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Learning Objectives
The student will be able to use the Fick equation to calculate O2 consumption and aerobic capacity, using proper units and citing ranges for average levels.
The student will be able to explain the relationship between pulmonary gas exchange and exercise intensity, and the effects of endurance training.
The student will be able to summarize the effects on aerobic capacity of genetics, normal aging, bed rest, and detraining.
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A subject's aerobic capacity is an important assessment in respiratory physiology because the lungs may comprise a primary limitation on O2 transport and V̇o2 (Chap. 1). Furthermore, abundant scientific evidence supports the contention that aerobic capacity is a very powerful predictor of both life expectancy and mortality from many diseases. Aerobic capacity has several synonyms in the literature, including exercise capacity, aerobic fitness, maximal or peak oxygen consumption (V̇o2max and V̇o2peak, respectively), metabolic equivalent of the task (MET) capacity (see below), and cardiovascular fitness. For consistency, the terms aerobic capacity and V̇o2max will be used throughout this chapter.
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DEFINITION AND CALCULATION OF AEROBIC CAPACITY
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Aerobic capacity is defined by the American College of Sports Medicine (ACSM) as "The ability to perform dynamic exercise that involves large muscle groups at a moderate to high intensity for prolonged periods." V̇o2max is the most widely accepted measure of such aerobic capacity. Recall that V̇o2 always is equal to the product of the amount of blood delivered to all body tissue, that is, the cardiac output, multiplied by the amount of oxygen extracted from the blood during each passage through the tissues, that is, its arterial-venous oxygen content difference. Mathematically this relationship is expressed by the Fick equation:
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Clearly an individual's V̇o2max may be limited by any factor that constrains either cardiac output or the (arterial-venous) O2 content difference. Each of these components will be discussed in this chapter. One general comment is worth noting at this point. When evaluating aerobic capacity using weight-bearing exercises like running, walking, or climbing stairs, the V̇o2max is expressed in mL/kg/min or L/kg/min to correct for differences in a subject's body weight (and from which can be calculated their body mass index, BMI). When evaluating aerobic capacity using non-weight-bearing exercises like swimming, rowing, or biking, then V̇o2max is expressed in mL/min or L/min to exclude influences of a subject's body weight.
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A useful alternative is to express V̇o2max in METs, with one MET being the average for a subject's resting V̇o2. Historically, 1 MET = 3.5 mL/kg/min, although recent research indicates that a truer estimate over a wider range of body weights is 2.6 mL/kg/min. Thus, a subject with V̇o2max of ...