Beatrice Lang
Fitbit devices are equipped with optical heart rate sensors that use flashing green LEDs to detect changes in blood volume in the capillaries of the user's wrist. This technology, known as photoplethysmography (PPG), calculates heart rate by measuring light absorption. While Fitbit's PurePulse technology has been generally well-received, there are concerns about its accuracy, especially during interval training and for people with darker skin tones. Fitbit also offers an ECG sensor on select models, providing medical-grade heart rate readings by measuring electrical signals from the heart.
| Characteristics | Values |
|---|---|
| Type of Sensor | Optical |
| Technology Used | Photoplethysmography (PPG) |
| Mechanism | PPG works by shining a light through your skin and measuring how much light is absorbed. |
| Light Colour | Green |
| Accuracy | Fitbit data has been found to be largely accurate against other sensors. |
| Limitations | May not accurately track heart rates in people with darker skin tones. |
| Factors Affecting Accuracy | Air temperature, stress level, alcohol or caffeine intake, immune response to illness, medication use, cold weather, etc. |
| Resting Heart Rate | Number of times your heart beats per minute when you’re still and well-rested. |
| ECG Sensor | Fitbit Charge 5 and Sense smartwatches offer ECG sensors for medical-grade heart rate readings. |
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What You'll Learn
Fitbit's heart sensor is optical
Fitbit trackers and smartwatches use optical sensors to measure heart rate. This technology is called photoplethysmography (PPG), which involves shining a light through the skin and measuring how much light is absorbed. When the heart beats, more blood flows through the veins, absorbing more light. The sensor on the back of the device, in contact with the skin, detects these changes in light absorption and calculates the heart rate. Fitbit uses green light for PPG as it is absorbed well by blood and works better than infrared light when the user is moving.
The optical sensor technology, PurePulse, uses green LEDs and photodiodes to detect the amount of blood flowing through the wrist. The lights flash hundreds of times per second to gain accurate BPM data. Fitbit's Versa 3 and Sense watches have improved the performance of this technology by increasing the number of light transmitters and receivers, allowing for a larger surface area and more reliable data.
While Fitbit's optical heart rate sensors are generally accurate, they may struggle with rapid fluctuations between high and low heart rates during interval training. Wrist movement and motion can also impact the accuracy of readings. To improve accuracy, it is recommended to wear the Fitbit snugly on the wrist, ensuring it doesn't move around, and positioning it slightly higher on the wrist during exercise.
It is important to note that Fitbit's heart rate tracking accuracy is not equivalent to medical devices. While Fitbit strives for high accuracy, it is always safer to consult a doctor for highly accurate heart rate readings.
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Fitbits use photoplethysmography (PPG)
Fitbit's PPG optical heart-rate sensor can detect changes in blood volume, which are used to determine the heart rhythm. This data is then analysed by an algorithm for irregularities and potential signs of atrial fibrillation (AFib). AFib is a form of irregular heart rhythm that affects millions of people globally, and individuals with AFib have a higher risk of stroke. Fitbit's PPG algorithm can passively assess heart rhythm while the user is still or asleep, and notify the user of any potential issues.
The Fitbit PPG algorithm has been clinically validated by the Fitbit Heart Study, which enrolled 455,699 participants over five months. The study found that the Fitbit PPG detections correctly identified AFib episodes 98% of the time, as confirmed by ECG patch monitors. Fitbit has also received clearance from the U.S. Food and Drug Administration for its PPG algorithm to identify AFib.
While PPG technology is generally reliable, it can be affected by factors such as skin tone, motion artifacts, and device placement. For example, light leaking in from the sides of the watch can disrupt the signal. Fitbit recommends a snug fit on the wrist to ensure accurate heart rate readings. Additionally, the accuracy of PPG readings can vary depending on the company's algorithms and how they interpret the data.
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Fitbits use green light for PPG
Green light is used because it is absorbed well by blood. As blood is red, it reflects red light and absorbs green light. By using green LEDs and pairing them with photodiodes, Fitbit's PurePulse technology can detect the amount of blood flowing through the wrist. The lights are flashed hundreds of times per second to gain the most accurate BPM (beats per minute) data.
However, green light has been found to experience problems when trying to get a signal, especially when reaching deeper into the body to pull out data. Skin tone and tattoos, for example, can cause issues for green light sensor technology. As such, some companies are opting for red light technology, which can recognise water signals at various depths of the body and provide richer data.
Fitbit has acknowledged the potential benefits of red light technology, but for now, their devices use green light for PPG. This is indicated by a flashing green light on the back of the device, which some users may find helpful to know as it confirms that their heart rate is being monitored.
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Fitbit's ECG sensor
Fitbit's heart rate sensor technology is called photoplethysmography (PPG). PPG works by shining a green light through your skin and measuring how much light is absorbed. When your heart beats, more blood flows through your veins, which absorbs more light. Fitbit's PurePulse technology detects the amount of blood flowing through the wrist by pairing green LEDs with photodiodes.
Fitbit's ECG app has received medical device clearances in the U.S. and Europe, with the company conducting a multisite clinical trial in the U.S. to evaluate its algorithm's accuracy. The study showed that the algorithm detected 98.7% of afib cases and was 100% accurate in identifying people with normal sinus rhythm.
It is important to note that the Fitbit irregular heart rhythm notifications feature is not continuously looking for AFib, so it may not identify all instances. Additionally, the results may not be accurate for individuals who take medication or substances that affect heart rate or blood flow.
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Fitbits may be inaccurate for people with darker skin
Fitbits use optical sensors to measure heart rate. This technology, called photoplethysmography (PPG), works by shining a light through the skin and measuring how much light is absorbed. When the heart beats, more blood flows through the veins, absorbing more light. Fitbits use green light for PPG because it is absorbed well by blood. However, green light has a shorter wavelength, making it more readily absorbed by melanin, a natural skin pigment prevalent in people with darker skin. As a result, Fitbits may struggle to accurately measure the heart rate of people with darker skin.
While Fitbit has boosted the current powering its green light to address this issue, researchers, engineers, and other experts have expressed concerns about the accuracy of Fitbits and similar devices for people with darker skin. This issue has broad implications for the growing body of scientific research relying on these wearables and for individuals whose employers offer incentives for using them.
The inaccuracy of Fitbits for people with darker skin also raises broader concerns about the objectivity of new technologies and whether implicit prejudices are shaping their development. For example, Kadija Ferryman, a cultural anthropologist studying the social, cultural, and ethical implications of health information technologies, stated:
> "It really is about the existing bias in medicine that we have already. No matter what [the] technology is, there is evidence that … inserting another tool, no matter how advanced it is, will likely continue on and continue to uphold the existing biases or exacerbate them."
In addition to heart rate tracking, a class-action lawsuit alleges that certain Fitbits inaccurately gauge blood oxygen levels in users with darker skin. The lawsuit claims that Fitbits are falsely advertised as they cannot accurately measure the blood oxygen (SpO2) levels of users with darker skin, despite these consumers paying the same premium price.
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Frequently asked questions
Fitbit devices have optical heart rate sensors that use photoplethysmography (PPG) technology. This involves flashing green LEDs to detect changes in blood volume in the capillaries of your wrist.
The Fitbit sensor uses light-sensitive photodiodes to measure changes in light absorption, which occur when blood flows through your veins. This data is then used to calculate your heart rate in beats per minute (BPM).
The Fitbit heart sensor is not intended to be a replacement for medical devices. While it can provide relatively accurate heart rate readings, it cannot measure heart rate perfectly 100% of the time.
Yes, several factors can impact the accuracy of the Fitbit heart sensor, including skin tone, wrist placement, personal physiology, and the activity being performed. For example, fluctuating between high and low heart rates during interval training can be challenging for optical sensors to track accurately.
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