With the arrival of the wave of intelligence, intelligent wearable devices have entered a period of rapid development. While wearable items such as smart bracelets, smartwatches, and smart rings are emerging one after another, the functions of the products themselves are also constantly improving. For example, more and more products are equipped with heart rate monitoring function as standard.
Nowadays, the heart rate monitoring function equipped with smart wearable products, represented by smart wristbands and smartwatches, is being increasingly accepted, and some even consider it as one of the essential features for product selection. So, how do intelligent wearable devices monitor heart rate? Today, let's talk about heart rate monitoring.
Why measure heart rate?
As one of the most important vital signs of the human body, each person's heart rate varies depending on age, gender, and other physiological conditions. Generally speaking, the average heart rate of a normal adult at rest is around 75 beats per minute (between 60-100 beats per minute). The same person's heart rate slows down during quiet or sleep, and increases during exercise or emotional stimulation. Under the influence of certain drugs or neurohumoral factors, the heart rate may increase or decrease. People who frequently engage in physical labor and exercise have a slower heart rate.
Specifically, the factors that affect heart rate changes are mainly the three major regulatory systems, namely self-regulation, humoral regulation, and neural regulation. The change in heart rate can directly or indirectly reflect various aspects of human health, which is the significance of measuring heart rate. In the field of intelligent wear, the significance of measuring heart rate mainly manifests in three aspects.
Firstly, in terms of exercise, heart rate can reflect the true information of the user's body during exercise. If the heart rate is too high and the exercise is too intense, and the user's body water evaporates too quickly, then this exercise is not beneficial to the body. If the heart rate is not high enough for mild exercise, the user cannot burn enough calories.
Secondly, in terms of diseases, monitoring whether the resting heart rate is within the normal range, monitoring cardiac arrest during daily activities, abnormal increase in heart rate, etc. can play a timely role in preventing diseases, and even detecting whether the heart rate is abnormal through electrocardiogram monitoring. In addition, the fingertip photoplethysmography can also monitor the pulse wave changes to analyze the pulse rate, blood oxygen concentration, and microcirculation peripheral vascular status of diabetes patients.
Finally, in terms of mental health, the monitored heart rate variability can be used to analyze the assessment of autonomic nervous function, such as mental stress, tension and relaxation levels, and sleep quality.
How to achieve heart rate monitoring?
As an emerging product of the 21st century - intelligent wearable products, due to their specific usage scenarios and wearing requirements, the heart rate monitoring technologies applied in this field currently mainly include photocapacitance pulse wave tracing, abbreviated as photoelectric method, electrocardiogram signal method, pressure oscillation method, image signal analysis method, and other categories.
Photoelectric method
Simply put, this method of measuring heart rate is based on the principle of material absorption of light. It uses the green LED light of an intelligent wearable device combined with a photosensitive photodiode to illuminate blood vessels for a period of time. As the blood is red, it can reflect red light and absorb green light. When the heart beats, the blood flow increases and the absorption of green light increases accordingly; When in the gap between heart beats, blood flow decreases and the absorbed green light also decreases. Therefore, heart rate can be measured based on the absorbance of blood.
Specifically, when a certain wavelength of light beam is irradiated on the surface of the skin, the beam will be transmitted to the photoelectric receiver through transmission or reflection. During this process, due to the attenuation effect of skin muscle and blood absorption, the detector will detect a decrease in the intensity of the light. The reflection of light by the skin, bones, meat, fat, etc. of the human body is a fixed value, while capillaries and arteries and veins constantly increase and decrease with the pulse volume under the action of the heart. When the heart contracts, the peripheral blood volume is the highest, the light absorption is also the highest, and the detected light intensity is the lowest; When the heart relaxes, on the contrary, the detected light intensity is the highest, causing the light intensity received by the light receiver to fluctuate accordingly.
Most smartwatches use photoelectric methods to monitor heart rate, and their obvious feature is that the sensor area is equipped with green LED lights.
There are many types of photoelectric sensors for this measurement principle, and the photoelectric method can be divided into two modes: transmission and reflection, depending on the location of the light signal reception.
1. Transmission photoelectric method
The transmission photoelectric method refers to the generator (emitter) and photosensitive receiver (detector) on wearable devices located on both sides of the tested area (usually fixed by a clamp). The incident light passes through the skin and enters deep tissues. In addition to being absorbed by the skin, muscles, blood, bones, etc., the remaining part of the light transmission is perceived by the photosensitive receiver. According to its principle, this method is suitable for measuring tissues with relatively short distances between the two sides of the human body, such as earlobes, fingers, toes, etc. Representative intelligent wearable products are ear clip heart rate monitors, nail oximeters, etc.
Intelligent wearable products using the transmission photoelectric method are usually fixed with a clamp.
The product of this monitoring method usually adopts a sealed cassette structure in appearance, which can effectively reduce external light interference and improve measurement accuracy and stability. Due to its high signal-to-noise ratio and stable signal, in addition to measuring heart rate, many cardiovascular physiological information such as cardiac function and blood flow can also be analyzed through waveform analysis. The disadvantage is that it is not suitable for use on smart bracelets and smartwatches, and products used in earlobes, toes, and other areas may have an uncomfortable feeling to wear.
2. Reflective photoelectric method
In contrast to the transmission photoelectric method, in the reflection photoelectric method, the generator (emitter) and photosensitive receiver (detector) on the wearable device are located on the same side of the measured area, mainly measuring the reflected light. The advantage of this method for measuring heart rate is that it is very simple and has low requirements for the measurement site. As long as the tissue is relatively smooth and there is less subcutaneous fat, it can be measured almost everywhere, such as the forehead and wrist. Therefore, most wearable devices such as smart wristbands and smartwatches use this method to measure heart rate. Moreover, appearing in the form of smart bracelets or smartwatches perfectly solves the dual requirements of transmission photoelectric method for center rate monitoring and comfortable wearing.
However, although the reflective photoelectric method performs well in a stable state, when the device is worn at the end of the wrist, it will swing up and down like a pendulum as the user walks or moves irregularly, and centrifugal force will cause significant changes in blood volume; When vasoconstrictive pressure and centrifugal force interact in the blood, it becomes more difficult to distinguish the amount of blood in the blood vessels. Therefore, it may reduce the accuracy of heart rate data. In addition, the tightness of wearable devices and the size of human skin blood flow can also affect monitoring accuracy.
ECG signal method
The electrocardiogram signal method is actually the most accurate method for measuring heart rate commonly used at the medical level. In each cardiac cycle, the heart is excited successively by the pacemaker, atrium and ventricle, accompanied by the action potential changes of countless myocardial cells. These changes in bioelectricity are called ECG, and the heart rate can be detected through the periodic changes of ECG. In addition to heart rate, electrocardiogram can also provide information on heart dysfunction, heart disease, and recovery of heart function, as well as the patient's physical and psychological stress.
For intelligent wearable devices, the sensors equipped can determine the user's heart rate by measuring the electrical signal of myocardial contraction. The principle is similar to electrocardiography, and this method has very high accuracy. However, the disadvantage is that the circuit is relatively complex, occupies a large PCB space, and is susceptible to electromagnetic interference. At the same time, the sensors must be close to the skin and placed in a relatively fixed position. Therefore, intelligent wearable products using this measurement method are not common.
ECG lead system.
Pressure oscillation method and image signal analysis method
The pressure oscillation method is mainly applied to electronic sphygmomanometers. The cuff of the sphygmomanometer pressurizes the arm, and the pulsation amplitude of arterial blood vessels is detected through a thin film pressure sensor for AD conversion, thereby measuring blood pressure and pulse rate (heart rate is calculated based on how many pulse waves are present within a certain period of time). The image signal analysis method mainly uses facial images to estimate heart rate. Because the color on the face changes slightly when the heart beats, and there are also subtle movements in the chest and shoulders, the collected images can be used to estimate heart rate and respiratory rate. The Vital Radio launched by the Massachusetts Institute of Technology in the United States uses routers to emit Wi Fi signals. When the signal encounters people or objects around it, it immediately bounces back. Through special algorithms, the reflection speed of each signal can be calculated to determine whether there is a living object. If it is a living object, this product will record the human heart rate and respiratory rate.
These two methods have high requirements for users and are limited to situations where the human body is relatively stationary. Improper methods can result in significantly worse results, and even inaccurate measurement results for patients with certain cardiovascular diseases. Therefore, there are very few products in the field of intelligent wearable devices that use these two methods to measure heart rate.
How to choose intelligent wearable products?
After reading the heart rate monitoring methods introduced above, which smart wearable device should consumers choose?
For smart wearable device manufacturers, this depends on the market positioning of the product. If positioned as a fashionable bracelet, its heart rate monitoring function is often just a decoration. Most of the functions used by users may be time reading, text message push, step taking, etc. At this time, only providing static heart rate function is sufficient. If it is a sports watch, the photoelectric method wins with its convenience and wearing comfort; Mainly used in the direction of movement, the accuracy of heart rate monitoring through photoelectric methods can basically meet the requirements. In addition, for ordinary exercise monitoring needs, a sports watch needs to have a dynamic heart rate function, be able to monitor heart rate in real-time for a long time, and be able to effectively eliminate heart rate signal interference caused by exercise during the exercise process. Therefore, using compensation algorithms such as accelerometers is a better choice.
For ordinary consumers, the heart rate monitoring function of the vast majority of intelligent wearable products in the world is relatively accurate, making it more than enough as a tool for monitoring exercise and fitness for ordinary people. Therefore, you only need to choose based on your estimated price and personal preferences. However, for relatively professional athletes, it is recommended to wear heart rate chest bands with electrocardiogram signals, as these products have higher accuracy and can provide more health indicators by monitoring athletes' heart rate variability.