08/02/2026
Your watch isn’t necessarily lying to you… but it might not be necessarily giving you the right heart rate zones.
Most watches use one of two methods.
Garmin and Polar typically default to % of Max Heart Rate. Apple and COROS generally use Heart Rate Reserve (Karvonen). Some brands let you change the method, but most people never do.
The issue with % Max Heart Rate is that it assumes everyone with the same max heart rate should train at the same intensities.
Imagine two athletes with a max heart rate of 190.
Athlete A has a resting heart rate of 40.
Athlete B has a resting heart rate of 60.
If you only use max heart rate, they’ll have identical training zones. Even though Athlete A has a much larger aerobic ability.
That’s where the Karvonen Formula can make the separation.
Instead of only using your max heart rate, it also factors in your resting heart rate. That makes your training zones much more individualized and much more reflective of your current fitness.
The most accurate way to set training zones is with a lactate threshold test. But if you’re not doing lab testing or threshold testing, the Karvonen Formula is one of the best ways to dial in your zones.
So is your watch telling you the truth?