The Baevsky stress index Baevsky stress index A number built from the histogram of heartbeat intervals. It rises as the rhythm gets steadier and less variable, which Baevsky read as strain on the body's regulation. More about Baevsky stress index is a number calculated from the gaps between heartbeats that rises as those gaps become steadier and more alike. Roman Baevsky developed it in Soviet space medicine as a sign of strain on the body’s control systems, and on his scale a normal resting value is about 80 to 150.
Baevsky stress index: A number built from the histogram of heartbeat intervals. It rises as the rhythm gets steadier and less variable, which Baevsky read as strain on the body's regulation. More about Baevsky stress index
Where it comes from
Baevsky was a Soviet physician who took part in preparing the first animal and human space flights, and later served as chief researcher at the Institute of Biomedical Problems in Moscow (Tank and colleagues, 2020). His team wanted a quick way to judge how a cosmonaut’s body was coping, using only the heart rhythm, and the stress index was one of their answers. It is still widely used in Russia (Baevsky and Chernikova, 2017). The full story is in HRV in Soviet space medicine.
How it is calculated
The index starts from a list of RR intervals RR interval The time from one heartbeat to the next, measured in milliseconds between the R peaks of an ECG. HRV is calculated from a series of these intervals. More about RR interval , the times between heartbeats. The intervals are sorted into a histogram with bins 50 ms wide, and three values are read from it:
- Mode (Mo): the most common interval, taken as the middle of the fullest bin, in seconds.
- Amplitude of the mode (AMo): the percentage of all intervals that land in that fullest bin.
- Variation range (MxDMn): the longest interval minus the shortest, in seconds.
The index is SI = AMo / (2 × Mo × MxDMn).
A bar chart of RR intervals from 0.6 to 1.2 seconds in 50 millisecond bins. On the left, three bars between 0.60 and 0.75 seconds, with a tall middle bar holding 60 percent of the intervals: a strained, fast and steady rhythm with a stress index of about 370. On the right, eight low bars between 0.75 and 1.15 seconds, the tallest holding 24 percent: a relaxed, variable rhythm with a stress index of about 34.
When the rhythm turns steady, the intervals bunch together. More of them fall in the fullest bin, so AMo goes up, and the range between the longest and shortest shrinks. Both push the index higher. A faster heart also has a shorter mode, which raises the index a little more. Baevsky described the index as a measure of regulatory strain, tied to the sympathetic Sympathetic nervous system The "fight or flight" branch of the autonomic nervous system. It speeds up the heart and makes it pump harder, but it acts too slowly to drive beat-to-beat HRV. More about Sympathetic nervous system (“fight or flight”) side of the nervous system (Huizinga and colleagues, 2025).
Go deeper The numbers behind the diagram
Relaxed: the fullest bin, 0.90 to 0.95 s, holds 24% of the intervals, so Mo is 0.925 s. The intervals run from 0.76 to 1.14 s, a range of 0.38 s. The index is 24 / (2 × 0.925 × 0.38), about 34.
Under strain: the fullest bin, 0.65 to 0.70 s, holds 60%, so Mo is 0.675 s. The intervals run from 0.62 to 0.74 s, a range of 0.12 s. The index is 60 / (2 × 0.675 × 0.12), about 370.
How to read the number
Baevsky and Chernikova (2017) give these values for a resting adult:
| Situation | Stress index |
|---|---|
| Normal at rest | 80 to 150 |
| Mild physical or emotional stress | 1.5 to 2 times higher |
| Strong stress | 5 to 10 times higher |
| Regulation under constant strain, at rest | 400 to 600 |
These figures come from Russian practice, and other researchers report different ranges. In a 2025 study of 73 healthy volunteers, the average index of those aged 16 to 35 rose from 131 lying down to 204 after 5 minutes of standing and 268 after 15 minutes (Huizinga and colleagues, 2025). So a high value just after you stand up, exercise or have a hard day is expected. As with any HRV number, your own trend, measured the same way each time, tells you more than one reading.
Stress index vs RMSSD
The two numbers look at the same beats in different ways. 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 compares each interval with the next one and mostly reflects the calming, vagal side of the nervous system. The stress index looks at the shape of all the intervals together and was meant to track the sympathetic side. Under stress they usually move in opposite directions: the index goes up while RMSSD goes down (Huizinga and colleagues, 2025). The lesson on RMSSD, SDNN and lnRMSSD explains the more common measures.
Its limits
- It needs a long, steady recording. A histogram of a minute or two of beats is rough. The 2025 study used 5-minute recordings.
- Artifacts Artifact An error in a heartbeat recording, such as a missed, extra or mistimed beat, often caused by movement or poor sensor contact. Artifacts can badly distort HRV numbers. More about Artifact distort it. The range uses only the longest and shortest intervals, so one missed or extra beat can change it a lot (Baevsky and Chernikova, 2017).
- Recording length changes it. A longer recording catches slow drifts in heart rate, which widen the range and lower the index.
- Programs differ. Baevsky never set out every step, and programs differ in how they bin the intervals and find the mode. Kubios HRV shows the square root of the index and removes slow trends first (Kubios, 2021; Huizinga and colleagues, 2025).
- It is less studied. Outside Russia the index has had little testing; the 2025 study described itself as “a needed start”. It is not a diagnostic test.