Be still my heart.

Wearable fitness technology has come a long way since the basic pedometers of yesteryear.  It's a relatively broad category that can range from simple wearable pedometers to flagship smart watches. Today’s wearable fitness tech is packed with myriad awesome and exciting sensors and software. One of the most cutting-edge categories is heart rate sensors and the methods of sensor fusion they use to improve data quality.  

A few years ago, the idea of a compact wrist-worn heart rate sensor was unheard of, and larger sensors were typically reserved for some gym equipment and medical purposes.  Today's heart rate monitors are an area of innovation for many manufacturers.   

Most wearables now use a heart rate measurement method called photoplethysmography, or PPG.  In its simplistic form, PPG works by emitting a wavelength of light, very often green, from LEDs, then measuring how much light is reflected back using photodiodes.  The higher the blood volume in an artery, the more light will be reflected. In order to effectively measure a heart rate, the sensors take measurements 100s of times each second.  The data from PPG sensors could be closely approximated by a biased sawtooth wave with a small amplitude. The image belows shows an output of a PPG sensor:  

A large flaw with wearable technology and PPG-based heart rate monitors is that the data can be compromised by the movement of the user. To combat this, several companies have come up with unique solutions.  

A solution covered in Fitbit’s patent #8945017 is to use data from other sensors to optimize the method of heart rate monitoring.  In the patent, two methods of heart rate data acquisition are discussed: one if the user is in motion and another if the user is still.  The fitness tracker uses the accelerator information to determine if the user is in motion or not.  In order to provide better data while the user is moving, the device will increase the sampling rate at the cost of using additional power. If motion is not detected, the sensor will sample at a lower rate to conserve power.  The diagram below is from Fitbit’s patent:

Another solution to improve the data from wearable heart rate monitors is from Apple, outlined in patent application WO 2015102589 A1. Apple’s solution uses two separate PPG sensors.  The device then looks at both the information coming from the sensors and compares them in an effort to subtract out the noise.  The patent states ”...two light sensors situated in a line parallel to the direction of the blood pulse wave....”, and looking at the back of an Apple Watch this sensor can be seen in implementation.

In contrast, one of the original companies in the wearable technology market, Polar Electro, uses electrocardiogram (ECG) technology in contrast to the PPG used by other companies. A large benefit to using the ECG technology is that the technology is more accurate when the user is in motion. The higher accuracy is due to the fact that the sensors often produce data that show the heart rate more distinctively. Polar was one of the first companies to market in 1982 with their Sport Tester PE2000 wearable heart rate monitor and the company is still a player in the wearable market. Polar’s patent US6775566 B2 discusses the use for ECG technology in various forms of wearables located on the user's chest and wrist. Today Polar sells wearables with both PPG and ECG technologies.

Wearable fitness technology is a multibillion dollar industry with many large players such as Fitbit, Polar, and Garmin. Now new innovations are coming out multiple times a year. What will the industry look like in 5 or 10 years? Let us know what you think in the comments below!