Why your heart rate and power zones don't match

Power and heart rate measure different things

Power is the work you produce at the pedals, in watts. It responds immediately and does not depend on how you feel. Heart rate is your body's response to that work. It lags behind changes in effort and moves with heat, dehydration, sleep, caffeine, stress and fatigue.

So the same 180 W can sit at one heart rate on a cool morning and a higher heart rate on a hot afternoon. Neither reading is wrong. They are answering different questions.

How the platforms build their zones

Zone numbers look similar across apps, but the inputs are not the same. Strava's help pages say its heart rate zones come from your maximum heart rate, estimated by default as 220 minus your age, or 190 bpm if no age is set, unless you enter your own. Its power zones come from your FTP, and if you have not set one it estimates it from your activity data or from your weight and gender.

Garmin devices calculate heart rate zones from the profile you set up, and let you enter your own maximum, resting and lactate threshold heart rate. Many Garmin devices also let you choose the method used to calculate zones. Check your own device settings, because the default may not be the method you assume.

Why heart rate zone 2 and power zone 2 are not the same thing

A power zone is defined by a percentage of FTP. In Coggan's original levels, endurance is 56 to 75% of FTP. A heart rate zone is defined by a percentage of a heart rate value, and different methods use different reference values. Two zones with the same number can therefore describe different efforts.

Even inside one method the maximum heart rate matters a lot. The arithmetic below is not a zone definition. It only shows how much a single zone boundary moves when the maximum changes.

A difference of a dozen beats is enough to put the same ride in a different zone. The age formula is an estimate for a population, not a measurement of you.

Why the numbers drift during a ride

• Heat and dehydration raise heart rate at the same power.
• Fatigue, poor sleep and caffeine can raise or lower it.
• In a long ride, heart rate often creeps up at constant power. This is called cardiac drift.
• Heart rate takes a few minutes to catch up with a change in power, so it is a poor guide for short intervals.
• Optical wrist sensors can read poorly in cold, on bumpy roads or with a loose fit. A chest strap is usually steadier.

How to choose what to train by

• On the bike with a power meter, use power as the main target for intervals and steady blocks. Use heart rate to see how your body is coping.
• When running or swimming, or riding without power, use heart rate, pace and perceived effort together.
• Check the inputs in each app: FTP, maximum heart rate and the zone method. Make them match your latest tests.
• Pick one reference for a block of training and stop comparing zone numbers between apps. Compare the same metric over time.
• If power and heart rate disagree for several sessions at the same effort, look at heat, sleep, fatigue and recent training before changing zones.

A simple check you can repeat

Ride a steady block at the same power on similar days, for example 40 minutes at the middle of your power zone 2, and note the average heart rate and how it feels. If the same power gives a clearly lower heart rate over several weeks, that usually points to improving aerobic fitness. If it creeps up, look at fatigue, heat and recovery. This is a general training idea, not a medical test. Talk to a doctor if you have concerns about your heart rate.