Heart Rate Variability (HRV)

Heart rate variability (HRV) is the fluctuation in the time between consecutive heartbeats (RR intervals). It is a key marker of recovery and load.

Definition

“Heart rate variability” (HRV) is the fluctuation in the time intervals between consecutive heartbeats, the so-called RR intervals. It refers not to the heart rate itself but to its fine beat-to-beat irregularity.

This variability is governed by the autonomic nervous system. A dominant parasympathetic branch (rest, recovery) raises HRV; sustained stress, fatigue or illness (sympathetic dominance) lowers it.

Metric	Calculation	What it reflects
RMSSD	Root mean square of successive differences between RR intervals	Short-term parasympathetic activity; the standard for daily readings
lnRMSSD	Natural logarithm of RMSSD	Same information, but normally distributed and therefore statistically easier to compare
SDNN	Standard deviation of all RR intervals	Overall variability; requires long recordings (5 min to 24 h)

RMSSD = √( (1/(N−1)) × Σ (RR_(i+1) − RR_i)² )

Why it matters

HRV is one of the few daily available markers of autonomic nervous system state. It responds to training load, sleep, alcohol, infections and psychological stress. It flags recovery and readiness before performance drops.

What matters is the comparison with your own normal range. Absolute values vary enormously between people (20 to over 100 ms RMSSD). A single night is equally uninformative, because natural day-to-day variation is 10 to 20 %.

HRV only becomes meaningful as a rolling average (typically 7 days) against a baseline built over several weeks. Interpretation depends on whether the current value lies within the individual normal range (mean ± standard deviation).

Position relative to normal range	Interpretation	Consequence
Within, stable	Load and recovery in balance	Train as planned
Within, slightly rising	Good adaptation, growing capacity	Intensity possible
Below, 1–2 days	Acute fatigue, short night, alcohol	Easy session, observe
Below, 3+ days	Sustained strain, oncoming infection, overtraining	Reduce load, find the cause
Well above with very low resting HR	Parasympathetic saturation, often with heavy fatigue	Do not misread as readiness

A high value is therefore not automatically good.

How obseed measures it

obseed calculates HRV from the RR intervals of your activities using over 60 metrics in nine physiological categories, including RMSSD, lnRMSSD and SDNN. This requires RR intervals recorded by a chest strap. On Garmin devices, HRV logging must be enabled.

The analysis places individual readings against rolling baselines, making deviations from the normal range visible. More in the HRV analytics overview.

Example

An athlete has a four-week mean lnRMSSD of 4.10 (equivalent to 60 ms RMSSD) with a standard deviation of 0.15. His normal range is therefore 3.95 to 4.25.

The last seven days:

Day       1     2     3     4     5     6     7
RMSSD    58    63    44    41    39    42    40 ms
lnRMSSD  4.06  4.14  3.78  3.71  3.66  3.74  3.69

Day 3 alone would be an outlier after a late evening. From day 5 onward, however, the 7-day average (lnRMSSD 3.83) is clearly below the lower limit of 3.95, and stays there for several days. Combined with a resting heart rate up 6 bpm, this is a signal to reduce load.

References

Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology (1996). Heart rate variability: standards of measurement, physiological interpretation and clinical use. Circulation.
Plews, D. J., Laursen, P. B., Stanley, J., Kilding, A. E., Buchheit, M. (2013). Training adaptation and heart rate variability in elite endurance athletes: opening the door to effective monitoring. Sports Medicine.