Cardiac Drift
Also known as: Heart-Rate Drift, Pa:Hr Decoupling, HR Decoupling
The gradual upward creep in heart rate during a prolonged steady-pace effort, even when speed, power, and perceived effort stay constant. Cardiac drift is the body's progressive struggle to maintain a target intensity as core temperature rises, plasma volume drops, and substrate availability shifts — it's the most reliable physiological signal of accumulating fatigue during a single long session.
Formula
HR drift % = (avg HR second half − avg HR first half) / avg HR first half × 100 [also called "Pa:Hr decoupling" when expressed as pace-to-HR or power-to-HR ratio drift]Example
90-minute Zone 2 run at a steady 5:30/km pace. First-half average HR: 142. Second-half average HR: 152. Drift = (152 − 142) / 142 = 7.0%. A drift over 5% on a session that's supposed to be aerobic indicates you've slid above Zone 2 — the prescribed pace has become too hard for the day's underlying state.
How Afitpilot Uses This
Afitpilot does not currently compute cardiac drift — it requires continuous HR sampling and time-aligned pace/power data, which we don't yet ingest. When endurance-modality TRIMP support lands (`tss-trimp` improvement), drift will be a natural follow-on metric to surface on long aerobic sessions. For now, drift is something endurance athletes track manually via Strava, TrainingPeaks, or Garmin — and is one of the cleanest at-home signals for "is my Zone 2 actually still Zone 2 today?".
Cardiac drift by training state
| Who / Context | Value | Note |
|---|---|---|
| Aerobically well-trained, fresh, cool day | < 3% drift | Cardiovascular system is well-matched to the effort |
| Healthy athlete, average conditions | 3-5% drift | Normal range for a 60-90 min Zone 2 session |
| Mismatched pace (too hard for state) | 5-8% drift | The session is above the prescribed intensity by physiology, even if the watch says otherwise |
| Hot or humid environment | +2-4% drift baseline | Thermal strain is the dominant driver — not fitness |
| MAF / strict aerobic-base training | Goal: keep drift < 5% across multi-month blocks | Improving aerobic base shows up as drift dropping at matched pace |
Known Limitations
- •Drift is only interpretable on steady-pace efforts at sub-threshold intensity. Intervals, hills, surges, and tempo work all produce drift signals that mean different things or no useful thing at all.
- •Environmental conditions dominate. A 5% drift on a 32°C humid day is normal physiology; the same 5% drift on a cool 12°C day at the same pace is a fitness or readiness signal. Without ambient temperature in the model, drift is hard to compare across sessions.
- •Hydration status, caffeine, recent meals, and sleep all bias drift. A single session's drift number is noisy — the multi-session trend at matched intensity and conditions is what's actually informative.
- •Drift requires at least ~45-60 minutes of steady work to be meaningful. Short sessions don't develop enough thermal and cardiovascular strain to produce a stable signal.
Science Context
Cardiac drift was first characterized by Coyle and González-Alonso (2001) as a consequence of progressive dehydration and rising core temperature reducing stroke volume — the heart compensates by increasing rate to maintain cardiac output. The Pa:Hr (or Pw:Hr) decoupling expression, popularized by Friel and TrainingPeaks, reframes drift as a ratio so it's directionally consistent across run/bike/row. Lambert et al. (2008) and Achten & Jeukendrup (2003) established the < 5% benchmark for well-matched aerobic work; values above that on a controlled-condition session are taken as a sign the prescribed intensity has slipped above the second ventilatory threshold.