How does Apple Health measure heart rate?

In this brief guide, we are going to answer the question “How does Apple Health measure heart rate?” and provide you with more information regarding heart health.

How does Apple Health measure heart rate?

The iPhone has no inbuilt sensors to practically measure your heart rate, however, the Apple Watch can measure your heart and feed the information to the Apple Health app on your phone. Apple Health can also retrieve information from other apps that measure the heart rate for you. Examples of apps that can measure your heart rate using just your iPhone include

  • Instant Heart Rate
  • Heart Rate Plus: Pulse Monitor

You can also use different fitness trackers that monitor your heart rate for you apart from the Apple Watch. Following are a few examples of devices that are compatible with your Apple iPhones and also track your heart rate.

  • Xiaomi Wear
  • Garmin Watch
  • Fitbit
  • Fossil Smart Watch
  • TicWatch
  • Motorola Watch
  • Huawei Watch

How does Apple Watch measure heart rate?

The Apple Watch is fitted with small sensors that track your heart rate. The sensors include infrared LEDs, visible-light LEDs and photodiodes that work in a combined manner to measure your heart rate. The average resting heart rate is between 60-90 (BPM) beats per minute. During workout or strenuous physical activity heart rate rises and can go over 150 BMP. The maximum heart rate is calculated by subtracting your age from 220. 

Max heart rate = 220  ̶  your age.

The mechanism behind how Apple Watch measures heart rate?

Apple Watch uses what is known as photoplethysmography (PPG) to measure heart rate. The word itself makes it seem like a very complicated mechanism but in essence, it is based on very simple facts. When light travels through body tissues, it is absorbed by skin, bone and blood. This is why we don’t appear transparent!

Blood is red in colour, which means it absorbs all other light but red. Apple Watch uses green LEDs combined with light-sensitive photodiodes to detect the amount of blood flowing through your wrist at a given time. When your heart beats, blood flows through the capillaries. After a heartbeat, the blood flow increases slightly, which translates to more green light being absorbed. 

Between consecutive heartbeats, the blood flow is comparably lesser, so the amount of light absorbed is also slightly lesser. Photodiodes are sensors that detect the reflected light from the wrist. When blood flow is more, more light would be reflected as compared to when the blood flow is lesser. 

By flashing the green LEDs hundreds of times in a minute and using a photodiode to detect the different intensities of light reflected back, a repetitive pattern of highs and lows is observed. The highs and lows are detected due to the increase and decrease in blood between consecutive heartbeats. 

This data is then analysed using algorithms and processed to present the heart rate. The optical sensor supports a heart rate reading between 30 and 210 BPM. In addition to this, the optical heart rate sensor is designed in a manner to compensate for low signal levels by increasing the brightness and sampling rate of LEDs, so that better readings can be observed.

The optical heart sensor also can also use infrared LEDs. Apple claims that it uses infrared light to measure your heart rate in the background, and when it gives random heart rate notifications. But if this infrared system isn’t giving a reliable reading, your smartwatch switches to the green LEDs. Apple Watch says “ it uses green LED lights to measure your heart rate during workouts and Breathe sessions, and to calculate walking average and Heart Rate Variability (HRV).”

How accurate are Heart Rate readings?

To get an accurate reading for your heart rate, make sure you wear the watch in a snug fit, too loose and it might not touch your skin for sensors to take a heart rate reading.

Apple Watch takes heart rate readings every ten minutes, these are termed as background readings. These readings along with data about your movement are used to calculate the number of calories you burn in a given day. You can easily view these calories on your Apple Health app on your iPhone.

However, there have been studies, that have suggested that the accuracy for the heart rate measurement is fine while used at rest, but during exercise, the heart rate measurement accuracy declines considerably.

Users with heavy tattooing around the wrist region have also complained about the Apple Watch not being able to measure their heart rate. This is due to the heavy ink absorbing all light from the LEDs.

Why is Heart rate important?

Heart rate is considered a vital sign in the medical field. It is important to know that your resting heart rate is within the suggested limit of 60-90 beats per minute. A higher heart rate can be a sign of a serious health issue and has been shown to be associated with a greater risk of heart diseases. Athletes and fitness experts are prone to have a lower resting heart rate. It is good practice to keep track of your resting heart rate and consulting a doctor if you observe any abnormalities. Another metric now commonly used for cardiac health is Heart Rate Variability (HRV).

What is Heart Rate Variability?

Along with Heart Rate, the Apple Watch also measures Heart Rate Variability. Heart rate variability or HRV is the variance in the time interval between consecutive heartbeats. Normally a heart does not have an even ticking interval like a clock. If looked closely, there is constant variation between heartbeats in the order of milliseconds.

HRV varies from person to person. Studies have shown that people with a higher HRV have better cardiovascular fitness and are more resilient to stress. Mindfulness, meditation, sleep, and physical activity can help improve your HRV. A significant drop in HRV may be an indicator of your body experiencing illness, stress, or depression or anxiety.

How can I view my HRV using Apple Health?

 To view your HRV on the Apple Health

  1. Open the Health app on your iPhone.
  2. Select the Health Data tab.
  3. Tap on the Heart subsection.
  4. Select Heart Rate Variability.

Conclusion

In this brief guide, we answered the question “How does Apple Health measure heart rate?” and provided you with more insight regarding heart health.

If you have any questions or comments please let us know.

References:

https://support.apple.com/en-us/HT204666#
https://core.ac.uk/reader/151162169

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