Cardiorespiratory fitness is related to the ability to perform large muscle, dynamic, moderate –to-high intensity exercise for prolonged periods. Performance of such exercise depends on the functional state of the respiratory, cardiovascular, and skeletal muscle systems. Cardiorespiratory fitness is considered health-related because (a) low levels of CR fitness have been associated with a markedly increased risk of premature death from all causes and specifically from cardiovascular disease, (b) increases in CR fitness are associated with a reduction in death from all causes, and (c) high levels of CR fitness are associated with higher levels of habitual physical activity, which in turn are associated with many health benefits. The assessment of CR fitness is an important part of a primary or secondary prevention program.

Cardiorespiratory fitness is a measure of how well your body is able to transport oxygen to your muscles during prolonged exercise, and also of how well your muscles are able to absorb and use the oxygen, once it has been delivered, to generate adenosine triphosphate (ATP) energy via cellular respiration (cellular respiration is a chemical process in your body’s cells that converts the energy stored in the food you eat into the ATP form of energy that is recruited for use by your muscles). Essentially, your cardiorespiratory fitness level is a measure of the strength of your aerobic energy system.