Key takeaways
Three things to remember
- 01
Heart Rate Reserve is the gap between your resting and maximum heart rate; a larger reserve means more precise, individualized training zones.
- 02
HRR-based zone targets (the Karvonen Method) are more accurate than max-HR-only formulas for anyone with meaningful aerobic fitness.
- 03
Zone 2 base training progressively lowers resting heart rate, widening HRR and improving zone accuracy over a training season.
How it works
The Mechanism
Heart Rate Reserve was formalized by physiologist Seppo Karvonen in 1957 in what became known as the Karvonen Method. The formula is straightforward: HRR equals maximum heart rate minus resting heart rate. A person with a max HR of 185 and a resting HR of 55 has an HRR of 130 beats per minute to work with.
Training zones are then assigned as percentages of this reserve, added back to resting heart rate. Zone 2 in the Karvonen framework corresponds to roughly 60-70% of HRR plus resting heart rate. Because resting heart rate is included in the calculation, an aerobically fit person with a low resting heart rate will have a wider reserve and therefore higher absolute zone targets than a less fit person with the same maximum heart rate.
As cardiovascular fitness improves, resting heart rate tends to drop, which widens HRR and shifts zone targets. This is why the same perceived effort can correspond to different absolute heart rates across different fitness levels. HRR-based zones are more sensitive to this change than maximum-heart-rate-only zones, making them more useful as fitness evolves over a training cycle.
In practice
Why It Matters
Your resting heart rate is half the equation. Zone targets that ignore it are less accurate.
HRR-based training zones are more accurate for highly fit individuals whose resting heart rate has dropped substantially through training. A runner with a resting HR of 40 and a max HR of 185 has a very different aerobic zone than someone with a resting HR of 70 and the same max HR, even though maximum-HR-only zone formulas would give them identical targets. Using HRR captures this difference and produces zone targets that reflect actual cardiovascular state.
Common misconception
Many people calculate training zones using maximum heart rate alone (the 220-minus-age formula), which ignores resting heart rate entirely. This produces zones that are accurate only for sedentary individuals. For anyone with meaningful aerobic fitness and a lower resting heart rate, HRR-based zones are more individualized and more useful.
Reference ranges
What a Healthy Range Looks Like
Small Reserve
40-70 bpm
Sedentary adults or those with elevated resting heart rate; limited cardiovascular headroom
Moderate Reserve
70-100 bpm
Moderately active adults; adequate range for most zone-based training
Large Reserve
100-130 bpm
Aerobically fit adults with low resting HR; wider zone spread for precise training
Athletic Reserve
130+ bpm
Endurance-trained athletes; resting HR often 40-55 bpm combined with high max HR
HRR improves primarily by lowering resting heart rate through aerobic training. A larger reserve gives you more precision in zone training. Compare your reserve to your own prior measurements across a training season rather than to population averages.
Signs it is disrupted
- Resting heart rate rising over several days without a corresponding training block, which narrows HRR and compresses zone targets upward
- Training heart rate hitting target zones at lower perceived effort than expected, suggesting cardiovascular fatigue or illness
- HRR stagnant across a training season despite consistent Zone 2 volume, which may indicate insufficient aerobic base work
- Heart rate not recovering quickly during rest intervals, a sign of autonomic fatigue narrowing functional cardiovascular range
What moves it
How to Improve It
Zone 2 cardio base. 150-180 minutes per week at conversational intensity progressively lowers resting heart rate over 8-12 weeks, widening HRR and improving zone precision.
Recalculate zones seasonally. As resting heart rate drops with training, recalculate HRR-based zones so targets stay accurate; a drop of 5 bpm in resting HR meaningfully shifts all zones.
Track resting HR trend. Monitor your 7-day average resting HR on Oura or WHOOP; sustained downward trends confirm aerobic adaptation is widening your reserve.
Avoid overtraining. Chronic overtraining raises resting heart rate over weeks through autonomic suppression, shrinking HRR and making all zones harder to hit accurately.
Where to see it
Which Devices Track It
- Garmin
- Garmin uses the Karvonen Method for zone calculation when resting heart rate is entered; it factors HRR directly into zone targets and updates them as resting HR changes in your profile.
- Oura
- Oura tracks resting heart rate and can inform HRR calculations, but does not display HRR directly; use Oura resting HR in the Karvonen formula manually.
- WHOOP
- WHOOP tracks resting heart rate trends but does not calculate HRR-based zones natively; external calculation using WHOOP resting HR data is straightforward.
- Apple Watch
- Apple Watch displays resting heart rate in the Health app and uses it for some fitness calculations, but zone-based training primarily uses max HR percentage rather than the Karvonen HRR method.
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