A fitness watch displays five colored heart rate zones and reports minutes spent in each. Those boundaries derive from an estimate of maximum heart rate, and the estimate carries substantial error.
The formula behind the zones
Most consumer devices predict maximum heart rate from age using a simple linear equation, then set zone boundaries as fixed percentages of that predicted value.
The equation describes the average relationship between age and maximum heart rate across a population. It was never intended to describe any particular person.
The spread around that average is wide, and two people of identical age can have genuinely different maximums without either being abnormal.
Percentage boundaries inherit the error
If the predicted maximum is off, every zone derived from it shifts by the same proportion. An athlete can therefore sit in the wrong zone across an entire session.
The practical consequence is that easy sessions can be run harder than intended, which is the failure mode aerobic base training is meant to avoid.
Because the error is systematic rather than random, it does not average out across weeks of training.
Laboratory anchors use physiology instead
Exercise physiology labs define intensity domains by ventilatory or lactate thresholds, points where the body's response to increasing workload changes character.
These thresholds are measured directly during an incremental test rather than predicted, and they correspond to meaningful shifts in fuel use and fatigue accumulation.
Heart rate is then read off at those thresholds, making it a marker of an individually determined boundary rather than the boundary itself.
Heart rate drifts even at constant effort
During prolonged exercise, heart rate rises gradually at unchanged workload as body temperature increases and plasma volume falls.
Caffeine, heat, humidity, altitude, illness and accumulated fatigue all move heart rate independently of how hard the muscles are working.
This is why heart rate is a lagging and context-dependent proxy for intensity rather than a direct reading of it.
Why perceived exertion still earns its place
Rating perceived exertion integrates breathing, muscular sensation and effort into a single judgment that automatically accounts for heat, fatigue and illness.
The talk test works on the same principle, using the point at which continuous speech becomes difficult as a rough marker of a ventilatory transition.
People with cardiac conditions, or taking medications that alter heart rate response such as beta blockers, should set training intensity with a physician rather than from a device estimate.