Mean RR

Mean RR is the average of all the RR (or NN) intervals in a recording, in milliseconds. It is the recording’s heart period, and dividing 60,000 by it gives the mean heart rate in beats per minute.

Why this measure exists

Every HRV program prints the mean interval first, because every other number is measured against it. The Task Force notes that terms built on the interval stress that the time between beats is what is analysed, rather than the heart rate, and the two are not related by a straight line. Adding 100 ms to a 500 ms interval takes the rate from 120 to 100 beats a minute; adding the same 100 ms to a 1000 ms interval takes it from 60 to 55. Sacha’s 2014 review shows how this bend in the curve ties HRV to heart rate: the same nervous wobble makes bigger millisecond swings when the heart is slow.

The formula

$$\mathrm{RR}_{\mathrm{mean}} = \frac{1}{N}\sum_{i=1}^{N}\mathrm{RR}_i,\qquad \mathrm{HR} = \frac{60\,000}{\mathrm{RR}_{\mathrm{mean}}}$$

Add the intervals up and divide by how many there are; then divide the number of milliseconds in a minute by that mean to get the heart rate. $\mathrm{RR}_i$ is the $i$-th interval in milliseconds and $N$ is the number of intervals.

Worked example

Mean heart rate step by step

Ten beats, every number shown

The ten intervals, in ms (300 to 2000)

Beat 1 Beat 2 Beat 3 Beat 4 Beat 5 Beat 6 Beat 7 Beat 8 Beat 9 Beat 10

Mean heart rate of these 10 intervals is 71.0 bpm.

Step 4 of 4

Back Start at step 1

1. Step 1 of 4 Start with the intervals

   Beat 1	880 ms
   Beat 2	826 ms
   Beat 3	812 ms
   Beat 4	823 ms
   Beat 5	848 ms
   Beat 6	877 ms
   Beat 7	880 ms
   Beat 8	866 ms
   Beat 9	828 ms
   Beat 10	815 ms

   10 intervals between 11 beats, in milliseconds.

2. Step 2 of 4 Add the intervals up

   Sum of the intervals	8455 ms
   Number of intervals	10

3. Step 3 of 4 Divide by their number

   8455 ÷ 10	845.5 ms

   The mean interval is the heart period: the average time from one beat to the next.

4. Step 4 of 4 Divide a minute by the mean interval

   60 000 ÷ 845.5	71.0 bpm

   Heart rate is the mean interval turned upside down, so a longer interval is a slower heart. The two are not a straight line: 1000 ms is 60 bpm, 500 ms is 120 bpm.

   Mean heart rate 71.0 bpm

With JavaScript on, the steps show one at a time and the beats can be changed.

A row of ten editable heartbeat intervals in milliseconds, then the calculation of Mean heart rate as numbered steps: start with the intervals; add the intervals up; divide by their number; divide a minute by the mean interval. The final step reads Mean heart rate 71.0 bpm. Back and Next buttons move between the steps.

The ten beats of the teaching recording average 845.5 ms, which is a heart rate of 71.0 beats a minute. Shorten every beat by 100 ms and the rate rises more than you might expect, because the curve is steeper at short intervals.

What moves it

Fitness, age, posture, temperature, caffeine, illness, sleep and emotion, through both branches of the autonomic nervous system autonomic nervous system Autonomic nervous system The part of the nervous system that runs the body without conscious control, including heart rate. HRV mostly reflects how it adjusts the heart from moment to moment. More about Autonomic nervous system and through hormones. Adults at rest mostly sit between 60 and 100 beats a minute, well-trained athletes lower.

When it misleads

A mean hides the spread: two recordings with the same mean RR can have very different HRV, which is the whole point of the other measures. Heart rate and HRV also move together, so a fall in HRV while heart rate rises is partly arithmetic; Sacha’s review suggests reading the two together, or correcting one for the other, before concluding anything. And an optical sensor on a wrist reports the pulse interval, which lags the electrical beat and jitters with blood pressure, so its mean is right but its spread can be off.

Where you will see it

The Task Force reports mean NN interval and mean heart rate. Kubios HRV prints Mean RR in ms and Mean HR in beats per minute, with min and max HR as 5-beat moving averages; most apps show heart rate only. HRV vs resting heart rate compares the two numbers, and the lesson What your heartbeat is really doing starts from the interval.