Heart-rate variability is one of the best daily windows into recovery, but raw numbers are meaningless without your personal context. This tool compares today’s reading to your 7-day rolling baseline and turns the deviation into a clear train-hard, hold, or ease recommendation.
What HRV actually measures
HRV is the variation in time between successive heartbeats. When your autonomic nervous system is in good parasympathetic balance — rested, recovered, not fighting illness or accumulated stress — the intervals between beats vary more, which registers as a higher RMSSD (Root Mean Square of Successive Differences). When the body is under strain, the sympathetic (“fight or flight”) arm of the ANS dominates, inter-beat intervals become more regular, and RMSSD drops.
A single number like “RMSSD = 58 ms” means very little without reference. An elite endurance athlete may baseline at 90+ ms; a sedentary person at 25 ms. Both can be healthy. What matters is the deviation from your own recent baseline, not a population table.
How it works
The tool builds a normal band around your baseline using the variability of your own readings:
band width = baseline × coefficient of variation (default ~6%)
deviation = (today − baseline) / band width (in standard deviations)If you supply a standard deviation from your 7-day readings, it is used directly as the band half-width; otherwise a typical 6 percent coefficient of variation (CV) is assumed. A CV of 6% means that if your baseline is 60 ms, a reading below about 56.4 ms (60 × 0.94) would be flagged as a meaningful drop.
Worked example
Suppose your 7-day rolling average is 65 ms with a standard deviation of 4 ms.
- Today’s reading: 59 ms
- Deviation: (59 − 65) = −6 ms, which is 1.5 standard deviations below baseline
- Interpretation: meaningfully suppressed — the tool recommends an easier day or active recovery rather than a hard training session
Contrast with a day where today reads 68 ms — just above baseline, within normal variation, suggesting you are well-recovered and ready to train as planned.
Training intensity guidance
| Deviation from baseline | Typical recommendation |
|---|---|
| More than 1 SD above | Green: proceed as planned or push slightly |
| Within normal band (±1 SD) | Amber: normal training as planned |
| 1–2 SD below | Amber/caution: reduce volume or intensity |
| More than 2 SD below | Red: active recovery, rest, or easy movement only |
These are guidelines, not rules. Athletes in a heavy training block often have chronically suppressed HRV; a single yellow reading mid-block is less alarming than a yellow reading during a taper week.
Practical measurement tips
- Consistency is everything. Measure at the same time (ideally within 5 minutes of waking), in the same position (supine or seated), before caffeine or any exertion. A measurement taken after rushing to the bathroom and climbing stairs will read differently from one taken lying still.
- Use a chest strap for reliability. Optical (wrist-based) sensors introduce more noise than a chest strap ECG, particularly at rest. For tracking trends, device consistency matters more than absolute accuracy.
- Build at least two weeks of data first. A 7-day baseline computed from only 3 readings is unreliable. The interpretation improves as more days of data normalise your personal baseline and SD.
- HRV complements — it does not replace — subjective feel. If you feel terrible but your HRV is fine, trust how you feel. If your HRV is low but you feel great, proceed with caution and monitor how you respond during warm-up.