What does cardiorespiratory endurance primarily measure?

Cardiorespiratory endurance is primarily concerned with the body's ability to supply oxygen efficiently to the muscles during sustained physical activity. This component of fitness is essential for overall aerobic performance, as it reflects how well the cardiovascular and respiratory systems work together to transport oxygen to the body’s tissues and remove carbon dioxide. High cardiorespiratory endurance indicates a well-conditioned cardiovascular system that can support prolonged exercise, making it a crucial measure for activities ranging from running to swimming and cycling.

The other choices represent different aspects of physical fitness that do not align with the definition of cardiorespiratory endurance. The ability to change body position pertains more to agility or coordination, flexibility concerns the range of motion in joints, and the relative amounts of body fat relate to body composition rather than endurance in oxygen delivery and use. Thus, the focus on oxygen supply during sustained physical activity clearly identifies why the correct choice pertains to cardiorespiratory endurance.