel vasodilation in the active skeletal muscles.33
• Blood pressure. Blood pressure (BP) is defined as the pressure exerted by the blood on the walls of the blood vessels, specifically arterial blood pressure (the pressure in the large arteries). Systolic BP normally increases in proportion to oxygen consumption and CO, while diastolic BP normally shows little or no increase, or may decrease. Long-term aerobic training can result in reduced systolic and diastolic BP. Failure of the systolic BP to rise with an increase in intensity, referred to as exertional hypotension, is considered abnormal, and may occur in patients with cardiovascular problems. The minimal change in diastolic BP is due primarily to the vasodilation of the arteries from the exercise bout. Thus, the expansion in artery size may lower BP during the diastolic phase.32
• Oxygen consumption rises rapidly during the first minutes of exercise and levels off as the aerobic metabolism supplies the energy required by the working muscles.
• Mitochondria: An increase in size and number of the mitochondria.
• Hemoglobin concentration. Oxygen is transported throughout the system attached to hemoglobin, an iron containing protein that has the capability of easily accepting or giving up molecules of oxygen as needed.33 The concentration of hemoglobin in circulating blood does not change with training; it may actually decrease slightly.33 However, because training for improving cardiovascular endurance produces an increase in total blood volume, there is a corresponding increase in the amount of hemoglobin.
• Myoglobin: Increased myoglobin content.
• The use of fat and carbohydrates: Improved mobilization and use of fat and carbohydrates.
• Lung changes that occur due to exercise.
  • An increase in the volume of air that can be inspired in a single maximal ventilation. Ventilation is the process of air exchange in the lungs.
  • An increase in the diffusing capacity of the lungs.

During physical exercise, energy turnover in skeletal muscle may increase by 400 times compared with muscle at rest and muscle oxygen consumption may increase by more than 100 times.34 The energy required to power muscular activity is derived from the hydrolysis of ATP to ADP and inorganic phosphate (Pi). Despite the large fluctuations in energy demand just mentioned, muscle ATP remains practically constant and demonstrates a remarkable precision of the system in adjusting the rate of the ATP generating processes to the demand.35 There are three energy systems that contribute to the resynthesis of ATP via ADP rephosphorylation. These energy systems are as follows:

• Phosphagen system. The phosphagen system is an anaerobic process—can proceed without oxygen (O2). Within the skeletal muscle cell at the onset of muscular contraction, phosphocreatine (PCr) represents the most immediate reserve for the rephosphorylation of ATP. The phosphagen system provides ATP primarily for short-term, high-intensity activities (i.e., sprinting) but is active at the start of all exercises regardless of intensity.36 One disadvantage of the phosphagen system is that because of its significant contribution to the energy yield at the onset of near maximal exercise, the concentration of PCr can be reduced to less than 40% of resting levels within 10 seconds of the start of intense exercise.37
• Glycolysis system. The glycolysis system is an anaerobic process that involves the breakdown of carbohydrates—either glycogen stored in the muscle or glucose delivered through the blood—into pyruvate to produce ATP. Pyruvate is then transformed into lactic acid. Because this system relies upon a series of nine different chemical reactions, it is slower to become fully active. However, glycogenolysis has a greater capacity to provide energy than does PCr, and therefore it supplements PCr during maximal exercise and continues to rephosphorylate ADP during maximal exercise after PCr reserves have become essentially depleted.36 The process of glycolysis can be in one of the two ways, termed fast glycolysis and slow glycolysis, depending on the energy demands within the cell. If energy must be supplied at a high rate, fast glycolysis is used primarily. If the energy demand is not so high, slow glycolysis is activated. The main disadvantage of the fast glycolysis system is that during very high-intensity exercise, hydrogen ions dissociate from the glycogenolytic end product of lactic acid.35 The accumulation of lactic acid in the contracting muscle is recognized in sports and resistance training circles. An increase in hydrogen ion concentration is believed to inhibit glycolytic reactions and directly interfere with muscle excitation–contraction and coupling, which can potentially impair contractile force during exercise.36 This inhibition occurs once the muscle pH drops below a certain level, prompting the appearance of phosphofructokinase (PFK), resulting in local energy production ceasing until replenished by oxygen stores.

• Oxidative system. As its name suggests, the oxidative system requires O2 and is consequently termed the “aerobic” system. The oxidative system is the primary source of ATP at rest and during low-intensity activities. It is worth noting that at no time during either rest or exercise does any single energy system provide the complete supply of energy. While being unable to produce ATP at an equivalent rate to that produced by PCr breakdown and glycogenolysis, the oxidative system is capable of sustaining low-intensity exercise for several hours.36 However, because of an increased complexity, the time between the onset of exercise and when this system is operating at its full potential is around 45 seconds.38

The relative contribution of these energy systems to ATP resynthesis has been shown to depend upon the intensity and duration of exercise, with the primary system used being based on the duration of the event:39

• 0–10 seconds: ATP–PCr
• 10–30 seconds: ATP–PCr plus anaerobic glycolysis
• 30 seconds–2 minutes: anaerobic glycolysis
• 2–3 minutes: anaerobic glycolysis plus oxidative system
• >3 minutes and rest: oxidative system

The performance of any activity requires a certain rate of oxygen consumption, so that an individual's ability to perform an activity is limited by the maximal amount of oxygen the person is capable of delivering into the lungs.33 Fatigue and recovery from fatigue are complex processes that depend on physiologic and psychological factors. The physiologic factors include the adequacy of the blood supply to the working muscle and the maintenance of a viable chemical environment, whereas the psychological factors include motivation and incentive.2 After an intense exercise session, anaerobic energy sources must be replenished before they can be called on again to provide energy for muscular contraction. The anaerobic energy sources of ATP–PCr and lactic acid are ultimately replenished by the oxidative energy system. The extra oxygen that is taken and used to replenish the anaerobic energy sources after cessation of the exercise effort was previously referred to as the oxygen debt, but is now more accurately referred to as excess postexercise oxygen consumption (EPOC).

The patient who is injured while performing an activity or exercise is likely to return to that activity once the symptoms subside.5 Thus, the body needs to be prepared for a resumption of the stresses and demands that the activity or exercise will place upon it. If not, when the patient returns to competitive sports or functional and work activities, fatigue may result in alterations in efficient movements making the individual susceptible to injury.

Before beginning any exercise program, the clinician should perform a health screening check or risk factor assessment as part of the initial assessment to identify individuals who should consult a physician before initiating an exercise program. The following are considered risk factors for cardiovascular disease:40

• High BP: >140 mm Hg systolic or >90 mm Hg
• Smoking
• Elevated serum cholesterol: a total serum level >200, LDL >160 (individuals without heart disease, >100 in individuals with heart disease), or HDL <40 in men or <50 in women
• Lack of regular exercise (three or more times per week of regular exercise or moderate physical activity)
• Family history (mother or father with heart disease or stroke before age 60)
• Stress (particularly personality factors of anger and hostility)
• Diabetes
• Obesity
• Sex: men are at greater risk than women until women reach menopause, then equal risk
• Age: increasing age increases risk

All patients should be monitored (i.e., HR, BP, and symptoms) during the initial assessment if they have been determined unsafe to exercise based on any risk factor or healthy lifestyle assessment.

During exercise, the clinician should note normal clinical responses to progressively increasing aerobic exercise. The magnitude of the change in cardiovascular responses during aerobic exercise is related to the physical activity status of the patient. For example, a patient whose HR takes longer than 5 minutes to return to resting levels after 2 minutes of exercise has poor cardiovascular function.41 Once the individual has demonstrated normal vital signs and lack of symptoms with the activities, it may not be necessary to monitor further activities.

A number of precautions need to be taken when exercising patients who have been determined unsafe to exercise. These include:

• An appropriate level of intensity must be chosen:
  • Too high a level can overload the cardiorespiratory and muscular systems and potentially cause injuries or severe complications.
  • Exercising at a level which is too high causes the cardiorespiratory system to work anaerobically, not aerobically. Initially the patient should be exercising at a level with the HR at 60% of his or her maximum (220–age of patient). If the patient is exercising within their target HR, they should be able to carry on a conversation.
• A sufficient period of time should be allowed for warm-up and cool-down to permit adequate cardiorespiratory and muscular adaptation.

Signs and symptoms of exercise intolerance include, but are not limited to:

• BP:
  • Systolic BP >200 to 210 mm Hg
  • Drop in systolic BP >20 mm Hg
  • Diastolic BP >110 mm Hg
• HR:
  • Increase in HR >50 bpm with low level activity
  • Significant arrhythmias
• Chest pain
• Nausea, vomiting
• Unusual or severe fatigue
• Syncope or moderate dizziness
• Marked dyspnea (2 +/4+)
• Severe claudication (grade III/IV)
• Cyanosis or severe pallor

Aerobic conditioning is especially valuable for those who participate in sports that involve endurance.46 Nirschl47,48 recommends general body conditioning for patients, which provides the following benefits:

• Increased regional perfusion
• Neurophysiologic synergy and overflow
• Neurologic stimulation
• Minimization of the domino effect of weakness of adjacent structures
• Minimization of negative psychological effects
• Obesity control

A number of different training factors must be considered when attempting to maintain or improve cardiorespiratory endurance:

• Continuous training—the F.I.T.T. (Frequency, Intensity, Time, and Type) principle:
  • Frequency: to see at least minimal improvement in cardiorespiratory endurance, it is necessary for the average person to engage in no less than three sessions per week.
    • If the intensity is kept constant, there appears to be no additional benefit from twice a week versus four times or three times a week versus five times per week.
    • If the goal is weight loss, 5 to 7 days per week increases the caloric expenditure more than 2 days per week.
  • Intensity: the recommendations regarding training intensity (overload) vary. Relative intensity for an individual is calculated as a percentage of the maximum function, using VO2 max or maximum HR (HR max). It is now recognized that an individual's perception of effort (relative perceived exertion or RPE) is closely related to the level of physiological effort (see Chap. 12).49,50 For aerobic activities, the exercise intensity should be at a level that is 40–85% maximal aerobic power (VO2 max) or 55–90% of maximal HR.51

Two other common methods of monitoring intensity are employed:

• Monitoring HR—two formulas are commonly used:
  • Maximum heart rate reserve (MHRR): this method uses the difference between the maximum heart rate (MHR) and the resting heart rate (RHR), referred to as the maximum heart rate reserve (MHRR). When using this formula, the recommended intensity level range is 50–85% of VO2 max. For example, for a 40-year-old with a RHR of 65 bpm who wants to train at an intensity of 70%:
  • 220–40 = 180 bpm (MHR)
  • 180–65 = 115 bpm (MHRR)
  • (115 × 0.7) + 65 = 145 bpm
  • The age-adjusted maximum heart rate (AAMHR)52: MHR = 220 – patient's age. For example, for a 40-year-old, the MHR is 220–40 = 180. The recommended level of intensity when using this formula is a range between 60–90% of an individual's MHR Therefore, the target heart range (60–90%) for a MHR of 180 is between 180 × 60% and 180 × 90% = 108 to 162 bpm.

Regardless of which formula is used, the clinician should keep in mind that formulas are merely estimations (and highly inaccurate when used with patients taking prescribed medications that may affect HR and BP responses), and therefore, it is more important to be conservative in prescribing exercise intensity for any sedentary individual, elderly individual, or someone at moderate to high risk for cardiovascular disease.3

• Type of exercise: the type of activity chosen in continuous training must be aerobic—involving large muscle groups activated in a repetitive and rhythmic manner. Examples of aerobic activities are running, cycling, swimming, and cross-country skiing. Aerobic exercises allow the individual to speed up or slow down the pace to maintain the HR at a specified or target level.33 Cross-training exercises, including cycling, upper body ergometer (UBE), and water running, increase cardiovascular endurance. However, it must be remembered that although cross training can produce a similar cardiovascular effort as the original sport, it does not necessarily produce the same musculoskeletal effects (Table 15-1).
• Time (duration): duration is increased when intensity is limited, e.g., by initial fitness level. For minimal improvement to occur, the patient must participate in continuous activity for at least 20 minutes with a HR elevated to its working level. Three to five minutes per day produces a training effect in poorly conditioned individuals, whereas 20 to 60 minutes, 3 to 5 times a week is optimal for conditioned people. Generally, the greater the duration of the workout, the greater the improvement in cardiovascular endurance.33 A number of pieces of exercise equipment can be used with continuous training:
  • Treadmill walking: progressing from slow to fast and short distances to longer distances with or without an incline.
  • Ergometers: these come in a variety of forms for both the upper extremities and the lower extremities. The pace progression is from slow to fast and the goal is to increase the time spent exercising.
  • Free weights and elastic resistance: the use of low resistance and high repetitions can produce an aerobic effect.