Comparative Study of Wearable Heart Rate Monitors

Abstract

Wearable heart rate monitors are widely used these days due to its easy mobility and sleekness. There are a lot of different heart rate monitors available in the market and consumers generally don’t have a clear idea about which device would be better. Objective: Compare the accuracy of heart rate monitoring technology in 4 different watches. Method: Heart rate monitoring feature of four latest models of smartwatches from different brands were compared with a clinical ECG device. These devices are: Fitbit Charge 3, Garmin Forerunner 235, Apple Watch series 5 and Polar Vantage V. Results: The Apple Watch series 5, which uses the ECG technology gave the most accurate reading. Conclusions: ECG based technology gives better results as compared to PPG based technology. Clinical Impact: This study will be helpful for individuals in specifically choosing the best training device for their health-related uses.

Index Terms—Heart rate monitor watch, Electrocardiography, Photoplethysmography, Heartbeat, Fitbit, Garmin, Polar, Apple

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I. Introduction

With the advancement in technology, recent developments have been seen in different fields of study such as education, communication, health, etc. In this fast-paced world, it is necessary to keep one’s heart rate in check in order to avoid any problems in the future. Since the world is developing, these days it is easy to measure the heart rate using the different devices available in the market. These devices may include many different types of activity trackers and heart rate monitors which are inbuilt in smartwatches to monitor various activities. However, there is no certainty whether these devices give accurate readings or not. The heart rate which give accurate readings are generally those used commercially in clinics. These are used for medical purposes. They are, however, too bulky to be used in everyday life since it consists of a lot of wires and circuits.[1] Presently, modern wearable devices are able of providing a lot of important health metrics such as step count, activity reports, Heart Rate Variability (HRV), sleep insights, glucose measure in the body, blood pressure readings, oxygen level of the body, etc.[2] Heart rate monitors are mostly used by sports persons and individuals who practice rigorous exercise. These individuals need to keep a regular check of their heart rate to avoid any harm. For such individuals, smartwatches would be the perfect alternative to the clinical equipment. This report analyses 4 different commercially available heart rate monitoring smartwatches.

A. Heart rate monitor

A heart rate monitor is a device through which one can measure their heart rate instantly and can also record this data. The recorded data can used to study for any discrepancies in the body since hate rate gives data relating to other different parts of the body too. A term relating to heart rate is Electrocardiography (ECG or EKG). It can be referred to as the computing of electrical heart data. [3] Electrical and optical are the two most common methods to record heart signals.

In the electrical methods, the heart rate is calculated by measuring the expansion and contraction of heart chambers which is controlled by electrical signals. These electrical signals generate bio-potential which is measured by ECG (Electrocardiography) sensors. [5]

In optical methods, light-based technology, also known as PPG (Photoplethysmography) is used to measure the blood volume controlled by the expansion and contraction of the heart. [5]

The heart rate data provided by these two methods is basically the same. The heart rate in both these methods is measured using computerized algorithms such as the Pan-Tompkins algorithm. [4]

B. Electrical HRM: Working

Chest straps are the main device employed to implement the electrical HRM. Chest straps work on the principle of ECG. It consists of an electrode pad which is activated when it comes in contact with moisture such as sweat or water. [6]

Figure 2: ECG components

When the individual is doing heavy exercise which results in sweating, the electrodes are activated and they grasp the electrical signals exuded by their heartbeat and send it to microprocessor where this data is analyzed for heart rate. There is a transmitter which serves the purpose of transferring this signal data to the mobile devices. It executes this using Bluetooth and chips. The individual gets the heart rate data on their mobile via the transmitter. Here, the smartphone acts as a receiver.

C. Optical monitors

Optical heart-rate monitors employ the principle of “photoplethysmography' (PPG). It measures the flow of blood using light. The main components of a PPG device are a light source and a photodetector. The light source emits light. This light is reflected to a tissue and then measured by the photodetector. The reflected light gives the data of the blood volume.[7] This data, along with motion information detected by the device's accelerometer, is later processed with algorithms to generate the comprehensible pulse readings. [6]

The light source in PPG sensor may be an infrared light emitting diode or a green LED. Infrared LEDs are generally used to measure the blood flow concentrated intensely in certain parts of body whereas green light calculates the absorption of oxygen in oxygenated and deoxygenated blood.[8]

Figure 3: Working of Optical heart rate monitor technology

The wearable PPG sensors can only be positioned at certain body locations such as the finger, earlobe and forehead. However, as the measurement sites are sites, their degree of accuracy may also change.[9] The most common wearable PPG sensor available in the market is positioned on the wrist. There are a few variations too, such as Moov’s latest fitness[10] tracker which consists of the optical heart-rate monitor on the temple.

D. Bioimpedance technology

Apart from the above discussed two technologies, there has been introduction of a new technology to measure the heart rate. It is known as bioimpedance technology. Bioimpedance measures the resistance of body tissue to minute electric current. This enables the capture of a wide range of physiological signals including the heart rate. This technique is similar to the measuring of the body composition such as fat content. Bioimpedance fundamentally tests the amount of opposition to the electrical current sent through your body tissues. [11]

Figure 4: Fig: measurement of resistance by body tissues in (a) vertical and (b) horizontal direction

The heart pressure is measurement of pulse wave velocity (PWV), i.e. the measurement of the distance and the transit time of the pulse between two arterial sites. [12]

This bioimpedance technology was used by a brand Jawbone for measuring the heart rate. However, the device only measures resting heart rate (just after you wake up) and passive heart rate throughout the day. There’s no option to track the active heart rate during a workout. [10]

This company was shut down in 2017 due to financial issues. The bioimpedance technology is still however, experimented and improvements are being made.

II. Mechanism

A. Fitbit charge 3

Characteristics:

1. Technology name: PurePulse – PPG. Although Fitbit doesn’t mention photoplethysmography, but the technology uses optical heart rate sensors that maintain extended battery life. [13]
2. SpO2 pulse oximetry sensor for sleep insights.. This sensor is useful in making Pulse oximetry, which is a test that measures what proportion of the oxygen-carrying molecules in the blood (called haemoglobin) are actually carrying oxygen. This is known as oxygen saturation or SpO2. Fitbit uses this sensor to generate the readings while sleeping and reports of any abnormality in the oxygen saturation while sleeping.[14]
3. Heart rate zones such as fat burn, cardio or peak zone
4. Automatic exercise recognition
5. All-day calorie burn
6. Real-time pace and distance: connect with phone's GPS to see pace and distance during outdoor runs.
7. Wireless sync to android, iOS and windows devices.
8. 24X7 Heart rate tracking

B. Apple watch series 5

Characteristics:

1. Technology name: ECG App. The Apple watch uses ECG app to measure heart rate. This feature replicates a single-lead ECG with a titanium electrode in the watch's Digital Crown and a layer of chromium silicon carbon nitride on the back of the watch. When you place your fingertip on the electrode, it creates a closed circuit from finger to heart to wrist and allows the watch to record the electrical impulses that make your heart beat. [15]
2. Alerts when noise level increases.
3. Compass and ground alleviation
4. Fall detection
5. International SOS trigger
6. Apps designed to keep health conditions like diabetes, stress in control
7. Advanced workout metrics

C. Garmin forerunner 235

Characteristics:

1. Technology name: Elevate- PPG [16]. Garmin works on the PPG technology to measure the heart rate.
2. Smartphone compatibility with iOS and Android
3. Live track
4. Step counter
5. Move bar: to alert period of inactivity
6. Sleep monitoring
7. Calories burned
8. Distance travelled
9. GPS based distance, time and pace
10. HR Zones

D. Polar vantage V

Characteristics:

1. Technology name: Precision Prime- PPG. Polar technology also works using the optical HRM technology. Its technology uses three methods to provide accurate heart rate: nine optical channels using several colors and wavelengths of light, 3D acceleration and an electrical sensor that measures the quality of sensor-skin contact. [17]
2. 9 LED Optical HR sensors
3. Electronic sensors for skin contact measurement
4. Extensive fitness features
5. Multisport mode to track several different sports in one session
6. Support for swimming metrics and cycling sensors
7. Bluetooth Connectivity

Comparision of heart rate monitors

The heart rate measurements of Fitbit Charge 3, Apple Watch Series 5, Garmin Forerunner 325, Polar Vantage V watch were compared with an ECG device. The ECG device gives readings with an accuracy of about 98%. These readings are compiled from various sources. [18][19][20]

Graph description: Mean changes in heart rate (HR) over time. Time represented in mins vs mean HR bpm on y-axis for both HRM watch and ECG device.

1. Fitbit Charge 3

Figure 5: image source: researchgate.net

Observation: We can see from the graph that the readings of FB Charge 3 are not in sync with that of the ECG device though it has a similar pattern like that of the ECG device.

2. Apple Watch Series 5

Figure 6: image source: exist.io

Observation: We can see from the graph that the readings of the Apple watch are quite in sync with that of the ECG device (Mio Alpha). This may be due to the fact that Apple watch uses the same technology as the ECG device. Therefore, their readings are similar.

3. Garmin Forerunner 325

Figure 7: image source: cnet.com

Observation: We can see from the graph that the Garmin Forerunner cannot cope up with readings of the ECG device under extreme conditions. The average reading is almost close to the ECG readings.

4. Polar Vantage V

Figure 8: image source: gadget.fitness

Observation: We can infer from the graph that the Polar Vantage V is lagging and takes time to warm up. The readings are, however, similar to the ECG readings.

III. Results and discussion

Comparing the accuracy rate and features of the four watches, we observe the following.

FITBIT CHARGE 3 APPLE WATCH SERIES 5 GARMIN FORERUNNER 325 POLAR VANTAGE V

HRM technology PPG ECG PPG PPG

Connectivity iOS and Android iOS iOS and Android Bluetooth

Accuracy Not very accurate Accurate Accurate only for normal activities Accurate but slow

Sleep insight SpO2 pulse oximetry Sleep tracking Sleep monitor Sleep tracking

Problems occurred Incorrect active heart rate reading Can only connect to iOS. Slow heart rate monitor, cannot cope up with chest strap. Lagging heart rate monitor. Takes time to warm up.

Additional features Heart rate zones, all day calorie burn Noise level alertness, Fall detection Move bar, Heart rate zones 9 LED Optical HR sensors

From the result table and graphs, we could observe that all the systems were capable of generating the accurate reading under normal circumstances, i.e. when the user is not doing any heavy exercise. However, some of the watches show inaccurate readings when the user is doing heavy exercise.

Apple Watch Series 5 has stood out among the other 3 with accurate readings even under extreme circumstances.

The heart rate monitoring accuracy of the four devices can be ranked in the following order:

1. Apple watch series 5
2. Polar Vantage V
3. Garmin Forerunner 235
4. Fitbit Charge 3

Challenges in measuring heart rate using wrist-based devices:

Since wrist-based HRM devices mainly depend on testing the light absorption of skin to determine the heart rate, it may face some challenges in order to get an accurate reading. The following are a few such challenges: [21]

• The light of the heart rate sensors may be blocked due to tattoos.
• It may not give accurate reading for dark skinned people since light sensors may not be able to penetrate through the skin.
• Wrong positioning of the band may result in incorrect readings.
• The band needs to be tightly worn around the skin
• Cold weather may infer with the readings of the device

IV. Conclusion

The present study compared four commercially available online heart rate monitoring systems with its main focus on their use in daily life of individuals. The Apple Watch series 5, which uses ECG technology gave the most accurate reading. The athletes and other individuals competing in extremely vigorous activity need to be able to track exertion levels and design training plans that are appropriate, in order to avoid injury, leading a healthy lifestyle and improving their fitness training.

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