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.


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.

Screenshot 2015-04-20 10.19.43

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.