HRV for runners, without the mysticism
Heart-rate variability has become the wellness industry’s favourite number: a single score that claims to know how ready you are. The claim is half true, which is exactly why it needs explaining.
What HRV actually is
Your heart doesn’t beat like a metronome. The gap between beats varies by tiny amounts, and that variation is steered by the autonomic nervous system — the balance between its stress side (fight, flight, race pace) and its recovery side (digest, repair, sleep).
More variability generally means the recovery side has room to work: your body isn’t busy fighting something. Less variability means the stress side is running the show — because of training, illness, alcohol, poor sleep, or a deadline. HRV doesn’t know which. It measures that your body is under load, never why.
The three rules that make it useful
1. Your baseline is the only baseline. HRV differs wildly between people — a 25-year-old and a 45-year-old, or two runners of the same age, can sit at completely different absolute numbers and both be fine. Comparing your number to a friend’s is meaningless. Comparing this morning to your own recent weeks is the entire method.
2. Trends beat mornings. A single low reading follows a hard session, a late dinner, a warm bedroom, or nothing at all. Noise is part of the signal. One low morning is a note in the margin; three or four below baseline in a row is a paragraph. React to paragraphs.
3. Measure the same way every time. Overnight averages from a watch or ring, or a morning reading taken the same way at the same time — either works, but they’re different numbers. Devices disagree with each other more than mornings disagree with each other. Pick one source and stay loyal to it.
What a low morning should actually change
Not panic, and usually not the whole day. If HRV is below your baseline and everything else — sleep, resting heart rate, how the legs feel — reads fine, run the plan. One quiet signal is not a verdict.
When the signals agree — low HRV and a short night and a resting heart rate a few beats up — that’s when today should bend: easier, shorter, or moved. The hard session isn’t cancelled, it’s rescheduled to a day when it will actually make you faster. Training on a suppressed system doesn’t build fitness; it builds a hole you spend next week climbing out of.
A useful mental model: HRV never tells you what workout to do. It tells you how much of the workout your body can currently turn into progress. (For the full morning decision, the checklist is here: Should I run today?)
The honest limits
Worth saying plainly, because most apps won’t:
- Not every setup measures HRV. Some watches keep it inside their own ecosystem, some don’t measure it at night at all. If your coach — human or software — doesn’t have your HRV, it should say so and lean on sleep, resting heart rate, and training load instead. A readiness score built on data that doesn’t exist is theatre.
- HRV lags reality sometimes. You can feel terrible with a fine HRV and feel great with a low one. It’s one witness, not the judge.
- It can’t diagnose anything. Persistently low HRV plus feeling unwell is a reason to see a doctor, not to buy a bigger training plan.
We built Faster around exactly these rules: baselines instead of absolute scores, trends instead of single mornings, and a readiness ring that names the signals it’s built from — and admits it when one is missing. HRV is a good witness. It just shouldn’t testify alone.
Training guidance, not medical advice. If you feel genuinely unwell, HRV charts can wait; a doctor shouldn’t.