Fitness watches and gym equipment display heart rate zones as though they were fixed physiological boundaries. They are calculated from an estimate, and the estimate is looser than the display suggests.
Maximum heart rate is a ceiling, not a fitness measure
Maximum heart rate is the highest rate the heart can reach during all-out effort. It declines with age and is largely determined by factors training does not change.
This surprises people who expect a fitter heart to beat faster at peak. Training improves how much blood each beat moves, not how high the rate can climb.
Because it is stable within a person, maximum rate works as a reference point for scaling effort even though it says nothing about conditioning.
Age-based formulas describe averages
The common practice is estimating maximum from age using a simple formula. Such formulas are fitted to population data and describe the average person at each age.
Individual maximums scatter widely around that average, with a spread of many beats per minute in either direction at any given age.
Someone whose true maximum sits well above the formula will find prescribed zones feel easy, while someone below it will find them punishing.
Zones divide the range into intensity bands
Zones are typically set as percentage bands of the maximum, with lower bands intended for easy aerobic work and higher bands for efforts sustainable only briefly.
The physiological reasoning is that different intensities emphasize different adaptations, from capillary and mitochondrial development at low intensity to peak oxygen uptake at high.
Some systems anchor the bands to heart rate reserve, subtracting resting rate before applying percentages, which accounts for differences in resting heart rate.
Heart rate lags and drifts
Heart rate takes time to respond to a change in effort, so during short intervals the displayed zone often reflects the previous minute rather than the current one.
During prolonged steady exercise, heart rate drifts upward at unchanged workload as body temperature rises and blood volume falls slightly with sweating.
Heat, altitude, dehydration, caffeine, illness and stress all shift the relationship between effort and rate, which means the same zone represents different work on different days.
What zones are good for
Their strength is enforcing easy days as easy. Left to perceived effort, many people drift into a moderate middle intensity that is neither restful nor demanding.
Laboratory testing can establish individual thresholds directly rather than by formula, and anyone with a cardiac condition should discuss intensity targets with a physician.
Treated as a rough scale rather than a precise boundary, zones do useful work; treated as exact, they mislead.