What does the resting heart rate (RHR) measure?

The resting heart rate (RHR) is a measure of the heart rate when a person is at rest, typically taken in a consistent manner such as first thing in the morning before getting out of bed. This measurement reflects the basal heart rate—the number of times the heart beats per minute without the influence of physical activity or stressors.

A lower resting heart rate is often associated with better cardiovascular fitness and efficient heart function, as a well-conditioned heart can pump a larger volume of blood with each beat, requiring fewer beats per minute to maintain circulation. Monitoring RHR can also provide insights into a person's overall health and can indicate changes in fitness levels or potential health issues.

In this context, the other choices do not accurately represent what RHR measures. Heart rate during maximum exertion refers to the maximum beats per minute achieved during intense activity, while average heart rate during physical activity does not represent the heart’s function at rest. Additionally, heart rate at the onset of fatigue pertains to the point when exercise can no longer be sustained, which is also not reflective of resting conditions. Thus, the defined measure of RHR is the basal heart rate with no exertion, making the chosen answer the most accurate.