Baevsky Stress Index (SI)

What is the Stress Index?

The Baevsky Stress Index (SI), developed by Russian physiologist R.M. Baevsky, estimates the level of sympathetic nervous system activation by analyzing the shape of the RR interval histogram. Instead of looking at individual intervals or successive differences, SI asks: how concentrated is the distribution?

A narrow, peaked histogram means your heart is beating very regularly (sympathetic dominance, stress). A wide, flat histogram means lots of variability (parasympathetic activity, relaxation).

First: What is a Histogram?

A histogram groups your RR intervals into bins (ranges) and counts how many intervals fall into each bin. It shows the distribution of your heartbeat intervals.

RR intervals grouped into 50 ms bins. The tallest bin is the Mode.

Bin Width Matters

The bin width determines how wide each range is. Common choices are 50 ms or 1/128 s (≈ 7.8 ms) (Baevsky's original). The choice affects how the histogram looks and the SI value.

Wider bins smooth the histogram; narrower bins reveal more structure but change AMo.

      Important: Because AMo depends on bin width, SI values are only comparable when the same bin width is used. Always report which bin width was used alongside your SI value.
    

The Three Ingredients

The Stress Index combines three properties of the histogram:

Parameter	Symbol	What it measures
Mode	Mo	The most frequent RR interval value (center of the tallest bin), in seconds
Amplitude of Mode	AMo	The percentage of intervals that fall in the modal bin (%)
Variation Range	MxDMn	Difference between the longest and shortest RR interval (max − min), in seconds

The three histogram features that define the Stress Index

The Formula

SI = AMo / (2 × Mo × MxDMn)

Where AMo is in %, Mo in seconds, and MxDMn in seconds. The result is in units of 1/s² (though often reported without units).

Intuition

High AMo (tall peak) → most beats are crammed into one bin → very regular → increases SI
Small Mo (fast heart rate) → shorter intervals → sympathetic activation → increases SI
Small MxDMn (narrow range) → little spread → rigid rhythm → increases SI

All three factors point the same way: a stressed, sympathetically-dominated heart produces a high SI.

Worked Example

Suppose we have 100 RR intervals with bin width = 50 ms:

Mo = 825 ms = 0.825 s
AMo = 35% (35 of 100 intervals fall in the 800–850 ms bin)
MxDMn = 950 − 710 = 240 ms = 0.240 s

SI = 35 / (2 × 0.825 × 0.240) = 35 / 0.396 ≈ 88.4

      Check: This is a moderate stress level — the heart has some variability (240 ms range) but a fairly concentrated peak (35%).
    

Visual: Stressed vs Relaxed

A relaxed state produces a wide, flat histogram (low SI); stress produces a narrow, peaked one (high SI)

Interpreting SI Values

SI	Interpretation
< 50	Low stress — parasympathetic dominance, relaxed state
50 – 150	Normal range — balanced autonomic activity
150 – 500	Elevated stress — moderate sympathetic activation
> 500	High stress — strong sympathetic dominance

      Context matters: SI values vary widely between individuals. Athletic, young adults at rest might have SI < 50, while the same person during a meeting might hit 200. Track your own baseline rather than comparing to others.
    

Key Points

SI is a geometric measure of histogram shape — not a statistical moment
Higher SI = more concentrated, peaked histogram = more sympathetic stress
Depends on bin width — always use the same bin width when comparing
Complementary to RMSSD: RMSSD captures parasympathetic activity; SI captures sympathetic dominance
Originally developed for space medicine — Baevsky used it to monitor cosmonauts