Tests of 3 heart rate monitors during interval training

heart rate monitors
The Polar Vantage 2 (left), the Polar Vantage M2 (center) for the Verity Sense sensor and the Polar Grit X Pro (right) for the H10 chest strap.

Heart rate is one of the most difficult parameters to analyze when running. This is because the heart rate takes time to stabilize when accelerating, but also because the measurement is not always precise.

To illustrate this difficulty, I conducted an interval training session with three sensors. These included an optical sensor on the wrist using the Polar Vantage 2, an optical sensor on the arm with the Polar Verity Sense, and the Polar H10 chest strap.

Which of these 3 methods is more accurate? What conclusions should we draw?

Description of the test session

I did a full session to show how the heart rate responds to each phase.

The weather conditions were cool: 6 degrees Celsius, with a light drizzle shortly after the warm-up.

The warm-up jog lasted 16 minutes and 35 seconds, followed by two 20-second accelerations to complete the warm-up. I usually jog a little slower, around 5:00/km, but I ran a little faster to warm up 🥶

The training sessions involved a 2000-meter run to demonstrate the time required to reach a steady-state heart rate. This was followed by 3×1000 meters on an 850-meter loop at increasing speeds within the Savoie Technolac campus, marked every 1000 meters on the ground.

The 2000 meters was run in 6:16 or 3:08/km, which is slightly faster than my half-marathon pace. Then the 3×1000 meters in 2:59, 2:55 and 2:52. The last 1000 meters at 21 km/h was certainly my current MAS.

Recovery was consistently 1 minute and 30 seconds. The first run at 4:10/km, the second involved walking, and the third at 4:39/km, demonstrating recovery effects on heart rate.

This was followed by a 6-minute recovery by jogging. 

Details of the laps measured with the Polar Vantage 2:

RoundDistanceTimePace
13,86km16:354:18/km
20,02km54s
30,11km21s3:16/km
40,17km56s5:21/km
50,11km19s2:46/km
60,20km01:51
72,04km06:163:04/km
80,36km01:304:10/km
91,02km02:592:55/km
100,21km01:347:29/km
111,01km02:552:52/km
120,32km01:294:39/km
131,02km02:522:47/km
140,97km06:386:50/km

The presentation of 3 tools for measuring heart rate

Polar sent us a Verity Sense sensor on the arm, but we did this test 100% independently. I have had the Polar H10 chest strap since 2017 as I believe it is the most accurate on the market. The 3 Polar watches we used for the test were lent to us by friends.

The Polar Vantage 2 was used with its optical sensor on the wrist, while the other two were used to connect external sensors via Bluetooth and collect heart rate data from the chest strap and arm sensor. I could have also used my Garmin Forerunner 935 or my Suunto 9 Peak, which are compatible with the external Polar sensors, but with three Polar watches it was easier to compare with similar TCX files.

The Polar H10 chest strap

The Polar H10 heart rate belt is one of the most accurate on the market, and the recorded heart rate has been validated in several scientific studies. Polar, with its recorded heart rate, has been validated in several scientific studies.  With its pioneering role and scientific approach, Polar remains the benchmark for heart rate measurement.

In this test, heart rate data was briefly missing for around 30 seconds at the warm-up’s start and roughly 20 seconds after the 1000 meters. Periodically, the watch signaled a low battery on the chest strap, possibly due to transmission or electrode reception issues. These brief interruptions didn’t impede our ability to distinguish the three methods, as we had heart rate data for over 95% of the session.

A heart rate monitor consists of two flexible electrodes, which measure the electrical signal from the heart, and a transmitter. The transmitter is often battery-operated and Bluetooth-enabled, as here with the Polar H10. Ideally, the 2 electrodes should be wet before use.

The optical wrist sensor of the Polar Vantage 2

The optical wrist sensor is the most practical, as it does not require any additional accessories.

Photoplethysmography is used, measuring vein volume changes with LEDs emitting and a light detector receiving light. These changes are notable at rest and low intensity but less during intense exertion. Consequently, algorithms may struggle to recognize these fluctuations, leading to less accurate optical sensor values during high-intensity fractional training.

The wristband must fit snugly on the wrist so that daylight does not interfere with the measurement. Hair, sweat and especially tattoos on the sensor can interfere with the measurement.

The Polar Verity Sense armband

The optical sensor on the upper arm uses the same technology as on the wrist. However, the sensor adheres better to the skin in the arm area. The Polar Verity Sense transmitter, which is Bluetooth-enabled, operates with a USB-charged battery

Which sensor is more accurate for heart rate?

In my training, you would expect my heart rate to get higher and higher as I complete the 3 x 1000 meters faster and faster.

During the warm-up and recovery phase, the 3 sensors provide similar results

However, optical sensors consistently lag behind the heart rate chest strap by 10 to 20 seconds during pace changes. This discrepancy arises from the complexity of interpreting light fluctuations, requiring algorithms more time for heart rate estimation.

The chest strap provides the most accurate reading, with fluctuations related to speed. The only drawback is the loss of data in 4 cases totaling about 1 minute 30.

The wrist sensor behaves rather arbitrarily here and only worked half the time in this test. It is impossible to use the data from this split session. It works quite well at lower intensities.

The arm sensor is a very good compromise between the two sensors, which is less uncomfortable than a chest strap. However, above 90% of maximum heart rate, it is less accurate than a heart rate monitor. For long-distance runners who don’t often run in these areas, it seems to be a very good alternative!

We conducted the test in cold temperatures (6 degrees Celsius), causing veins to contract slightly more. This resulted in reduced fluctuation in vein volume and lower accuracy of the optical sensors.

How can you use heart rate during training?

Although it is difficult to draw conclusions from a single training session, I have already observed the same behaviors in my various measurements and those of other runners. We can draw 3 lessons for all runners.

The time it takes to reach a steady-state heart rate value

When I change gear, it takes a while for my heart rate to stabilize. At rest, it typically takes about 2 minutes to reach my steady-state heart rate within the basic endurance range (60 to 140 bpm). These 2 minutes may vary among runners, so it’s advisable not to monitor heart rate until at least 2 minutes of jogging.

Zoom on the interval training part
Zoom on the interval training part

During the 2000-meter run, it took around 3 minutes to reach my steady-state heart rate of 178 bpm. Towards the end, I unintentionally sped up due to a turn just before, possibly accelerating excessively. There are 45 seconds between the start of this acceleration and my maximum heart rate for this section.

In summary, altering the pace does not produce an immediate impact on heart rate. Instead, it requires a relatively lengthy period to stabilize, contingent on the extent of the speed variation and the new pace. An optical sensor only shows this change 10 to 20 seconds later. It is therefore best to rely on your sensations at the start of a change of pace before checking after 3 or 4 minutes whether you are in the right range.

Active recovery is more beneficial than passive recovery

Recovery highlighted rapid heart rate drops while walking, requiring an extra 20 seconds to reach 180 bpm.

In shorter sessions like 10×1 minute, it’s challenging to hit max heart rate if walking during recovery.

For these sessions, it is sometimes better to run a little slower on the fast sections so that you can jog during the recovery phase instead of walking. Or if you walk, at least jog the last 30 seconds of the recovery phase.

This way, the time you spend in the higher heart rate zones is greater and more beneficial for training in the first place.

Invest in an external sensor to monitor your heart rate

You don’t necessarily need to track your heart rate while exercising, you can simply stick to the paces recommended in your training plan.

But if you choose to measure your heart rate, make sure the readings are accurate, otherwise what’s the point? Should I have accelerated during my 2,000-meter run when my heart rate briefly dipped to 140 bpm? Certainly not!

If your heart rate and speed curves match the optical sensor on your wrist, you are one of the lucky few – well done!

If, like me, you’ve always felt that the sensor on your wrist isn’t accurate enough, you’re better off with an external sensor, such as a heart rate chest strap if you don’t mind wearing it on your chest (the Polar H10 chest strap is available for $89), or an optical sensor on your arm (the Polar Verity Sense, also for $89).

In principle, the precision of the optical sensor on the arm is even better in warmer weather, so you have to wait a few months before trying again in midsummer 😉

I have already recorded the “jumps” achieved here 4 times for the heart rate chest strap. This was also an issue with my 2013 Suunto chest strap; it’s crucial to moisten the electrodes to avoid inaccuracies. Additionally, the chest strap isn’t always convenient.

If you stay below 90% of your heart rate, the arm sensor is a viable alternative. Keep in mind a 10-second pace delay.

The maximum heart rate achieved in this test was 189 beats per minute with the chest strap. My maximum heart rate values in a maximal effort test were 193 in 2014 (with chest strap) and 191 in 2021 (with Polar Verity Sense). In the last 1000 meters I was very close to my MAS.

In the RunMotion Coach app, you can find these equivalences between speed and heart rate by entering your resting and maximum heart rate in your profile. Now you know how to optimize your heart rate monitor during training 😉 

I’ll continue training by feel, occasionally checking pace on my watch. Since I’m not fond of chest straps, I’ll often wear an arm optical sensor for zone stats, especially during trail runs. Additionally, I’ll monitor heart rate periodically during trail runs to gauge intensity, whether for endurance or anaerobic threshold splits.

FYI: The devices used, such as the Polar Verity Sense optical arm sensor and the Polar H10 heart rate chest strap, are available on the official Polar website. With the code RUNMOTION you will receive a 10% discount.