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Currently all GPS watches (and I would say almost 100% of activity wristbands) incorporate an optical heart rate monitor. But many of you have suspicions on how they work and their reliability; some are true but some are totally unfounded.
The leading manufacturers equip their watches with optical heart rate monitors, sensors that with every new model the brand cares to improve. Those same manufacturers also design and sell chest straps. It's not that they keep selling the old monitors they already offered before OHR became popular, it's that they are making and releasing new versions of those same sensors.
These chest straps can be very reliable during training, but it is not possible to wear them 24/7. Keeping a daily record of our heart rate and knowing our rest heart rate is a key factor in determining the health level not only of those who are athletes, but also of those who aren't.
Monitoring heart rate changes throughout the day can provide valuable information that will help you lead a healthier life and recognize when it's a good time to train and at what intensity.
Optical heart rate monitors and chest straps are the two most common measurement types used in devices today. However, there are differences in the way they are designed and in how they work, and you need to know them to know which one to use depending on what training you are going to do.
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Chest straps have been the most common way to measure heart rate for a long time. However, they have not been without problems.
The common critics they have received has been regarding their comfort. The cheaper models were often made of thick and rigid rubber on which the electrodes were placed. That rubber strap could cause rubbing and burning when running. The more expensive models were somewhat more comfortable, being an elastic textile band with more comfortable electrodes. But despite their better design they could also produce abrasions and burns with use.
The way heart rate chest straps work has nothing to do with their optical homonyms (which we usually carry on the wrist because they are integrated into the watch). The measurement is carried out through a technique called electrocardiography. Basically it's a constant electrocardiogram.
It's a direct method of measurement of what happens in the heart, as it amplifies its electrical impulses. The electrodes (which are those gummy touch areas that you find on your chest strap) are responsible for recording those electrical impulses.
The electrodes must be moistened in order to capture the electrical signal. When we are practicing sports and sweating, we don't need to do anything special to capture the data correctly; but it is for the same reason that it is advisable to wet them when we are going to start training, or we may have problems on cold and dry days.
So electrodes capture electrical signals, but how do they get to our device? Through the transmitter, which is that “pill” we put in the band.
Inside that pill are the electronic components that transmit the signal to the watch, bike computer or mobile phone (usually via ANT+ or Bluetooth). But also the chip that processes the signal it receives from the electrodes and turns it into information that we can understand.
The transmitter simply emits a heart rate figure after analyzing and processing the data it has received from the electrodes. It does this at a rate of 60Hz (that is, once every second).
Powering all this we also find a battery, usually a 2016 or 2032, with a duration of about 8-12 months.
Getting good results from this type of monitors is very simple because there are really not many aspects were we can go wrong. But don't doubt that there might be issues. These are the most common ones:
- The electrodes are dry. If this happens because you are not sweating the electrical conductivity will not be good, so no heart rate recording is done. You can moisten the electrode zone with water or just spit on it.
- Sensor battery is low. Obviously this can affect data transmission due to lack of emission power. Depending on the monitor and its communication, it will usually notify the connected device that the battery is low, no matter if it's connected by ANT+ or Bluetooth.
- Wear down electrodes. Yes, they can also be spoiled and are not eternal. The rubber cracked, or you bent the strap too much damaging the internal wiring.
- Transmission failure because the transmitter broke. Just as the electrodes may get damaged, the transmitter can also do it. It still is a board with electronic components that, like any other electronic board, is subject to breakdowns. Beyond falls or temperature changes, what can usually cause the transmitter to die is a bad seal (from the factory or when changing the battery) that lets sweat inside. That'll kill it almost instantly.
- Interferences. It is no longer common because now communication is digital, but when most of the sensors were analog the simple fact of passing near high-voltage wiring or any signal transmission equipment (telephone antennas, etc.) could disrupt operation.
Bottomline, if you keep the strap electrodes moist and worry about changing the battery every 6-8 months, you'll be doing everything in your power to get good heart rate monitor performance.
Optical heart rate monitor
Currently the optical heart rate monitor is the one we see in 99% of GPS watches or any other wearable. But before we go into specifics let's go to the technical aspects of this matter.
An optical heart rate monitor uses “photoplethysmography” (or PPG) to obtain heart rate data. Simplifying it much and in language that any human being is able to understand, optical heart rate measurement uses light to measure changes in the tissues where the sensor is placed. These changes occur when blood circulates in the body.
Depending on the number of beats the volume of blood in circulation increases. Everything will depend on how much light is absorbed or how much is reflected. Blood reflects less light, so when there is greater volume of blood in circulation, less light will be returned to the sensor.
That's why optical monitors are built surrounded by LEDs, usually green.
Then there are more complex designs such as the Polar Precision Prime heart rate monitor, which features four optical sensors and four electrodes that confirm that the watch has good skin contact, as well as using more colored LEDs to vary the wavelength.
Different manufacturers have been working the sensors in different ways. Another special case is Apple, which uses green lighting for workouts but has an infrared sensor to record heart rate throughout the day.
It's also possible to estimate oxygen saturation in the blood, but that is a completely different topic that may be interesting for another day.
This technique (photoplethysmography), although we have begun to see it in watches recently, is not new at all. Indeed, on more than one occasion, when you went to the doctor, they put a fingertip pulse monitor to measure your heart rate. The technique used is exactly the same.
Then why hadn't it been adapted to the sport yet? Because until recently this technology worked correctly while at rest and static, but as soon as movements were introduced, the accuracy dropped tremendously.
To alleviate this issue, the devices now use accelerometers to measure the type of movement and intensity as well as other elements (for example, in the case of Polar, electrodes to ensure contact with the skin). Accelerometers identify the movements we perform in space to try to discern whether volume changes occur by your heart pumping blood or caused by movements of your arms.
This is what has changed recently. Now all such data can be processed by the device to estimate your heart rate.
You'll see that I've emphasized the word “estimate”. That's what I want to make clear to you, unlike the chest strap, it's not doing a direct measurement, but a series of algorithms try to eliminate “noise” and focus on what it really has to measure: heart rate.
To understand it, it can be said that the sensor on the chest measures the heart rate “live”; while the optical heart rate monitor does it “deferred”, because it is necessary to work all such data to reach a final value.
This is where we find the first issue, since it is necessary to process a quantity of data that is being recorded constantly it is necessary that the device has sufficient processing power to do so and not delay the response of the data. This is why we usually see that optical heart rate monitors take longer to identify changes in intensity during our workouts, for example running intervals.
For that same reason they usually don't show any issues when registering workouts at constant intensity, because what is happening “right now” is the same thing that was happening “five seconds ago”.
Measurement also depends on which part of the body the sensor is placed. No matter where the device is located, the PPG process is exactly the same. What changes is the movements that occur and also that some areas are better than others for measuring.
Polar conducted a study among the different sensors they have on sale. Considering the Polar H10 chest monitor as their golden standard, the Polar OH1+ optical heart rate monitor could have an error of less than 1% in all activities. On the other hand, the sensor we find on watches ranged from 1% error at rest to up to 13% practicing floorball.
Tips on how to use your optical heart rate monitor
Historically, optical heart rate monitors have had bad press. Those bad opinions have not been without motive, but technology continues to advance and improve. The improvement is slow and progressive, but it's there.
One of the reasons we see improvement is because of the increased data processing capacity we have now. There's nothing in common between the processor we found on a TomTom Runner Cardio to the one used on a Garmin Fenix 6X Pro.
But some tips remind to be told regarding the correct use of optical heart rate monitors:
- Wear the strap tightly
- One finger above the wrist bone
- Centered on the arm, neither higher nor lower
Carrying the device correctly positioned and tight on the wrist, the movement should not be an issue. Here the adjustment is tremendously important.
If you put the strap too hard, it is clear that it will not move out of place, but you can interfere with blood flow and have false data, because blood does not flow naturally into your wrist. And if he's too loose it'll move out of its place.
Basically, firm but without becoming annoying. I personally can put a finger under the sensor by pulling the watch up, and when it comes to workouts I tighten the strap by one hole, and then I'm no longer able to fit that same finger under the sensor.
Finally, it should not be forgotten that there are other factors that can make the measurement more or less accurate. Every person is different and what it works for me may not for you. Body hair, skin tone, tattoos... everything can affect heart rate measurements.
Which one should you use?
It all depends on the type of training and the data you want to get. If you want to have all the data possible and in the most precise way, the chest strap is the option you should choose, especially in workouts with changes in intensity such as HIIT or running intervals.
But the fact is that some people won't find comfortable with straps on the chest, especially women because of sports bras. In that case we can sacrifice some precision at certain times in exchange for greater comfort.
For constant pace long runs you can perfectly rely on the optical heart rate monitor. Usually you will do it at a fairly stable pace so there are no changes in intensity or movements, and in these conditions the optical heart rate monitor is usually just as reliable as the one on your chest.
However, if you go cycling, you have no choice but to opt for the chest strap. The movements and vibrations that occur are totally different from those we find in running or any other sports, so it is common for the algorithm to be confused. Road vibrations or road bumps have nothing to do with typical sports movements, so in most cases an optical heart rate monitor is not the best solution.
In the same way, for strength exercises in the gym the chest strap is a better option because by exerting strength with the arms we can alter the way blood flows. However, in this type of exercise, heart rate data is less important.
It can be important while doing Crossfit workouts, in which case it is better to use the chest strap.
The best heart rate monitors
Well, after all this information and what you have learnt, what do you need? You have quite a few options from different manufacturers.
Best chest straps
As for reliability, they are all very even. In fact I have never noticed differences between different manufacturers, as long as they are reliable. I have had problems with Chinese straps purchased on Aliexpress, even if I use a transmitter of a well known brand.
It's what has happened to me with a 4iiii HR monitor that I used occasionally on the bike trainer with a strap I bought on that site, when it works well everything is fine, but when it goes wrong it blows all the training data.
As for things you need to look at in a chest strap, it will depend primarily on which watch you use.
It's the first thing you need to keep in mind. There are currently two transmission methods: ANT+ and Bluetooth. Depending on the manufacturer of your watch you will have to use one or the other, although it is less and less important because the most modern monitors all incorporate dual ANT+/Bluetooth connectivity.
In fact, my recommendation is to opt for such sensors to ensure compatibility in the future (in the hypothetical change of watch brand), unless you are looking for other types of functions.
Heart rate monitors in the chest are not only used to measure heart rate. Its usefulness has been growing over time and it is now common to find HR monitors that provide additional running metrics (Garmin HRM-Run and HRM-Tri) or memory to store heart rate data and transmit it afterwards (Polar H10, Garmin HRM-SIM and HRM-Tri).
We can also take into account the number of simultaneous connections that we want to have via Bluetooth, in case we want to connect to more than one device at a time, for example with the phone and the bike computer.
The best chest straps
Right now my favorite heart rate monitor (the one I almost always rely on), as long as you don't need special functions, is the Polar H10. Reasons? A very comfortable strap with a handy clip, dual ANT+ and Bluetooth HR monitor, two Bluetooth channels for transmission to two devices, 5kHz analog band and internal memory for use independently without any other device.
If you want to save a little money and do not need so many features, within the same brand you have the Polar H9. The strap and hook are simpler, it has no internal memory and although it is compatible with ANT+ and Bluetooth simultaneously, it does not have dual Bluetooth band.
But it is true that it will cover the use of the vast majority of users, and if it does so by spending less... then that's a win-win.
The other HR monitor I use regularly, alternating it with the Polar H10. And it's the one that I bring with me in triathlon races.
The main issue with the Garmin HRM-Tri is that it doesn't have Bluetooth connectivity. It broadcasts only via ANT+. Then why is it here? Because if you do triathlon, it's THE HEART RATE STRAP.
And it is because if you look at the starting line of a triathlon (especially medium or long distance) the vast majority of participants will have a Garmin on their wrist. With Garmin's high-end multisport watches, we have access to two important functions:
- Advanced running metrics
- Internal memory on the sensor that synchronizes at the end of the workout (useful for the swimming leg)
Plus, with advanced race metrics, you can access the Garmin Running Power thanks to a Connect IQ app. I'm looking forward to the dual-connectivity version arriving very soon.
The HRM-Dual lacks all the advanced features of the HRM-Tri or HRM-Run, but in return it can boast dual connectivity. It is very similar to the Polar H9 we have seen earlier and, in fact, they sit at very similar prices.
Basically Garmin launched this chest strap for those who want to train using Zwift (syncing via Bluetooth) and also want to record or view training data on their bike computer or watch (via ANT+).
Optical heart rate monitors
Talking about optical heart rate monitors is primarily limited to what you have on your own watch and depends on the manufacturer of the watch. There are external monitors that also use this type of measurement, although this is not something as common as chest straps.
Grading an optical heart rate monitor integrated in the watch as reliable or unreliable is not as easy as saying that one brand's is better than another, for the simple fact that there are other factors that should also be taken into account.
The main one is the weight of the watch. The higher the weight of the watch, the more difficult the heart rate measurement will be, because due to the larger mass it will be easier to move on the wrist. So in that case it's more a question of looking at per watch performance, rather than determining that those of one brand are always superior to those of another.
Of the last watches I've reviewed there have been two with which I have had very good results in while running: Suunto 7 and Apple Watch Series 5. If you notice there is one common denominator in both cases, and it's that they are both smart watches.
Why have they worked so well in these two cases, especially in terms of interval running? Because both have much more powerful processors than what we can find in traditional GPS watches (at the cost of the battery life of the watch), which makes them able to process more data and at higher speed. This means that when comparing the delay in the increase or decrease of heart rate with the chest strap, it is shorter than usual in this type of sensor.
As I was saying, there are external optical heart rate monitors, with several manufacturers offering them: Wahoo, Polar or Scosche, for example. All of them are compatible with ANT+ and Bluetooth, but as special functions they only offer the memory to record workouts autonomously.
The best optical heart rate monitors
Another of my favorite monitors that accompanies me in many of my reviews is the Polar OH1+. It's a very reliable sensor (almost par with the Polar H10) that stands out for its lightness and comfort, but also because it allows you to record an activity autonomously very easily. Simply by double-clicking the button it has it will start recording, and then you can download it to your phone. If you only want the heart rate data of your workout it's the perfect choice.
It can be placed in various places (it even incorporates an adapter to place it on your swimming goggles and measure your heart rate on the forehead), but where it will be most common to see it is on the forearm.
Here it is barely subject to movement or vibration and offers very high performance regardless of the kind of workout, including use in gym or cycling.
Wahoo TICKR Fit
It's very similar to the Polar OH1+, except that it does not have a memory to store a workout. In return what it offers is a much greater battery life, reaching up to 24 hours of use.
I would only opt for Wahoo's sensor if that level of battery life is really necessary for your use case (OH1+ has 12 hours).
I hope you've learnt everything about the operation of heart rate monitors and the difference in direct ECG and indirect measurement made by PPG sensors. However, if you still have any doubt, here are the comments so you can shoot me your question and I'll answer right away.
And with that, thanks for reading!