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Apple details the technology and functionality behind Apple Watch’s heart rate monitor

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Ahead of the Apple Watch making its way onto the wrists of consumers, Apple has published a new support page detailing the device’s heart rate monitor. As we know, the Apple Watch includes a heart rate reader to measure a person’s intensity during workouts.

With knowledge of this intensity data, the Watch is able to more accurately measure the amount of calories a person burns per day. Additionally, a user can check their heart rate at any time using a feature known as the Heart Rate Glance. But beyond these two user functions, this new support document details the technologies behind the hardware as well as some little known software features.

According to the document, the Apple Watch will silently measure your heart rate every 10 minutes. This data will be stored in the iOS 8 Health application for later viewing and integration with third-party health tracking applications and hardware.

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Beyond software, Apple says that the Apple Watch uses fascinating mechanics to actually get the heart rate reading:

The heart rate sensor in Apple Watch uses what is known as photoplethysmography. This technology, while difficult to pronounce, is based on a very simple fact: Blood is red because it reflects red light and absorbs green light. Apple Watch uses green LED lights paired with light‑sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment. When your heart beats, the blood flow in your wrist — and the green light absorption — is greater. Between beats, it’s less. By flashing its LED lights hundreds of times per second, Apple Watch can calculate the number of times the heart beats each minute — your heart rate. The heart rate sensor can also use infrared light. This mode is what Apple Watch uses when it measures your heart rate every 10 minutes. However, if the infrared system isn’t providing an adequate reading, Apple Watch switches to the green LEDs. In addition, the heart rate sensor is designed to compensate for low signal levels by increasing both LED brightness and sampling rate.

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As with other heart rate reading devices, the Apple Watch’s functionality is not perfect, but Apple notes that how you wear your Apple Watch can affect readings. In order to get the most accurate reading possible, Apple recommends wearing the device closer to the skin. If users are looking to get the best heart rate reading possible, it seems likely that they should use one of the more tight fitting rubber Sport bands. Of course, other aspects of the human anatomy could negatively affect heart rate readings, as Apple also details:

A fancy way of describing how much blood flows through your skin, skin perfusion varies significantly from person to person and can also be impacted by the environment. If you’re exercising in the cold, for example, the skin perfusion in your wrist may be too low for the heart rate sensor to get a reading. Motion is another factor. Rhythmic movements, such as running or cycling, give better results compared to irregular movements, like tennis or boxing.

Besides detailing the heart rate reading feature, Apple has posted another support document detailing the materials within the Apple Watch. Interestingly, Apple notes the breadth of testing conducted in order to ensure that the device is safe to be worn.

A great deal of care and research go into choosing materials for all our devices. In addition to ensuring that all materials adhere to existing regulations, we developed our own specification for Apple Watch that goes beyond those requirements.1 In fact, every material that touches your skin has gone through extensive evaluation in accordance with our specification. This includes:

  • Thousands of material composition tests

  • More than a thousand prototypes worn for trial studies

  • Hundreds of toxicological assessments

  • Consultations with board-certified dermatologists

The Apple Watch begins delivering this Friday, April 24th.

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Comments

  1. spiralynth - 9 years ago

    Good stuff.

  2. iamreyjr - 9 years ago

    I wonder if there is a way not to read heart rate every ten minutes seems like that would be a battery drainer even better would be give us the choice. But then again its apple so yeah.

    • tmeesseman - 9 years ago

      HUR DUR APPLE NEVER HAS PREFERENCES

    • o0smoothies0o - 9 years ago

      Why do you think it’s every 10 minutes? Why do you think the every 10 minutes check uses only the infrared LEDs? Read the article, they’ve designed it so that it can still check, but save battery. I’m guessing the every 10 minutes check takes next to no battery…seeing as how exercise is rated for like 6.5 hours, and that is using visible light LEDs constantly.

  3. bennynihon - 9 years ago

    does it sample your heart rate at a higher frequency than once every 10 minutes when you start a workout? I would imagine, right?

  4. Robert Gauthier - 9 years ago

    The sampling is to reduce error, all medical HR determinations average. ECG’s typically average every 9-1- beats, pulse oximetry about the same. The intermittent sampling not only saves battery to filters out “noise” due to activity, motion, etc.

  5. It will be interesting to see how accurate the PPG-derived heart rate measurement is through high-motion activities. This is a topic near and dear to us and one that few wearables have got right.

  6. workoutcancer - 9 years ago

    “As you exercise, your muscles need more oxygen, and your heart beats faster to deliver it.” Heart rate (HR) increases due to psychological stressors too – e.g. commodity pit traders – I assume that the Apple watch records ‘exercise’ based on motion detection (accelerometer) plus HR, not HR alone. FirstBeat HR monitors’ technology distinguishes between sedentary HR stress and exercise HR using respiratory rate (I’m not an officer, employee, or stockholder).

    Apple, please publish some White Papers on the accuracy of the watch compared to ECG/ECK heart rate. Similarly, what validated non-proprietary formulas does the watch use for determining calories burned? Additionally, how accurate is the accelerometer for detecting activity/distance/calories burned? Many GPS devices are good at getting distance accuracy, but accelerometers (when indoors) can vary significantly.

    More information please, I’m very interested but not sold.

  7. anthonymaw - 9 years ago

    If you go on vacation and you forgot the charger you’re screwed. I’m out.