Changes in the CR System

An improvement in CR functioning, or fitness level, requires adaptation of the system. Remember, the point is to more effectively generate ATP so more work can be accomplished. In order to process more oxygen and deliver more oxygenated blood to the cells, the overall system must undergo changes to make this possible. Here is a list of adaptations that occur to the CR system as a result of consistent aerobic exercise:

• Resting heart rate may decrease. The average resting heart rate hovers around 70–75 beats per minute. Elite athletes may have resting heart rates in the high 30s. Generally, resting heart rate may decrease by approximately 10 beats per minute with chronic exercise.
• Pulmonary adaptations, such as increased tidal volume (the amount of oxygen entering the lungs with each breath) and increased diffusion capacity (the amount of oxygen that enters the blood stream from the lungs). This allows for more oxygen to enter the pulmonary circulation en route to the left side of the heart.
• The heart muscles, specifically the left side of the heart, increase in size making it possible to contract more forcefully. As a result, more blood can be pumped with each beat meaning more oxygen can be routed to the systemic circulation.
• More oxygen is delivered and transported into the cells where ATP production can occur. This is called the arterial-vein difference (a-VO_2diff)

These changes in the system are not permanent due to the principle of reversibility. Following a period of inactivity, the benefits from chronic aerobic exercise will be reversed.

Assessing CR Fitness

To adequately prepare for starting a personal fitness program, it is important to first assess your current level of fitness. There are multiple methods for assessing a person’s level of fitness. Each of the walking/jogging assessments discussed here attempts to estimate a key physiological marker of the heart’s and lungs’ functioning capacity and maximal oxygen consumption. Maximal oxygen consumption, or VO_{2 max}, measures the body’s maximum ability to take in and utilize oxygen, which directly correlates to overall health and fitness. A good estimate of VO_{2 max} provides a one-time glance at a person’s health and fitness level and a baseline measurement for reassessment at future dates to gauge improvements.

Some of the most common walking/jogging assessments used to estimate VO_{2 max} include the 12-Minute Walk, 1.5-Mile Run/Walk Test, the 20m Leger Shuttle Run, 3-Minute Step Test, and 1-Mile Walk Test. Unfortunately, these field assessments, although practical and inexpensive, only provide estimations. More accurate assessments require a lab-based VO_{2 max} test using equipment that measures the volume of oxygen and carbon dioxide being moved in and out of the air passages during exercise. Although this test is more accurate, the expense and availability make it impractical for most. Unlike the lab test, the field assessments are relatively cost free, user-friendly and require very little expertise to conduct or perform. In addition, the key point of the assessment is measuring differences rather than absolute values, and the field tests accurately meet that objective.

Several examples of tests that assess cardiorespiratory endurance can be found in the Fitness and Stress Assessments chapter at the end of this textbook.