Why We Don’t Use Heart Rate Zones to Prescribe Training Intensity

Wearables have made heart rate data ubiquitous, and platforms like Garmin Connect conveniently translate that data into predefined training zones. But there is a problem with this approach: how those zones are created—and how athletes respond to them.

The Problem with Default Heart Rate Zones

Most apps, including Garmin Connect, estimate heart rate zones using percentage-based models derived from predicted maximum heart rate. These approaches may work reasonably well across large populations, but they are unreliable at the individual runner’s level.

Physiologically meaningful zones are defined by thresholds, not percentages—specifically, the first and second lactate thresholds (LT1 and LT2, sometimes called the aerobic and anaerobic thresholds). Two athletes with the same maximum heart rate can have very different thresholds, meaning the same zone can represent very different internal loads.

In short, default heart rate zones are classifications, not suitable for individual training prescriptions.

Why I Rarely Prescribe Training by Heart Rate

Even when heart rate zones are correctly set, I rarely use them to prescribe training intensity. Instead, I prefer to regulate training intensity based on perceived exertionPerceived exertion describes an athlete’s subjective sense of how hard an effort feels at a given moment. Commonly… More (RPE). RPEPerceived exertion describes an athlete’s subjective sense of how hard an effort feels at a given moment. Commonly… More has been validated as a reliable and sensitive measure of internal training load, integrating physiological stress, fatigue, and context more effectively than single physiological markers such as heart rate. At Top Of The Trail, we use the 1-10 RPE Scale (Modified Borg Scale).

Heart rate is influenced by many factors, including heat, hydration status, fatigue, altitude, stress, and accumulated training load. Heart rate will inevitably drift during a training session—especially during long runs or ultra trail races. RPE automatically integrates all of these factors.

Just as importantly, psychology matters. Numbers on a screen can create strong behavioral responses: athletes start chasing or avoiding a heart rate value rather than regulating their effort. Over time, this external focus disrupts pacing and erodes the internal awareness endurance athletes critically need.

Heart rate remains useful as information, but it becomes problematic when it turns into a constraint. It is best suited for observation and post-session analysis—not for real-time intensity control.

Zones are Not Forever

Heart rate thresholds shift over time, which limits the validity of the zones. As an athlete becomes fitter, thresholds typically shift upward. With detraining, periods of illness or injury, or prolonged fatigue, they shift downward. Consequently, heart rate zones are only valid for a limited time.

To remain accurate, zones need to be reassessed:

• Every 8–12 weeks during periods of structured training or performance development
• After major interruptions (illness, injury, prolonged detraining)
• When training context changes (heat, altitude, major volume or intensity shifts)

In practice, people rarely reassess their zones, making zone-based prescriptions increasingly outdated over time.

How to set Heart Rate Zones without an App

If heart rate zones are used at all, they should be anchored to physiology.

• Laboratory testing is the gold standard, using lactate and/or gas exchange to identify LT1 and LT2.
• If lab testing is not available or not desired, validated field tests offer a far better alternative than percentage-based algorithms:
  • 30-minute time trial (average HR of the final 20 minutes ≈ LT2)
  • Talk test (a robust proxy for LT1)
  • heart rate deflection point protocols
  • Incremental treadmill or uphill running tests (especially relevant for trail runners)

Field tests are less precise than lab testing, but they are individual, easily repeatable, physiologically grounded, and scientifically validated.

Takeaway

Heart rate zones aren’t inherently wrong—but they’re often inaccurate, overtrusted, and quickly outdated. Training quality improves when athletes learn to regulate effort internally rather than rely on an algorithm. Longevity, consistency, and performance are built by understanding effort, not by chasing numbers.

That’s why I prefer perceived exertion (RPE) to prescribe training intensity: it adapts instantly to fitness, fatigue, and context—no recalibration required.

• Use perceived exertionPerceived exertion describes an athlete’s subjective sense of how hard an effort feels at a given moment. Commonly… More to guide training.
• Use heart rate for post-session analysis rather than for real-time intensity control.
• In case you use heart rate zones, make sure they are set correctly and updated regularly.

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Cover photo by Fellipe Ditadi on Unsplash