After completion of this chapter, the physical therapist should be able to do the following:
Explain the relationships between heart rate, stroke volume, cardiac output, and rate of oxygen use.
Describe the function of the heart, blood vessels, and lungs in oxygen transport.
Describe the oxygen transport system and the concept of maximal rate of oxygen use.
Describe the principles of endurance and high-intensity interval training and the potential of each technique for improving aerobic activity.
Describe the difference between aerobic and anaerobic activity.
Describe the principles of reversibility and detraining.
Describe caloric threshold goals associated with various stages of exercise programming.
Although strength and flexibility are commonly regarded as essential components in any injury rehabilitation program, often relatively little consideration is given toward maintaining aerobic capacity and cardiorespiratory endurance. When musculoskeletal injury occurs, the patient is forced to decrease physical activity, and levels of cardiorespiratory endurance may decrease rapidly. Thus, the physical therapist must design or substitute alternative activities that allow the individual to maintain existing levels of aerobic capacity during the rehabilitation period. Furthermore, the importance of maintaining and improving functional capacity is becoming increasingly evident regardless of musculoskeletal injury.1 Recent research has shown that increased levels of physical activity reduce the risk of cardiovascular disease.2 Sandvik et al3 reported mortality rates according to fitness quartiles during 16 years of follow-up. The number of deaths in the least-fit portion of the study outnumbered the deaths of the most fit by a margin of 61 to 11 deaths from cardiovascular causes.3 Myers et al studied 6,213 patients referred for treadmill testing and concluded that exercise capacity is a more powerful predictor of mortality among men than other established risk factors for cardiovascular disease.4
By definition, cardiorespiratory endurance is the ability to perform whole-body activities for extended periods without undue fatigue.5 The cardiorespiratory system provides a means by which oxygen is supplied to the various tissues of the body. Without oxygen, the cells within the human body cannot function and, ultimately, cell death will occur. Thus, the cardiorespiratory system is the basic life-support system of the body.5,6
TRAINING EFFECTS ON THE CARDIORESPIRATORY SYSTEM
Basically, transport of oxygen throughout the body involves the coordinated function of four components: heart, blood vessels, blood, and lungs. The improvement of cardiorespiratory endurance through training occurs because of increased capability of each of these four elements in providing necessary oxygen to the working tissues.7,8 A basic discussion of the training effects and response to exercise that occur in the heart, blood vessels, blood, and lungs should make it easier to understand why the training techniques discussed later are effective in improving cardiorespiratory endurance.
Adaptation of the Heart to Exercise
The heart is the main pumping mechanism and circulates oxygenated blood throughout the vascular system to ...