Cardiorespiratory fitness can best be defined as the ability to:

Cardiorespiratory fitness is fundamentally about the body's ability to supply oxygen to muscles during sustained physical activity and to use that oxygen effectively. This type of fitness is primarily developed through aerobic activities, which are exercises that increase heart rate and promote better cardiovascular and respiratory efficiency over longer periods.

Engaging in activities such as running, swimming, or cycling at a steady pace trains the heart, lungs, and circulatory system to deliver oxygen efficiently to the working muscles. Therefore, the ability to perform aerobic activity for sustained periods of time directly correlates with one's level of cardiorespiratory fitness, making this the most accurate definition.

In contrast, resistance training focuses on building muscular strength rather than improving the efficiency of the cardiovascular system, which is why it doesn't align with the definition of cardiorespiratory fitness. Although high-intensity interval training can contribute to overall fitness and enhance cardiovascular capacity, it is not primarily characterized by sustained aerobic activity. Flexibility exercises, on the other hand, involve stretching muscles and joints but do not impact cardiovascular or respiratory function in the same way, thus falling outside the scope of defining cardiorespiratory fitness.