What physiological measurement might indicate a person’s fitness level?

Heart rate is a key physiological measurement that can indicate a person's fitness level. A lower resting heart rate typically indicates better cardiovascular fitness, as the heart becomes more efficient at pumping blood. In trained athletes, for example, it's common to see resting heart rates lower than those of untrained individuals, reflecting a strong heart and efficient circulatory system. Additionally, heart rate response to exercise—how quickly it increases during activity and how quickly it returns to resting levels after exercise—also provides insights into fitness.

Other factors, such as body temperature, body weight, and height, do not directly measure the efficiency of the cardiovascular system or overall fitness level, making them less appropriate indicators in this context. While body weight may reflect changes due to fitness programs, it does not account for the composition of that weight (muscle vs. fat) or overall health, and height provides no direct correlation to fitness level.