Heart Rate Reserve
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Heart rate reserve is the difference between your maximum heart rate and your resting heart rate, representing the full range your heart can work through. It's used in the Karvonen method to set more individualized training zones than simple percent-of-max calculations.

Two people with the same max heart rate but very different resting heart rates have different reserves, so using reserve rather than just max heart rate better reflects each person's true cardiovascular fitness. A fitter person's lower resting heart rate widens their reserve and shifts their training zones accordingly.

You multiply the reserve by the desired percentage (like 60-80%) and add back your resting heart rate, rather than just taking a percentage of max heart rate directly. This Karvonen formula produces target zones that sit higher than simple max-HR percentage zones at the same intensity level.

Resting heart rate is lowest and most stable right after waking, before caffeine, stress, or movement raise it. Measuring at the same time and conditions daily for a few days gives a reliable average to use in the calculation.

Generally yes, since aerobic training strengthens the heart's stroke volume, letting it pump the same blood with fewer beats. However, certain medications, thyroid conditions, or genetics can also lower resting heart rate independent of fitness, so it's not an absolute measure on its own.