Heart Rate Lies in the Heat, After Illness, and in Overreaching. RPE Has Known This for 50 Years.

Heart rate monitors are now universal. The belief that they accurately quantify training load is not warranted — and the situations where heart rate is most misleading are exactly the situations where accurate load measurement matters most.

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What Heart Rate Actually Measures

Heart rate is a response to multiple simultaneous demands:

• Exercise intensity (the target variable for load monitoring)
• Core temperature (rises with heat and duration independent of intensity)
• Hydration state (dehydration elevates HR by 3–7 bpm per 1% body weight fluid loss)
• Emotional arousal (competition vs. training at identical intensities show different HR profiles)
• Caffeine and stimulants (elevate resting HR and alter the HR-intensity relationship)
• Illness (inflammatory cytokines elevate HR independently of work output)
• Autonomic fatigue (parasympathetic withdrawal in overreaching elevates resting and submaximal HR)

This means that the same absolute heart rate — 150 bpm — can represent Zone 2 training in cool conditions, Zone 3+ effort in hot conditions, and near-maximal effort in a dehydrated, fatigued, or ill athlete. The watt output, pace, or metabolic load can be completely different across these states.

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Cardiac Drift: Where HR Fails During Long Sessions

Cardiac drift is the progressive increase in heart rate during prolonged exercise at constant power output or pace, driven by core temperature rise and plasma volume reduction. In a 90-minute steady-state effort:

• The last 30 minutes of a session at identical pace will show 10–20 bpm higher HR than the first 30 minutes
• HR-based load metrics will assign more "hard training" to the second half of the session despite identical mechanical output
• A session scored as 140 average HR might represent entirely Zone 2 work by metabolic standard but appear as Zone 3 by HR zones

In hot weather, cardiac drift begins earlier and is more severe — a 25°C session vs. a 10°C session at identical pace will differ by 8–15 bpm across the final third of a long run or ride.

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The Session RPE Solution

Gunnar Borg developed the RPE (Rating of Perceived Exertion) scale in the 1970s as a subjective effort descriptor. Carl Foster's adaptation for session RPE in 2001 provided a practical training load quantification method:

1. Record session duration in minutes 2. Ask athlete "how hard was the overall session?" using a 10-point modified Borg CR10 scale, 30 minutes post-session 3. Multiply: Load (AU) = RPE × Duration

Session RPE-based load (arbitrary units, AU) correlates r=0.85–0.93 with TRIMP (HR × time × exponential weighting) in research validation studies — comparable predictive validity with lower equipment requirements.

The critical advantage: session RPE integrates all the variables that distort HR-based load. When an athlete has a harder session than their heart rate reflects (high autonomic fitness, cool conditions), RPE captures it. When an athlete has an easy session that HR overstates (heat, illness, emotional stress), RPE captures it.

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Combining Both Signals for Diagnostic Value

The most powerful use of HR and RPE together is divergence detection:

Training monotony — calculated as the ratio of mean weekly load to standard deviation of daily loads (Foster's monotony index) — is a practical metric derived from session RPE that predicts illness and overuse injury risk. Monotony >2.0 consistently precedes illness in athletic populations.

For athletes wanting to establish and calibrate individual heart rate training zones — the baseline against which cardiac drift and load divergence is detected — the heart rate zone calculator at winsport.uk/tools/performance/heart-rate-zone-calculator generates personalised zones from age, resting HR, and optional field test data, with VT1/VT2-referenced boundaries for training-load management.

In your monitoring practice, do you track the RPE-HR divergence as a daily marker — or rely on one signal alone?