How do COROS watches measure heart rate?

We all know how important measuring an accurate heart rate during exercise is, no matter the sport or hobby. In this article, we want to provide our best to explain how COROS watches measure heart rate, what limitations wrist-based sensors may have, along with best practice tips to receive the most accurate heart rate readings possible.

Research Gate: How do PPG Sensors Measure HR

The Technology

All COROS watches are equipped with a photoplethysmography (PPG) sensor which is a common technology sensor to measure heart rate within the wearable tech industry. In an over-simplified explanation, the PPG sensor estimates heart rate by emitting light into to area directly into the skin of your wrist and then measuring the amount of light reflected or scattered by blood flow. This process is commonly referred to as optical heart rate monitoring or OHR/OHRM. 

The raw data which the PPG gathers is then interpreted by unique algorithms built for the COROS system which is turned into an accurate heart rate estimation during the process. This requires removing any "noise" that would influence the data to provide an accurate measurement.

Challenges of Optical Heart Rate Monitoring

There are challenges of measuring heart rate from the wrist, the major factor being motion. As described above, since PPG sensors measure blood flow volume, the amount of blood that flows through the vessels in the wrist can be artificially inflated depending on the activity which you are doing.

COROS has designed our activity modes to interpret your data with specialized algorithms taking into account things such as accelerometer data (motion) in order to sift through what may be inaccurate measurements in heart rate so that you can rest assured that the data you are seeing is an accurate estimation. Other factors that may present challenges to measuring heart rate from the wrist with a PPG sensor are as follows:

• Tightness: As described above, the PPG sensor measures light to determine heart rate. If a device is not worn tightly, external light can reach the sensor and influence the readings.
• The "Crossover" problem: When heart rate measurements lock with cadence, particularly on steep downhill runs
• Personal Physiology: The density and number of capillaries near the surface of the skin can play a factor in how well a device is able to read the blood flow through on your wrist
• Tattoos and skin tone: Humans have a wide range of skin tones that all absorb light differently. For example, darker skin and tattoos both absorb more light, while lighter skin absorbs less. 
• Excessive sweat/water: If too much liquid gets beneath the PPG sensor, this will impact heart rate readings
• Temperature: In very cold weather, your body will not send as much blood to your hands and feet as on warm days which may cause heart rate readings to appear lower than what is accurate

Best Practice Tips

To ensure that your watch is able to gather the most accurate data possible, here are a few best practice tips to follow:

• Wear the watch tightly, but not so tight as to cut off circulation. It should fit snug and not slide around with movement.
• Leave a 1cm gap between your wrist bone and the watch. The higher the watch is above the wrist bone, the more flush the surface of your skin, and the less likely the sensor is to be impacted by flexion in the wrist.
• Keep your hands warm: The OHR signal comes from your blood reflection. So, the more blood flow you have, the better signal your watch will receive. The weather doesn’t need to be freezing to impact your OHR accuracy. As long as your hands feel cold, your blood flow is reduced significantly. The best way to solve this is to wear gloves. Another suggestion is always warm up your body ( and your hands ) before starting your workout in the winter.
• Wait for your watch to display a stable HR reading: Just like your body, your OHR sensor requires some warm-up too. It’s recommended to stay on the “Start” page and wait a little longer so the watch can filter some noise by itself. Our watches also provide an alert beep to let you know when it’s ready to go ( if sounds are turned off, you will see a heart symbol that will be solid and steady when ready )
• Select the proper training mode: The regular running, cycling, and swimming algorithms are not specifically built for tracking rapid changes in heart rate. When you train with intervals, please select the Interval Training mode under Run mode for enhanced heart rate tracking sensitivity.

For the Most Accurate Measurements

Optical heart rate sensors are generally considered as an estimate of your effort. However, to provide transparency, athletes following HR-based training programs are recommended to use heart rate straps and armbands ( which can be paired to COROS watches via ANT+/BLE connection ).

Here is why: In the technology world, all sensors including OHR will receive two things: signal and noise. The signal includes everything that’s helpful to your sensor reading, while noise is what keeps disturbing your sensor from providing correct measurements. If the sensitivity of a sensor is enhanced, it often increases both the signal and noise, causing an imperfect result. Most HR chest straps are equipped with technologies to receive less noise and stronger signal resulting in superior heart rate tracking performance than OHR during interval training.

Optical HR sensors rely on the reflection of the remaining green light after certain light got absorbed by the blood under your skin to determine the pulse frequency. COROS watches have four OHR algorithms built-in: running, cycling, swimming, and interval running. They are customized to different types of workouts to give you the best accuracy possible. Since heart rate sensors worn on your wrist are more prone to noise interference, the wearer should take the following steps to increase the signal and reduce noise as much as possible.