HRV (Heart Rate Variability)
Also known as: Heart Rate Variability, rMSSD, Vagal tone
The beat-to-beat variation in time between heartbeats, measured in milliseconds. HRV is a window into autonomic nervous system balance — a relatively high, stable morning HRV suggests parasympathetic dominance and good recovery; a depressed or volatile HRV often signals accumulated training stress, illness, or poor sleep before subjective fatigue catches up.
Formula
rMSSD (root mean square of successive RR-interval differences) is the most-used field metric — calculated from a 1-5 minute morning reading. Daily values are noisy; the actionable signal is the 7-day rolling average vs. your personal 30-90 day baseline. Coefficient of variation (CV) over the rolling window is sometimes added to detect rising day-to-day instability.Example
Athlete baseline rMSSD ~65 ms (7-day average). Three nights of poor sleep + a hard interval session → 7-day average drops to 48 ms (-26% vs. baseline). Plan the next hard session for after the rolling average recovers above ~58 ms, or sub a Zone 2 / mobility day. A single 48 ms reading on one morning is not actionable by itself.
How Afitpilot Uses This
We do not yet ingest HRV from devices (Garmin, Apple Watch, Whoop, Oura, HRV4Training). Subjective readiness via the Hooper-Mackinnon scale is our default because it doesn't require a wearable and works for every athlete. HRV is on the roadmap as an optional objective overlay for athletes who already track it — the intended surface is a 7-day rolling rMSSD trend next to the Hooper drift chart, with low-HRV days flagged the same way as high-Hooper days. We will not auto-modify sessions from HRV alone; the signal informs the plan, the athlete and coach decide.
HRV in practice
| Who / Context | Value | Note |
|---|---|---|
| Healthy adult baseline (rMSSD) | 30-100 ms (wide individual range) | Compare to your own baseline, not to others |
| Trained endurance athlete | Often 60-120 ms | Higher vagal tone is a common endurance adaptation |
| Drop signalling recovery debt | 7-day average ~10% below baseline | Useful threshold to flag a deload or easy day |
| Acute illness onset | HRV drops 24-48h before symptoms | One of the few early-warning signals available to athletes |
| Heavy alcohol night | rMSSD often -30 to -50% next morning | Among the most reliably HRV-suppressing single inputs |
| Active aging (60+) | Baselines lower (~20-50 ms is normal) | Trend changes still matter; absolute values fall with age |
Known Limitations
- •Single daily HRV values have a coefficient of variation around 7-10% in healthy adults — comparable to the noise floor of the measurement itself. The 7-day rolling average is the smallest meaningful unit; isolated dips are not actionable.
- •Measurement context matters enormously. Position (supine vs. seated), time of day, breathing rate, caffeine, alcohol, and movement during capture all shift the reading. HRV from a wrist optical sensor while moving is closer to a guess than a measurement.
- •Absolute HRV values are not comparable between athletes. A trained endurance athlete might baseline at 90 ms while a strength athlete of the same age baselines at 45 ms — both healthy. HRV is only useful relative to your own baseline.
- •HRV can be acutely elevated by parasympathetic rebound after exhaustive training — a one-day spike is not necessarily a green light. Look at the trend and pair with subjective readiness.
- •Chronic stress, poor sleep, illness, alcohol, and hard training all suppress HRV. The signal tells you something is taxing the system; it does not tell you which thing.
Science Context
HRV as a recovery and training-readiness marker has a substantial evidence base in endurance and team-sport literature (Buchheit 2014; Plews et al. 2013). The consensus is that the 7-day rolling rMSSD (or its natural log, lnRMSSD) is more informative than any single value, and that HRV-guided training can produce equal or slightly better adaptations than fixed programmes at matched volumes in trained populations. Strength-sport evidence is thinner. The practical translation: HRV is a useful, not magical, recovery proxy — it adds an objective layer to subjective readiness but does not replace planning judgment, and the cost (daily morning measurement compliance) is non-trivial for self-coached athletes, which is why we treat it as optional rather than core.