The wearable health and fitness technology industry is booming and for obvious reasons. Not only do we have more information on how to improve and maintain optimal levels of health, but in the present information and digital age, we now have the power to take control through action while measuring our vital signs and fitness progress.
Over the years since the launch of wearable health and fitness devices many users have, however, noted a major flaw in the function of the heart rate monitors integrated into these trackers… the readings just simply aren't the same for everyone1.
The most prominent cases mentioned on various online platforms pertain to individuals with darker skin tones or tattoos who report that the devices cannot detect contact with the skin or read a pulse. When the same devices were used by lighter skinned individuals or those without tattoo ink covering the skin where the device made contact with the skin, they worked perfectly.
While there has been very little research conducted on the reasons for these discrepancies in the consumer fitness tracking device space, the medical community has been facing the same issues in the development of consumer tracking devices for medical purposes.
Trials on biosensing technologies using pulse oximetry which were being tested in medical wearable tech for the remote monitoring of health conditions like heart disease, Parkinson's and PTSD (post-traumatic stress disorder) have reported similar user issues and inaccuracies2 based on skin tone and body mass. These types of medical devices often use the same optical technology as most commercial smartwatches and fitness trackers to record heart rate. As such, further research is being conducted into just how to account for and correct these issues in order to garner approval for use.
What exactly causes the issues with heart rate monitors in health and fitness trackers?
The issues with biosensing devices when it comes to heart rate monitoring lie not in some technologically racist or discriminatory agenda, but rather in how the technology that powers these devices works and the myriad of complexities that lie within human skin. These include everything from the skin's tone to the presence of tattoos and arm hair, perspiration and even body fat.
Health and fitness trackers as well as many wearable medical heart rate monitors use photoplethysmography to determine their readings. This is the process of shining light onto the skin, when light particles (photons) enter the skin they scatter. These are then absorbed by the skin and photodetectors (i.e. light detectors) in these devices then measure how much light leaves the skin.
Blood due to its colour readily absorbs green light, heart rate monitors in health and fitness trackers generally use green light from LEDs to obtain a measurement. The greater the volume of blood present, the higher the green light absorption. This means that absorption levels are higher when the heart pumps and lower in between pumps, and heart rate can thus be measured accordingly.
While this is a relatively simple concept, human skin and tissues are not. They vary from one person to the next and anything that interferes with the amount of light that reaches the photodetectors reduces the signal within these devices which, in turn, leads to inaccurate readings.
Each physiological reaction or state has its own influence on readings, as follows:
- Perspiration creates a layer of moisture between the skin and the LED sensor, creating noise
- Darker complexions contain more melanin (the dark brown to black pigment that occurs in the hair, skin, and iris of the eye, giving these a darker appearance in some people) which absorb more light and interferes with heart rate readings, which sometimes may not even be detected at all
- Tattoos in areas where the tech is worn also absorb greater amounts of light and like darker skin complexion require different specifications to produce accurate readings.
- Body fat may also influence readings as extra layers of fat influence light absorption by creating greater distances between the blood vessels and the light sensors.
Trials of other types of optical technology face similar issues. For example, those that employ near-infrared spectroscopy (NIRS) for non-invasive brain monitoring that require light sources and sensors to be worn on the head have shown that both skin and hair colour affect the quality of the signal and readings. Individuals with characteristically thicker, darker hair cannot get reliable signals because their hair simply absorbs the majority. Newer device have evolved to include fibre-optic lightguides which make contact with the scalp between hairs to avoid this issue and avoid the need to shave the head in order to get accurate results3.
So how exactly did flawed heart rate monitors and fitness trackers make it to market? The most likely explanation is that due to the sheer diversity in the global popular, companies that sell health, wellness and fitness trackers simply have to select specifications for their device sensors that perform optimally for their general target markets.
What is being done to address these issues?
Some fitness device manufacturers take tracking very seriously and have already integrated features to try and counteract the issues. Fitbit has reportedly boosted the current that powers the green light in an attempt to address the issue while Apple has included an infrared light reading approximately every five minutes to provide additional readings4.
Medical research teams, in the meantime, are examining the following possible solutions:
- Using multiple light wavelengths: The theory is that if a certain wavelength is more readily absorbed by the melanin that causes darker skin tones, another may offer better functionality when it comes to those with darker skin tones or tattoos and could be used instead.
- Improved designs offering a better fit: Compromised contact between the skin and sensor can cause interference from outside light sources and movement. Devices with designs that improve contact and shield the sensor from light can help to improve readings.
- Multiple sensors: Instead of relying solely on optical technologies for heart rate readings, next generation devices may employ multiple sensors to combine and cross-check readings and data. This means they could make use of both optical and ECG (electrocardiography) technologies for greater accuracy. Electrical sensors such as those used in ECGs have no issues when it comes to skin tone, although sweat and the presence of hair do have a greater influence on these readings. Thus, a multiple sensor approach may prove to be more effective.
What's for certain is the wearable tech engineers have a considerable challenge to face, especially when it comes to ensuring accuracy in consumer tracking devices for medical purposes. The hope is that physiological factors such as skin tone, moisture and body fat soon pose less of a significant problem in both health and fitness tracking.
1. Shcherbina A, Mattsson C, Waggott D et al. Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort. J Pers Med. 2017;7(2):3. doi:10.3390/jpm7020003
2. Bickler P, Feiner J, Severinghaus J. Effects of Skin Pigmentation on Pulse Oximeter Accuracy at Low Saturation. Anesthesiology. 2005;102(4):715-719. doi:10.1097/00000542-200504000-00004
3. Sen A, Gopinath S, Robertson C. Clinical application of near-infrared spectroscopy in patients with traumatic brain injury: a review of the progress of the field. Neurophotonics. 2016;3(3):031409. doi:10.1117/1.nph.3.3.031409
4. Hailu R. Fitbits, other wearables may not accurately track heart rates in people of color. STAT. https://www.statnews.com/2019/07/24/fitbit-accuracy-dark-skin/. Published 2019. Accessed July 26, 2019.