SDNN (Standard Deviation of NN intervals) is a key measure of heart rate variability (HRV). It reflects overall autonomic nervous system activity, capturing both sympathetic and parasympathetic influences on the heart.

Step-by-Step Calculation of SDNN

Let’s take an example of five heartbeat intervals (measured in milliseconds):

800, 810, 820, 830, 840

1. Find the Average NN Interval

(800 + 810 + 820 + 830 + 840) / 5 = 820 ms

2. Calculate the Squared Differences from the Average

(800 - 820)² = 400

(810 - 820)² = 100

(820 - 820)² = 0

(830 - 820)² = 100

(840 - 820)² = 400

3. Find the Variance (Average of Squared Differences)

(400 + 100 + 0 + 100 + 400) / 5 = 200

4. Take the Square Root of the Variance

√200 = 14.14 ms

Thus, in this example, SDNN = 14.14 milliseconds, indicating the average variability in heartbeats over time.

How to Measure SDNN

To measure rMSSD, SDNN, and BPM, athletes must use a Polar H10 heart rate strap.