Baevsky stress index
Also called Baevsky's stress index, stress index, SI, Baevsky index, tension index, index of regulation strain
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.
Roman Baevsky created the stress index in Soviet space medicine to judge how hard the body’s control systems were working. It comes from a histogram of the 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 in a recording: the intervals are sorted into bins 50 ms wide, and three values are read off the chart.
- Mode (Mo): the most common interval, taken as the middle of the fullest bin, in seconds.
- Amplitude of the mode (AMo): the share of all intervals that land in that bin, as a percentage.
- Variation range (MxDMn): the longest interval minus the shortest, in seconds.
The index is SI = AMo / (2 × Mo × MxDMn). When the rhythm turns steady, the intervals pile into one tall, narrow peak: AMo goes up, the range shrinks and the index climbs. Baevsky linked this to 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”) activity. Baevsky and Chernikova (2017) give 80 to 150 as the normal resting range, and say mild stress raises it 1.5 to 2 times and strong stress 5 to 10 times.
Go deeper A worked example
Take six intervals: 800, 810, 820, 830, 860 and 900 ms. Four of the six fall in the 800 to 850 ms bin, so Mo is 0.825 s and AMo is 66.7%. The range is 900 minus 800, or 100 ms, which is 0.1 s. The index is 66.7 / (2 × 0.825 × 0.1), about 404. A real reading uses hundreds of beats, not six.
The index needs a steady resting recording of several minutes; a 2025 study used 5-minute recordings. The range depends on just the two most extreme intervals, so one missed or extra beat can change the result a lot, and 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 must be removed first. Slow drifts in heart rate also widen the range and lower the index.
Baevsky never spelled out every step of the calculation, and programs differ in how they find the mode and bin the intervals (Huizinga and colleagues, 2025). Kubios HRV reports the square root of the index, on which 7 to 12 counts as normal. Compare values only from the same app and recording length.
The index is widely used in Russia but has had much less testing elsewhere; the 2025 study, of 73 healthy volunteers, described itself as “a needed start”. It is a research and wellness measure, not a diagnostic test. What is the Baevsky stress index? explains it in more detail, and HRV in space medicine tells where it came from.