The custom heart rate monitor on the back of the Apple Watch helps the device get readings for your daily activity and specific workouts. Apple has launched a new support page detailing how the monitor on the watch works. The technology at work is called photoplethysmography, which uses light to measure the blood flow in your wrist.
From Apple:
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.
Apple also highlights some tips to get a good reading from the monitor. The fit of your Apple Watch is important. If your watch sits too loosely on your wrist, the heart rate monitor won't be able to get an accurate reading, so Apple recommends a snug fit.
The company also notes that things like your environment might impact your reading, based on how much blood flows through your skin. Rhythmic motions like those from running or cycling will give more consistent results than motions from activities such as tennis, where motions are irregular.
