What is the most accurate? The optical heart rate monitor in the watch or the classical heart rate belt? Our running expert, Jesper, has tested the two methods on the same run. See the heart rate curves and read his recommendations and conclusions here.
A built-in heart rate monitor is today standard equipment in almost all modern sports - and GPS-watches. And a lot of people probably think its nice to get rid of the belt around the chest when you want to measure your heart rate on the run or in the gym. But how well does the optical heart rate monitor in the watch actually work when we compare it to the classical heart rate belt. I intended to find out.
Heart rate measuring: How we tested
In our little un-scientific test, I went for a run for about 10 km where I also tested my heart rate using the two methods.
For the test I used two Garmin Fenix 5 Sapphire, both updated with the latest software version which currently is version 7.6.. I had paired one watch with a Garmin HRM-Run 4 Heart rate belt, while the other watch made heart rate measurements via the optical heart rate monitor built into the watch.
To test the two methods during different types of running, my run included both slow running, pacy running and intervals. The run lasted 52 min. in total and was structured in the following way:
- 15 min. slow running
- 10 min. pacy running (5 km pace)
- 5 min. slow running
- 6 repeats of: 30 sek. hill sprints (with a 1 min. active break)
- 13 min. slow running
Results
The figure below shows the heart rate curves measured during the run. The red curve shows heart rate measured with the Garmin HRM-Run 4 heart rate belt, and the blue curve shows the heart rate with the wrist heart rate monitor in Garmin Fenix 5 Sapphire.
My immediate assessment is that there is not that much difference between the two measurements. In fact, at a first glance they are much more similar than I had expected. I read a number of reviews where the optical heart rate monitor in the watch had received semi-poor ratings but here it looks pretty good overall. The measurements are incredibly alike especially in the three time slots of slow running (0-15 min, 25-30 min and 39-52 min) and in the time slot with pacy running (15-25 min).
Here you can see the average heart rate and max. heart rate for the two methods during the run.
| Wrist heart rate monitor | Heart rate belt | |
| Average heart rate | 157 | 156 |
| Max. heart rate | 184 | 183 |
These measurements are very similar and if you therefore are very interested in the average heart rate over an entire training session, then one method can apparently be as good as the the other.
Heart rate measuring during intervals
On the figure below we have enhanced the heart rate curves measured during the last 6 hill sprints.
On the figure you can see that the wrist heart rate monitor doesn´t react quickly enough to register sudden changes in the heart rate to provide the correct results. The red curve (heart rate belt) makes 6 obvious peaks while the blue curve (wrist heart rate monitor) creates a more messy result.
The first thing I notice is that the heart rate "peaks" later in the wrist heart rate monitor. On the graph, it may not look so bad but in the first 5 intervalthe the wrist heart rate monitor in average peaks no less than 12 seconds later than the heart rate measured by the heart rate belt. And at the 6th and last interval it went completely wrong with a delay reaching 53 seconds. That is far, far too much when you take into account that the sprint lasts only around 30 seconds. During the run I did notice that something was wrong with the wrist heart rate measurement as my heart rate kept rising despite the fact that I was on my way down the hill at jogging-pace...
In addition it can also be seen on the graph that the wrist heart rate monitor in the two central intervals doesn´t register the same minimum and maximum information as the heart rate belt. The wrist heart rate monitor simply doesn´t manage to register the minimum and maximum measurements before the heart again moves up or down.
Therefore I must coclude that the built-in optical heart rate monitor in the watch doesn´t work for short intervals. At least not for this little test..
Why is there a difference between the heart rate belt and wrist heart rate monitor (in the heart rate watch)?
The explanation is probably that even though both methods are indirect measurements of the heart rate, the heart rate belt is much closer to the heart and in addition you are measuring on the basis of two different physiological parametres.
The heart rate belt detects the electric impulses of the heart using 2 electrodes placed on the chest directly above the heart. The wrist heart rate monitor transmits light into the blood vessels running through the wrist and registers how the light, which reflects back, changes when a heart beat sends blood round the body. The two methods uses two different physiological parametres which both indirectly provide the speed of the heart beats. And in this case the technique used by the heart rate belt works faster and better to perceive changes in the heart rate.
Conclusion: Heart rate belt vs. Wrist heart rate measuring
Even though a simple run is a very fragile foundation to conclude anything, our little test does give a very clear result which is clearly in line with what others have previously shown.
- Wrist heart rate measurement overall works quite well. If you primarily are interested in the average heart rate during an entire workout and if you primarily do training sessions where there are no rapid fluctuations of the heart rate, the optical wrist heart rate monitor will provide satisfactory measurements.
- However, if you do intervals or other exercises where the heart rate fluctuates rapidly over a short amount of time, the wrist heart rate measurements are NOT good enugh. The optical heart rate monitor in the watch cannot detect changes in the heart rate fast enough and therefore this method will provide an inaccurate measurement.
- Therefore you should use the heart rate belt when you do intervals or if you generally want as accurate measurements as possible. As it is not possible to place measuring equipment directly on the heart, the heart rate belt (tightened around the chest) is currently the most accurate method to measure your heart rate during running and other physical activity.
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