How fitness bracelets measure blood pressure, pulse, calories burned

Health 2023
How fitness bracelets measure blood pressure, pulse, calories burned
How fitness bracelets measure blood pressure, pulse, calories burned

A journalist from Popular Mechanics tested various models of these gadgets on himself, and then shared his valuable experience. At the same time, he told how the most balanced cat food is obtained.

He alth under control: how fitness trackers work
He alth under control: how fitness trackers work

The last couple of months I have been walking around with a garland of fitness trackers on both hands, getting technical questions from specialists and now I know almost everything about them. Let's face it, the fitness tracker of my dreams doesn't exist yet, and that's bad news. But before his appearance there is not so much time left - and that's good. And yes, now I always wear these gadgets: they really make my life better.

There are three types of devices worn on the wrist that help monitor he alth. Professional ones like Garmin and Suunto are bought by fitness enthusiasts. The most expensive segment is smart watches, such as the Apple Watch and Samsung Galaxy Watch, which are essentially media devices where fitness is a nice addition. We are interested in the most massive segment of fitness trackers, which you see on the wrist of every second fit person.

Second Generation

Fitness trackers have gone through three stages. The first, which lasted almost until 2014, is conditional pedometers based on accelerometers. Then there were optical sensors that allow you to read the pulse and the concentration of oxygen in the blood. These two functions close almost 99% of all devices that are on the market. I actively used three of them, the most popular in our editorial office: Mi Smart Band 4 (2290 rubles), Huawei Band 3 Pro (4290 rubles) and Samsung Galaxy Fit (6990 rubles). They are almost identical and differ in the screen interface and charging method (Samsung has, for example, wireless). Color touch screen, GPS receiver, different training modes, 50m water resistance, sleep and heart rate monitoring are the main features of these devices. To read the activity, the bracelets use GPS data, built-in accelerometers and gyroscopes, as well as optical heart rate sensors. The latter are the most interesting.

How heart rate monitors work

Optical sensors in the trackers track the blood supply to the vessels located in the hand. By contracting, the heart muscle provokes an increase in blood pressure, capillary blood flow increases, as a result of which more light is absorbed. In trackers, two LEDs illuminate the skin, and a photodiode located between them (a single-pixel camera with a very large light-sensitive pixel) measures the light flux. This method is called photoplethysmography. If you remove such a tracker from your hand and look at the inside, you will see that the LEDs, with rare exceptions, “shoot” in green, since it is it that gives the optimal ratio of penetration depth and reflection. If you use blue light, it will not penetrate the skin. Red and infrared, on the contrary, penetrate too deeply under the skin and at the same time capture the vascular beds, in which pulse waves propagate differently. This is a general theory, the practical implementation is, of course, much more difficult.

The fact is that the signal reflects fluctuations in the blood vessel very poorly and various artifacts are superimposed on it in the form of respiratory waves, reflections from inhomogeneities, the movement of the person himself and side illumination from a loosely fitting bracelet. For signal processing, each company uses its own proprietary algorithms, up to neural networks. In recent years, technology has made a big leap and covers almost all amateur needs. We note right away that all optical sensors built into wrist trackers will always be inferior in accuracy to chest heart rate monitors, for example, Polar, which measure the pulse from the electrical activity of the heart (ECG). But such accuracy is only needed for professional athletes or for medical purposes.

It is almost impossible to estimate the accuracy of the above three trackers without special devices, so we can conditionally say that they all provide approximately similar information. Does this mean that you need to buy the cheapest and not bathe? And here it is not. The fact is that most of the functionality in trackers is hardwired into software, which is usually integrated into the smartphone itself. You can, of course, install such a program yourself, but using the built-in one is much more convenient. Therefore, the most correct choice is to buy a tracker of the same brand as the smartphone. But Xiaomi and Huawei have a nice addition. You can buy smart scales from the same manufacturers in the kit, and every morning they will drop your weight and body fat percentage into a proprietary application that combines this data with tracker data and allows you to monitor your fitness much better. I got myself such scales, and they have kept me in shape for a month.

Third Generation

The third generation of trackers is represented by the GoBe2 model (13,900 rubles) from Healbe. Interestingly, the company has Russian roots and uses a different technology to take data - the measurement of complex electrical resistance, that is, impedance, and galvanic skin response. The device has the size of a decent watch, two large contacts are clearly visible on the reverse side. Since the technology is controversial, we decided to ask Healbe CEO Artem Shipitsyn about it. The most interesting functions of GoBe2 (for which I dragged this tracker for a couple of months) are the measurement of calories consumed and stress level. So, Healbe uses the impedance to implement the function associated with digested calories and the presence of water in the tissues, and the galvanic skin response - to determine the level of emotional status. In fact, the latest technology is the sister of film lie detectors.

Healbe started out as an attempt to make a non-invasive blood glucose meter. The mechanics of measurement looks like this in general terms: when we eat something, sugar must enter the cell, insulin opens the cell, sugar enters, water leaves. This process of osmosis, the transition of water from the cellular to the intercellular space, can be fixed through impedance - the resistance of tissues at different frequencies, and from it, using complex mathematics, you can recalculate the actual consumption of sugar and carbohydrates by the body. Unfortunately, current legislation does not allow household appliances to show blood sugar levels. To do this, the company must conduct a million studies on diabetics with different types of complications and on different diets, says Artem, and the private company simply does not have the money for this. Although Healbe issues such data for internal use. And for me, real-time data on digested and expended calories is available. This is the fundamental difference between the Healbe approach.

Incineration method

How is the standard calculation of calories eaten? Weigh a portion, scan a barcode or enter it manually into a special application and in the end you get the result. Wrong. The program will calculate the calorie content of food, which has nothing to do with how many calories your body actually absorbed. This is where statements like this come from: to burn the calories contained in one cheeseburger, you need to run hard for 35 minutes.

For example, in the laboratories of Royal Canin, a company specializing in diet food for dogs and cats, digested calories are calculated in a more correct way - they measure the calorie content of food eaten and feces (usually by the burning method, which many took in schools), and then subtract the second from the first. As a result, the company manages to produce amazingly balanced feed - the animals on them are both full of energy and not obese.

But these are laboratory measurement technologies, you cannot apply them at home. So, GoBe2 does not count calories in general, which I swallowed with food, but only digested, and this is very important. One can argue how accurate these measurements are, but they show exactly what we need. Researchers from the Medical and Sports Center of the St. Petersburg State Institute of He alth conducted the GoBe test and found that the device measures calorie intake, nutrients and calories burned with an accuracy of 84-93%. This is a very good result for a household appliance, quite suitable for monitoring its own activity.


It is important that I see the process of assimilation and burning of calories in dynamics. For example, today I have a peak assimilation - 317 kcal - fell at 4 in the morning. That is, the body assimilates food at different times, and not directly during a visit to the refrigerator. The GoBe2 software provides quite a lot of different analytics, such as a breakdown of fat, carbohydrate and protein intake, as well as calorie expenditure by activity. Today, for example, I lost 64 kcal while working on this text, 41 kcal while walking the dog, and another 40 kcal - I don't know how. That is, solving differential equations can burn your calories more efficiently than running in the park.

The accuracy of measuring GoBe2 body parameters, as Artem explained, lies within +/- 15% on he althy people. It may not be enough for medical applications, but for weight control and stress levels, it’s just right. GoBe2 gave me a strategy to reduce my weight by 8 kg (in 3 months) and turned my attempts to achieve it into an entertaining game. The first two months I lost completely (minus 2 kg instead of 5 kg), and the program recalculated my goals, extending my time for another couple of months. As you would expect, the maximum result is achieved more with the right diet, rather than grueling workouts. The only thing GoBe2 loses to optical trackers is in compactness and battery life: it needs to be recharged once every two days, while the second - once a week.

Under Pressure

Colleagues, seeing the garlands of trackers on my arm, as a rule, immediately ask the question: do they measure blood pressure? Next in descending order are questions about sugar and ECG. It is understandable, these are the main parameters of he alth and continuous monitoring can save a huge number of lives, and my colleagues already have elderly parents. These questions haunt me too.

Tracking these parameters requires more data than what pedometers and optical heart rate sensors can provide. Impedance sensors extend this set to provide calorie and sugar related data, but are not accurate enough for medical purposes.

There are devices for mobile cardiogram recording, but they must be worn on the chest (since the device must measure the electrical activity of the heart). And people need a compact device in the form factor of a lightweight wristband so that it can be worn around the clock. But the hand, especially the hand, is one of the most unfortunate places to measure anything, in principle, says Artem Shipitsyn.

Some time ago, various companies, with varying degrees of pathos, announced successes in mobile pressure measurement. But today the noise has died down. Why? The maximum that can be obtained is an assessment of the dynamics. All algorithms come down to the following: they try to calculate the pressure from the shape of the pulse wave. But since in parallel there is a huge amount of noise generated by the movement of the hand, it is not yet possible to correctly process the signals. Moreover, if you take an ordinary home crimping tonometer and take several measurements in a row, they will differ from each other. This is the answer to the question, with what accuracy do you measure pressure. The most that can be expected from this technology in the foreseeable future is the emergence of solutions related to tracking dynamics, when the wrist device gives a signal to check the pressure with a traditional blood pressure monitor.

Big Data

But even existing measurement technologies allow us to learn much more about our he alth. While manufacturers almost do not process the gigantic arrays of data generated by wrist trackers. Although the current level of medical systems with artificial intelligence makes it possible to make many fairly accurate diagnoses based on the data received, this requires closer interaction between manufacturers of trackers and medical systems, which is fantastically difficult due to the extremely confusing legislation of most countries on the secrecy of personal medical data. Nevertheless, the number of trackers is growing, data is being collected, processed, analyzed, built and revalidated models. The potential demand for he alth trackers is huge - not only from users, but also, for example, from insurance companies that could change the risks for their client online depending on their lifestyle. In general, wristwatches are not dead, just instead of keeping track of time, they began to keep track of he alth.

Alexander Grek

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