Glossary
lnRMSSD
Also called ln RMSSD, ln(RMSSD), log RMSSD, natural log of RMSSD
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.
lnRMSSD is RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next. It mainly reflects vagal activity. More about RMSSD passed through the natural logarithm (“ln” on a calculator). An RMSSD of 20 ms becomes about 3.0, 40 ms becomes about 3.7 and 80 ms becomes about 4.4. The log has no unit of its own, though people often still write “ms”.
The log is there for statistics. In many studies, HRV values do not follow a neat bell curve, and their spread is uneven, while standard statistical tests assume the opposite. Researchers fix this by log-transforming the data first, and many studies now report lnRMSSD for that reason.
In sport science, lnRMSSD is often tracked as a rolling average. A 2012 case study of two elite triathletes followed the 7-day rolling average of lnRMSSD and how much it varied from day to day. Both measures trended down in the athlete who became overreached and stayed stable in the other. That was two people, so treat it as an illustration, not a rule.
Because the log squeezes numbers together, small gaps matter. A rise of 0.3 in lnRMSSD means RMSSD went up by about a third. The lesson RMSSD, SDNN and lnRMSSD explained shows the conversion step by step, and Building your HRV baseline shows how rolling averages are used.