Glossary
Baseline
Also called HRV baseline, personal baseline
Your own usual HRV level and range, built from weeks of readings taken the same way. Comparing a reading with your baseline says more than comparing it with other people.
HRV differs a lot from one person to the next, so a value of 40 ms can be ordinary for one person and unusually low for another. A baseline solves this by making you the reference. It is your typical value, plus the range you normally move within, built from repeated readings under the same conditions: same time of day, same position, same device.
Single days are noisy. In athletes, lnRMSSD lnRMSSD The natural logarithm of RMSSD. Taking the log evens out the spread of RMSSD values, which makes day-to-day changes easier to compare and analyze. More about lnRMSSD typically varies by 10 to 20% from one day to the next as training load changes. Sport scientists therefore lean on averages. Buchheit’s review notes that links between HRV and running performance showed up only when at least 3 to 4 days of readings were averaged, not in single-day values. A common approach is a 7-day rolling average, which a 2012 case study of two elite triathletes used to follow their training.
How this helps you read your numbers:
- One low morning means little on its own. Several days below your usual range deserve more attention.
- Small moves inside your normal range are mostly noise.
- A new device, a new time of measurement or a big change in routine can shift the baseline, so give it a few weeks to settle before you trust it.
The lesson Building your HRV baseline and reading trends shows how to build and read one, and Why does my HRV change so much day to day? covers the daily swings.