Physical activity has been defined as “any bodily movement produced by the contraction of skeletal muscles that results in a substantial increase in resting 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 VO2 max, 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 If our sedentary society is to change to one that is more physically active, clinicians must play their role in communicating with 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 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. Patients should be routinely counseled to adopt and maintain regular physical activity. 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 regard 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 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 of 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,1 vigorous exercise, and sports.25 The total amount of time spent engaging in a physical activity normally declines with age.1,26 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.1,27 Elderly African Americans, and other ethnic minority populations, are less active than white Americans,28–30 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.1 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 of 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, rowing, 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 (VO2 max). It is also common to see aerobic capacity expressed in METs. A number of adaptations occur within the circulatory system in response to exercise: In contrast, deconditioning, which occurs with any extended illness or prolonged inactivity results in a number of negative changes to the cardiovascular system: 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 this muscular activity comes from a number of energy systems (see Chapter 1). 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 environmental (room temperature, air quality, and altitude), physical, physiologic and psychological factors, including the patient’s health status, diet, and lifestyle (sedentary or active). 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 Certain disease processes can also affect fatigue. For example, multiple sclerosis typically allows a patient to function well during the early morning, but by midafternoon the patient can often become notably weak. Cardiopulmonary fatigue is likely to be caused a decrease in blood sugar (glucose) levels, a decrease in glycogen stores in the muscle and liver, and a depletion of potassium. The threshold for fatigue is the level of exercise that cannot be sustained for indefinitely. 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 (see Chapter 1). 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). Adequate time for recovery from fatigue must be built into every intrasession and intersession exercise progression. In addition, 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. As part of the initial assessment to identify individuals who should consult a physician before initiating an exercise program, the clinician should perform a health screening check or risk factor assessment. The following are considered risk factors for cardiovascular disease35:
CHAPTER 15
Improving Cardiovascular Endurance
OVERVIEW
PHYSICAL FITNESS
Muscle strength, which is the ability of muscles to exert or resist force (see Chapter 12).
Muscle endurance, which is the ability of the muscle to perform work (see Chapter 12).
Muscle power, which is the ability of a muscle to exert high force at high speed (see Chapter 12).
Balance, which is the ability to maintain equilibrium when the body is static or moving (see Chapter 14).
Agility, which is the ability to perform functional or powerful movements in opposite directions (see Chapter 14).
Flexibility, which is the ability to stretch, easily bend, or be pliable (see Chapter 13).
PHYSICAL ACTIVITY
CARDIOVASCULAR PHYSIOLOGY
Heart rate (HR). As the body begins to exercise, the working tissues require an increased supply of oxygen to meet increased demand. Monitoring HR is an indirect method of estimating oxygen consumption as, normally, these two factors have a linear relationship (this relationship is consistent with 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 max, 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 (SV). 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 (CO). CO is the amount of blood (approximately 5 L) 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 four times than that experienced during rest. Factors that influence the magnitude of change in CO include age, posture, body size, the 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 the 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 (BP). 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 a patient with a cardiovascular problem. 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. Myocardial oxygen consumption is a measure of the oxygen consumed by the myocardial muscle.1
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.
A decrease in maximum oxygen consumption.
A decrease in CO/SV.
A decrease in total blood and plasma volume.
RECOVERY
PRECAUTIONS WITH AEROBIC CONDITIONING
High BP: >140 mm Hg systolic, or >90 mm Hg diastolic
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 the age of 60 years)
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 riskRelated posts:
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