How does a heart rate monitor work and which one should you get?
Heart rate = The number of times per minute that the heart contracts, measured in the number of heartbeats per minute [bpm].
Pulse is not exactly the same as heart rate, although they are closely related.
Pulse = The mechanical pulse of blood flow through capillaries caused by contractions of the heart per minute. It is the pulse that you feel over an artery following each heart stroke.
electrocardiography = The measurement of electrical activity in the heart and the recording of such activity as a visual trace.
Photoplethysmography = or in short PPG is a simple and low-cost optical technique that can be used to detect blood volume changes in the microvascular bed of tissue.
Heart rate monitor (or HR monitor for short) is a device that measures your heart beats in beats per minute [bpm]. Heart rate monitors are arguably one of the most useful training equipment you can get. Knowing your heart rate is essential for efficient training, but it also tells a lot about your overall health. Your heart rate should go up significantly during physical exercise and a rough maximum heart rate can be calculated by subtracting your age from 220 bpm. Physical exercise can help you raise your maximum heart rate but also lower your resting heart rate. An average resting heart rate varies between 60-100 for healthy adults. Cardiovascular fitness will not just improve your physical performance but it also has many health benefits. For example, lower blood pressure, reduction in bad cholesterol, increase in good cholesterol and many others.
Long gone are the days when heart rate monitors were complicated and clumsy to use. Not surprisingly, modern gadgets such as smartphones and sports watches have revolutionized the way we use heart rate monitors. The recordings can be saved for later usage or paired with other data such as power output while cycling and GPS speed. This makes efficient training much easier and accessible to people who can’t get personal coaching.
But how do they work and what should you know about them before getting one?
Types of heart rate monitors
Two most popular types of heart rate monitors are chest traps and optical monitors.
People either love or hate them for various reasons. As the name suggests, chest wraps are long elastic loops that wrap around your chest. On the inside surface, there’s one or multiple electrode pad/s that is supposed to sit tightly against your skin. The actual heart rate “monitoring” and processing the data is done by a transmitter that clips onto the strap. The transmitter sends out the data to be displayed in the main unit, whether that is a sports-watch, computer, or mobile device.
Your heart has a lot of electrical activity going on in it, and chest straps take advantage of electrocardiography to measure the heart rate. Before any electrical activity can be detected, the electrolyte pads need to be moist. The electrolytes located in the electrode pads simply cannot sense anything unless moisture is present. That’s why you are supposed to soak the pads prior to putting on the chest strap. Once the electrolyte pads are moist, the transmitter can detect the electrical activity caused by the contractions of your heart. Because there’s a lot of electrical activity going on in your body at any given time, there is going to be some background “noise” that needs to be processed out first. A small microprocessor inside the transmitter takes care of that before transmitting the heart rate data, usually via Bluetooth or ANT+, to be displayed in the main unit.
The technology used in chest straps is the same used in hospitals for performing cardiology tests. The main difference being that chest straps generally use only one or two electrolyte pads whereas, in a conventional 12-lead ECG procedure, ten electrolytes are placed on the patient’s limbs and on the surface of the chest. If the medical professionals rely on the same technology that is used in your chest strap, it should be able to meet the needs of even the most demanding athlete.
Optical monitors are most commonly found in wrist-based fitness trackers which measure the heart rate directly from the wrist. They are less of a hassle to set up since you don’t need to put on a separate chest strap nor remember to take it with you. Unlike chest straps, they do not detect electrical activity. Instead, they use optical sensors to measure the pulse directly from the skin, a technique known as Photoplethysmography. What basically happens is the sensor uses light to detect the capillaries in your skin pulsating shortly after a heartbeat when the blood passes through. In simple words, it doesn’t feel the pulse like you would with your fingers, but it sees it.
A more complicated explanation is that in the optical sensor, there are three small LEDs which emit green light onto the skin underneath the sensor. The emitted light is not equal but has multiple different wavelengths of light. Different wavelengths behave differently when coming in contact with your flowing blood and either reflect or refract from it. Another sensor in the optical sensor is used to catch all that information. The gathered data needs to be processed before it can be used. The microprocessor in the wearable takes advantage of algorithms and uses other information, such as GPS speed and motion information from device’s accelerometer, to display comprehensible heart rate data.
While optical monitors are most commonly found in wrist-based fitness trackers, it doesn’t stop the technology from being used on other devices too. For example, did you know that there are earbuds that measure your heart rate using the same technology? The wrist is the most common place for the optical monitor to be placed on. However, nothing stops you from having it on another part of your body. For instance, there are products on the market that place the sensor on your head, on a headband.
Advantages and disadvantages
Naturally, both systems have their advantages and disadvantages. To make it simple for you, here’s a quick rundown of the main advantages and disadvantages of both systems.
- Considered more accurate due to their placement closer to your heart.
- Less prone to user error, although make sure to check the Dos and Don’ts!
- Can be bought separately, thus more affordable.
- The displaying device can be placed anywhere, like on the handlebars of your bicycle.
- More of a hassle to set up. Have to be put on separately and the electrolyte pads need to be soaked.
- A separate battery on the transmitter, so it will run out of battery when you need it the most.
- Easy to forget to take with you.
- Commonly built into a wearable, so it’s always with you (if you wear the wearable as your daily watch) and much harder to forget home.
- Convenient, much less of a hassle to use.
- Not quite as accurate as chest straps, but the accuracy has improved significantly over the years and the modern devices are pretty good.
- More prone to user error, because they need to be tight against your skin. Too loose, and the monitor might move around which will mess up the pulse readings.
- Often built into a device, so the cost of getting one is higher.
- In winters a long sleeve shirt/jacket will cover the device, and you can’t wear it on top of any clothing.
Dos and Don’ts
Let’s start with chest straps. Did you know that you should wash the chest strap regularly? Not washing it can increase your chances of getting acne mechanica, for example. It’s not just a hygiene thing either, but dirt can affect the strap’s elasticity and overall functionality. You can also wear the strap incorrectly. Firstly, don’t wear it upside down because the strap may not work quite as well as when worn correctly. Secondly, make sure the strap is centered on your chest. Although, there are individuals that might find the strap working better if the electrolyte pads are positioned on their backsides. Everyone’s different after all. If you are having problems with your heart rate data spiking up like crazy, especially in the beginning of the exercise, you may want to consider using some conductive gel on the electrolyte pads. Sometimes water alone just isn’t enough.
Optical monitors can be quite picky when it comes to wearing it right. The optical monitor needs to sit tightly against your skin with no gaps around it. If it’s too loose, the monitor might move around or light may find its way in to mess with the pulse readings. Don’t adjust it too tight either because that may disrupt the natural blood flow to your wrist and alter the readings. You should always try the device on before purchasing it, especially if you have got fairly thin wrists. A larger sized device might simply be too big for you, and no amount of adjusting is going to make it fit well. A correct sized device when adjusted properly should fit snuggly on your wrist without moving around easily, yet, cause no discomfort whatsoever.
The correct location for the device is around two fingers width from where your hand meets your wrist, or just above your wrist bone. If you place it too high or low on your wrist, it might mess up the readings. The correct tightness and location can be tricky to achieve, but hopefully, it will be easier after reading these tips.
Which heart rate monitor type should you get?
I highly recommend chest straps, especially for cycling. The better accuracy and cheaper price are both nice bonuses, but the main reason is the freedom to place the displaying device wherever you want. Checking your heart rate from the wrist is convenient for running, but quite tricky when holding onto the handlebars. Having accurate heart rate data displayed in your field of vision is a must for efficient training, especially if it involves training at a certain heart rate zone. For active training that demands the best accuracy, a chest strap is a way to go.
If you are not an athlete but rather take a more casual approach to exercise, then wrist-based devices with optical monitors might be just for you. They can achieve good enough accuracy for general activities and certainly fit the bill for someone looking for a simple device to track their heart rate during exercise and everyday life. The latter is one of the selling points for wrist-based devices with optical monitors. Unlike a chest strap that no one will be wearing after training, a sleek looking fitness tracker will always be with you in your everyday life, tracking your health and exercise.
For more info about the accuracy question, see the study conducted by The Journal of the American Medical Association.