Cardiorespiratory Endurance Basics

Cardiorespiratory (CR) fitness, sometimes called CR endurance, aerobic fitness, or aerobic capacity, is one of the five basic components of physical fitness. CR fitness is a condition in which the body’s cardiovascular (circulatory) and respiratory systems function together, especially during exercise or work, to ensure that adequate oxygen is supplied to the working muscles to produce energy. CR fitness is needed for prolonged, rhythmic use of the body’s large muscle groups. A high level of CR fitness permits continuous physical activity without a decline in performance and allows for rapid recovery following fatiguing physical activity.

Activities such as running, hiking, bicycling, swimming, cross-country skiing, rowing, stair climbing, and jumping rope place an extra demand on the cardiovascular and respiratory systems. During exercise, these systems attempt to supply oxygen to the working muscles. Most of this oxygen is used to produce energy for muscular contraction. Any activity that continuously uses large muscle groups for 20 minutes or longer taxes these systems. Because of this, a wide variety of training methods is used to improve cardiorespiratory endurance.

Physiology of Aerobic Training

Aerobic exercise uses oxygen to produce most of the body’s energy needs. It also brings into play a fairly complex set of physiological events.

To provide enough energy-producing oxygen to the muscles, the following events occur:

  • Greater movement of air through the lungs.
  • Increased movement of oxygen from the lungs into the blood stream.
  • Increased delivery of oxygen-laden blood to the working muscles by the heart's accelerated pumping action.
  • Regulation of the blood vessel's size to distribute blood away from inactive tissue to working muscle.
  • Greater movement of oxygen from the blood into the muscle tissue.
  • Accelerated return of veinous blood to the heart.

Correctly performed aerobic exercise, over time, causes positive changes in the body's CR system. These changes allow the heart and vascular systems to deliver more oxygen-rich blood to the working muscles during exercise.

Some of the main cardiorespiratory benefits of regular exercise include:

  • Stronger, larger heart muscle (enabling more blood to be pumped with each stroke)
  • Increasing the number of small arteries in the muscles of the body (to supply more blood to working muscles)
  • Larger, stronger lungs (more oxygen gets into the blood with each breath)

Also, those muscles regularly used during aerobic exercise undergo positive changes. By using more oxygen, these changes let the muscles make and use more energy during exercise and, as a result, the muscles can work longer and harder.

During maximum aerobic exercise, the trained person has an increased maximum oxygen consumption (VO2max). They are better able to process oxygen and fuel and can therefore provide more energy to the working muscles.


VO2max, also called aerobic capacity, is the most widely accepted single indicator of one's CR fitness level.

The best way to determine aerobic capacity is to measure it in the laboratory. It is much easier, however, to estimate maximum oxygen uptake by using other methods.

It is possible to determine a person's CR fitness level and get an accurate estimate of their aerobic capacity by using their 1.5-mile-run times. Other tests - the bicycle, walk, and step tests - may also be used to estimate one’s aerobic capacity and evaluate one’s CR fitness level.

In the presence of oxygen, muscle cells produce energy by breaking down carbohydrates and fats. In fact, fats are only used as an energy source when oxygen is present. Hence, aerobic exercise is the best type of activity for attaining and maintaining a low percentage of body fat.

A person’s maximum aerobic capacity can be modified through physical training. To reach very high levels of aerobic fitness, one must train hard. The best way to improve CR fitness is to participate regularly in a demanding aerobic exercise program.

Last modified: Thursday, 14 June 2012, 4:20 PM