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Heart rate sensor – everything about it and how can it help you

15.6.2021 | 12 MIN

I would say the heart rate sensor is nowadays a basic standard of smartwatch equipment. However, a lot of users reject it with the words "Well, I'm not interested in knowing my heart rate". Such an answer is often more related to (forgive my sincerity) ignorance. It is not just a number on the display, it is an indicator of our health, condition and also how long will we stay here in the world before our body resigns. In addition, it can help us a lot in sports so that we do not destroy ourselves completely (and unnecessarily).

It's that flashing diode on the bottom of the watch or band. Yes, that one. That is the optic heart rate sensor. Although it's just a permanently flashing light, it reveals more than you think. This article will be long. But that's because it's important.

What will you learn about heart rate sensor in this article:

How does the heart rate sensor actually work?

Technology is much simpler than it may seem at first. By pumping in regular intervals, the heart pours blood into our entire body and with that, the blood gets to our skin. The skin changes its color a litlle with the blood flow. You won't notice it with human eye, but the LEDs on the bottom of the watch can do it. The sensor literally detects regular changes in the color of your skin and from that it determines the heart rate.

It may seem like something smartwatch makers came up with, but it's not the case. This method is in medicine called photoplethysmography. Smartwatch manufacturers have succeeded in implementing this technology on a smaller scale and a slightly different principle into smart devices. In medicine, photoplethysmography is used to detect circulatory disorders. Based on the illumination of the skin and the reflection of light, it recognizes the places where the blood stays (varicose veins) or where there is bad blood circulation. To get better results, doctors use ultrasound.

Are there any better or worse heart rate sensors?

Of course. The quality of the sensor depends on the LED diode performance, which is determined by the intensity of light and the efficiency of skin transmission. Optical sensors with more diodes are generally considered to be better as they can effectively illuminate the skin.

The weight of the device is often an important factor in the quality of the recording. That doesn't really relate to the quality of the sensor, but to the whole construction of the watch. We all know something about the law of gravitation, so you have to correctly assume that a heavier body is attracted to the ground more. Therefore, heavier watches may "jump" more on the hand and the results may not be completely accurate. For that reason, you should always make sure that the device is firmly attached to your hand.

What is a resting heart rate and why should I care about it?

The resting heart rate is called RHR for short and it tells us a lot. For example, if we'll live a long life. Low resting heart rate is closely related to longevity. The average heart rate is around 72 beats per minute. But a heart of an athlete can beat just about 40 times per minute when resting.

A low heart rate is not always good. If someone is not exactly an athlete and their resting heart rate is below 60 beats per minute (does not apply when sleeping), we speak of bradycardia, which can be associated with some diseases such as myocardial infarction. The opposite of brachycardia is tachycardia. The heart beats more than 90 times per minute when resting. It can be caused by stress or it can be physiologically iduced and it can lead to a heart attack.

Since the heart of an athlete is strong enough, it is sufficient to just beat about 40 times per minute to get the blood flowing throughout his whole body. The heart of an untrained individual is weaker and has to compensate the strength of the heart with higher frequency. By doing that, the heart wears out over time, so it might not beat as long as a less worn out heart of an athlete.

People with a resting heart rate higher than 70 have almost twice the risk of myocardial infarction than individuals with a lower heart rate.

You can lower your resting heart rate through a healthy diet, more exercise, reduced smoking and drinking alcohol or caffeinated beverages.

Aerobic / anaerobic exercise – why should we monitor our heart rate during sports?

Our body needs fuel – oxygen for any physical activity. And depending on whether we have enough of it or not, we divide the exercise into aerobic and anaerobic. These two types of exercise depend on our heart rate.

If the body has enough oxygen, it is an aerobic exercise. It is typical for lower heart rate and the body primarly burns fat. With this type of training, our body is able to last longer without getting tired or sore.

Like everything, our body has its limitations. And it also applies to the amount of oxygen that our body can receive and use. If our body is not oxygenated sufficiently, we talk about an anaerobic exercise. It is typical for higer heart rate and the body primarly bruns sugars. This type of exercise is more suitable for trained athletes.

A wrongly selected heart rate is a common problem when you try to lose weight by running. It can be a demanding activity for the body, the heart is beating out of control and you mainly burn sugars. That's why it might be a good idea to start with walking first. The body will work at lower frequencies, you will not unnecessarily overload the heart and you will burn fat. When walking is "too easy" for you, you can slowly start running. The watch can calculate the correct heart rate for you or you can use this formula.

I want to start doing sports – Should I choose aerobic or anaerobic training?

If your goal isn't losing weight, but simply to be a good runner, a heart rate sensor may be even more important to you. Aerobic and anaerobic exercise is not only about what we burn, but also in whether we train properly. Learning to run is not just about running slowly at first and then fast. A lot of people may say: "The faster I run, the better I'll be in the future." But that's not how it works.

For a beginner athlete, it is best to train in aerobic exercise. The body forms a "base", ie condition, and gets used to the movement. As my colleague Matěj says: "If you want to run fast, slow down." The moment you feel (yes, running is about your feelings too) that you are comfortable, you run fast and your heart rate is still low, you can gradually incorporate anaerobic training – ideally in intervals. But it can take months to get to this part.

And what is the ideal heart rate for aerobic training? I can't really tell you that. It depends on your age and your past training. But I recommend using a simple calculation.

180 – (your age) = ideal heart rate when exercising. If you are a complete sports beginner, I recommend subtracting another 5. A 25-year-old sports beginner should therefore run at a heart rate of 150. This formula is called the MAF and yes, it is not the best. Everything is about the way you feel. If it's too much for you, slow down. If it's not enough, speed up.

How to run so that my whole body does not hurt? Will monitoring my heart rate help me?

Yes, it is a simple and I believe quite common question. And it is another thing an aerobic training can help you with. During aerobic training, the muscles receive enough oxygen and the body does not create an oxygen debt (the so-called EPOC value). During anaerobic training, this debt is created and the muscles begins to excrete lactic acid (so-called lactate) causing them to stiffen faster and your whole body to feel fatigue.

You may be wondering: And where does the lactic acid come from? Lactic acid is a by-product of the fat burning process.

All this does not happen to such an extent during aerobic exercise. So if you run at a low heart rate, your body won't produce lactate in large amounts and you will be able to run for a longer period of time and you probably won't be extremely sore the next day. You didn't create that much lactate and the oxygen debt is minor. So, can heart rate monitoring help you prevent muscle pain the next day? Yes, it can.

The transition between aerobic and anaerobic exercise is also called the lactate threshold. With aerobic exercise, the body takes care of removing lactic acid. With anaerobic exercise, the body no longer manages to remove lactate which begins to accumulate in the muscles. This leads to muscle stiffness and a growing feeling of fatigue. The lactate threshold can only be measured by a chest sensor, but you can consider the transition between the aerobic and anaerobic heart rate zones as your lactate threshold, which can also be determined by a smartwatch.

Why is anaerobic training important?

Your body has become accustomed to the movement, you feel good while running and your heartbeat does not jump to the heights of heaven. Yes, now it might be the right time to start flirting with anaerobic training as well. Our body can only process a certain amount of oxygen, and if it can't do that anymore, we're talking about anaerobic training. I said that a couple of times. But did you know that the amount of oxygen used can be increased?

The amount of oxygen that an athlete can use during an activity is called VO2 max. And this ability can be improved by anaerobic training.

To increase VO2 max, you have to overdo yourself. You have to get that heart pumpung and get your body into discomfort. And over time, all that suffering will turn to gold – you'll get another step further and improve your ability to get more oxygen – you will increase your VO2 max.

Forcing your body to work "above standard" increases lung capacity and your body's ability to take in more oxygen. For example, when you force your lungs and heart to work at 140% of your VO2 max, it gradually literally forces them to increase their capacity and ability to receive oxygen. The body is preparing to be exposed to this atypical burden again.

8.4.2021
What is VO2 max?

VO₂max is closely related to aerobic and anaerobic exercise as this value (milliliters of oxygen per kilogram of body weight) precisely shows the transition between these exercises. Our body can only process a certain amount of oxygen and the closer we get to this limit, the higher our heart rate is.

So if I workout on the aerobic and anaerobic exercise treshold, I know that I'm currently taking the ceiling amount of oxygen (100% of our VO2 max) into my muscles. Consequently, a trained individual has this treshold at higher heart rate.

Imagine you have a VO2 max of 38 and you run really fast next to an elite athlete who has a VO2 max of 75. Your heart will probably work at a high heart rate, your body will burn sugars and lactate will start to form in your muscles, causing the muscles to stiffen. You can only receive 38 milliliters of oxygen per kilogram of your weight.

In contrast, the elite athlete will receive the same amount of oxygen, but it will only be half of his total capacity as he is able to receive up to 75 milliliters per kilogram of weight. So his heart rate will be low, he won't feel tired or nor his muscles won't be stiffened, and he'll be able to run that fast much longer than you. Therefore, VO2 max is the most commonly used indicator of an athlete's condition.

Heart rate variability – what is that?

The optical heart rate sensor can not only measure heart rate, but also its variability. Heart rate variability (otherwise also HRV) is the time between individual heartbeats. Between inhaling and exhaling, the heart beats a little faster or slower, creating different lengths in the heart rate graph.

R-R interval je doba mezi dvěma údery srdce.

R-R interval is the time between two heart beats.

The heart rate variability is originated in two nerve centers in our brain – the sympathetic and parasympathetic system. The sympathetic nervous system (although by name it acts sympathetically) puts our body on alert. Our heart rate rises and the heart beats more regularly – so the HRV is lower. The opposite is the parasympathetic nervous system, which is the real cool guy that appears when our body is not tense. The heart beats irregularly, only when it needs it to – HRV is then higher.

How does the heart rate sensor measure stress?

Stress is measured by the variability in heart rate described above. When the variability is lower – the heart beats regularly – the watch automatically recognizes that our body is alerted. Our brain has a tendency to keep our entire body ready, including our heart, which does not take a break and does not "rest" from beatings.

Our body relaxes when the heart rate variability is higher. Arrhythmia is sometimes good, exactly. The heart does not need to beat regularly because the brain doesn't feel tense. The watch assumes a low level of stress when recording a higher HRV.

Is sleep monitoring more accurate with a heart rate sensor?

Ano, definitely. Devices without the heart rate sensor (this also applies to smartphones) only use accelerometer for sleep monitoring. The principle is simple – the less you move, the deeper your sleep. But that's not how it works in real life. That's why sleep monitoring without an optical heart rate sensor is completely useless. In reality, it works: the lower the heart rate – the deeper the sleep.

Imagine going to bed after a birthday party where you drank a lot of alcohol. The body sees it as a toxic substance that it has to deal with. All the processes in the body are activated and your heart rate increases.

When you go to bed in this state, you won't be able to reach that low heart rate as normal, because the body has to deal with alcohol. If you monitor your sleep without a heart rate sensor, the accelerometer will determine that you are in deep sleep because you have not moved for a long time. But with a heart rate sensor, the device would not record it as a deep sleep because it knows that you have a higher heart rate than normal and you are therefore in lighter sleep.

So if you are sacrificing your evening to "alcoholic pleasures", expect a higher heart rate during the night and the next day. But it's worth it here and there, isn't it? :)

"Deepness" of the sleep can be determined by the watch pretty accurately, but it is way harder to record REM sleep. REM is typical for the fact we have dreams in this phase and our eyes move a lot. It is also medicaly proven that during REM sleep, the sympathetic nervous system is activated – the variability of our heart rate is therefore lower. The watch is then able to determine REM sleep based on the HRV. During sleep, the HRV gradually increases and the parasympathetic nervous system is activated – the body rests.

Can we tell we are getting sick based on our heart rate?

Let'a be real, our body is smarter than we are. Maybe you knew it, maybe you didn't, but it's important to realize it. Before any disease manifests itself, our body immediately reacts internally to it. Many processes in our body are activated to destroy the "uninvited guest". So our whole body has to be on alert, which requires energy, and our heart has to beat faster. Therefore, increased resting heart rate (RHR) is a good indicator that something is wrong in our body. The same applies for an unusually high heart rate during sports performance.