cycling-training/references/data-confounds.md

Data confounds — when to trust HR, RHR, HRV, and decoupling

Load this doc when: interpreting heart rate, resting HR, HRV, sleep, or aerobic decoupling — i.e. any time a number is about to drive a training decision.

Data/knowledge split: MCP gives you the raw and computed signals (HR streams, RHR, HRV, sleep, readiness, decoupling). This doc governs whether that signal is trustworthy today and forbids naive single-number rules.

---

Core stance: power governs; HR is conditional

Prescribe from power. Heart rate is a response variable contaminated by many non-training factors. Use HR only where it is genuinely informative:

• Aerobic decoupling on steady rides (Pw:HR drift) — valid only on steady efforts.
• Recovery trends (RHR, HRV) — as trends, screened for the confounds below.

Do not use HR to prescribe or judge intensity for intermittent efforts — see the MTB dissociation below.

The confounds (screen every HR-based signal against these)

Any of these can move HR, RHR, HRV, or decoupling independent of fitness or fatigue:

Confound	Effect	Implication
Heat / dehydration	↑ HR, ↑ cardiac drift, ↑ decoupling	High decoupling on a hot ride ≠ poor fitness
Alcohol (prior evening)	↑ RHR, ↓ HRV, disrupted sleep	A bad HRV morning after drinking is not overtraining
Travel (jet lag, altitude, transit)	↑ RHR, ↓ HRV, poor sleep	Expected; not a training signal
Poor / short sleep	↑ RHR, ↓ HRV, ↑ perceived effort	Fatigue signal, but from sleep — adjust the day, don't panic
Illness / stress / caffeine timing	↑ RHR, altered HRV	Interpret in context, not in isolation
Intermittent effort (MTB)	HR lags surges; VO2 stays high on descents	HR mis-reads MTB intensity entirely (see below)

⚠️ The MTB / intermittent-effort dissociation

In intermittent off-road riding, HR, power, and VO2 dissociate: VO2 stays elevated on descents where power drops, and HR lags the short bursts that define the effort. In XCO, %VO2max did not correlate with %HRmax or %MAP (Hays et al. 2018, Front Physiol 9:1062; PMID 30158873; DOI 10.3389/fphys.2018.01062). For MTB, HR is unreliable for intensity — power governs. See mtb-xco-demands.md.

Do NOT hardcode naive rules

These single-number rules are wrong as written because they ignore the confounds above:

• ❌ "Decoupling < 5% = fresh / fit." Decoupling is inflated by heat, dehydration, fueling, and any non-steady effort. It is only interpretable on a steady ride in controlled conditions, and even then as a trend, not a threshold. High decoupling on a hot, long, or surgey ride tells you little about fitness.
• ❌ "RHR + 5 bpm = overtrained." A single elevated RHR morning is far more often alcohol, short sleep, travel, heat, or illness. Overtraining/non-functional overreaching is a multi-signal, multi-day picture (RHR trend + HRV trend + performance + mood + sleep + training load), never one morning's number.

The correct pattern: look at trends across days, corroborate across signals (RHR + HRV + sleep + performance + subjective), and explain-away confounds first before attributing a change to training fatigue.

Using decoupling for durability (the one HR job that's valid)

On a steady endurance ride, Pw:HR decoupling is a reasonable aerobic-durability signal: as aerobic fitness/durability improves, HR drifts less for the same power late in a long ride. Use it:

• Only on steady rides (not intervals, not MTB).
• Only when conditions are controlled (not a heat outlier, not dehydrated, not post-alcohol).
• As a trend across comparable rides, alongside the power-based durability field test (../assets/field-test-durability.md), which is the more direct measure.

Practical workflow

1. Pull the signal from MCP (HR stream, RHR, HRV, sleep, decoupling, readiness).
2. Screen for confounds (table above). If a confound is present, discount the signal.
3. Corroborate across signals and across days before acting.
4. Prescribe from power; use HR only for steady decoupling and recovery trends.
5. When signals conflict and confounds are present, default to caution (easier day) rather than trusting one number in either direction.