Comparing chest strap-based and wrist-based heart rate measurements

December 29, 2017

Wrist-based heart rate measurement is on the rise. But is it really time to ditch the good-old chest strap to determine your heart rate? Most new and high-end running GPS watches are either wrist-based and chest strap compatible or at least chest strap compatible. Which one is “better” or at least more suited for your needs?

Wrist-based heart rate measurement is light based. A small LED at the back of the watch emits green light which is reflected more from the wrist vein when there is less blood blood present in the vein. When the heart pumps, more blood arrives in the wrist vein and the green light reflection is decreased. These small “reflective cycles” are detected by the watch. The time between the cycles is used to calculate the heart rate. However, this repetitive pattern analysis is challenging and prone to measurement “noise”. This noise might be introduced by the individual wearing the watch. For example some people’s veins might reflect light better than others. Also the position of the watch matters. It should be worn relatively tight and about 1-2 cm above your wrist. Finally, the “blood signal” measured at the wrist might be weaker than when measured directly at the heart, simply because it is further away from the source. As a result, changes between maximum and minimum reflection might be small and difficult to analyze. This is especially true during situations with rapidly changing heart rates where the reflective “peaks” and “valleys” occur in difficult to analyze irregular patterns.

Chest strap based heart rate measurement on the other hand relies on the detection of an electrical signal emitted from the heart. Weak electricity propagates through the heart muscle in order for it to contract and to be able to pump blood. Measuring this signal directly at the “source” is very accurate and less “noise” is introduced by the biology of the user or other factors. Despite this, also a chest strap needs to worn correctly (tightly, right under the chest muscle).

So which technology is better for your workout? To check this I wore both a chest-strap based Garmin Foreunner 610 and a wrist-based Garmin Foreunner 735XT and went for a 11km run. This run included some sprints in the beginning (100m, 200m, 500m, 200m, 100m) and a 1km uphill tempo run towards the end. Here’s the bottom line: Both technologies are very comparable! Fig. 1 below shows the two differently measured heart rates (FR610 = chest strap, FR735 = wrist). Especially at “steady state” when the heart rate does not fluctuate much both technologies are very comparable. Before the run when I was just standing around both showed about 70 bpm (beats per minute, not plotted). Also the averages of both technologies either over the entire run of 11 kilometers or per finished kilometer are very comparable. Overall both curves look very similar, but the difference is significant when analyzed with a Student’s t-test (p = 0.027, two-tail, paired). Despite this, the difference of on average 1.4 bpm is so low that it has no practical relevance when your heart rate is around 160 bpm during a workout. Even the sprints around kilometer 3 and the long high-intensity hill sprint around kilometer 9 did not introduce a large difference on average.


Fig. 1: Comparison of the average heart rates per kilometer over the course of a 11km run obtained with a chest strap-based technology (FR610) or a wrist-based technology (FR735). Both technologies are comparable when averaged over longer time periods, although the wrist-based measurement seems to measure slightly lower values.

But how do things look like when you zoom into the data? Especially, when concentrating on situations with a lot variability? I did the pyramid sprints in the beginning to find this out. I sprinted 100m, 200m, 500m, 200m and finally again 100m with 200m breaks in between to let my heart rate go repeatedly up and down. For both watches, in Fig. 2, you can clearly see the black lines, indicating the speed, go up and down as you would expect for the five sprints and recovery phases. In both cases also the heart rate, represented by the gray lines, react as expected: Similar peaks as the speed, but with a slight delay. It is, however, surprising that the chest strap-based heart rate seems to be measured in a much smoother way. In addition the recovery phases between the sprints are picked up as expected. The wrist-based technology seems to have trouble here. Heart rate maxima are very similar, but drops during the 200m recovery phases lasting about 1 minute are less pronounced.


Fig. 2: Comparison of heart rate and speed during pyramid sprints obtained with a chest strap-based technology (FR610) or a wrist-based technology (FR735). GPS-based speeds are similar as expected, but the heart rate fluctuates much more when measured at the wrist and does not respond as accurately as chest strap-based measurements to rapid changes, for example during the recovery phases between two sprints.

During short time scales wrist-based heart rate measurements might therefore have more trouble to pick up rapid heart rate changes, especially when large differences between maxima and minima are present. Despite this, the overall performance of both technologies is very comparable and probably accurate when averaged over longer times such as minutes or an entire run, even if high-intensity phases such as sprints or hill running are present.

For you as an athlete this means the following: Wrist-based measurement is a hassle-free and accurate way of gaining insights into training status and workout intensities during longer training runs or training runs involving longer intervals such as Fartleks or hill sprints. If your training requires accurate heart rate measurements during very variable high intensity workouts, you might want to consider using the good-old chest strap. Classical track-based workouts involving 200m or 400m repeats with relatively short recovery phases in between, would fall into this category.

Happy training with all this technology around your wrist and chest!


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