A lot of us use our smartwatches and fitness trackers to track important health metrics, like how well we slept or whether our stress levels are high. Many of those insights wouldn’t be possible without heart rate tracking. “It’s really the basis for so much,” says DeCarlos Love, a wearables product manager who works on Google Pixel Watch and Fitbit devices.

When Google acquired Fitbit in 2019, the wearables team knew that bringing in deep expertise with wearables and a history of heart rate tracking would benefit future products — like the Google Pixel Watch. Now, the just-launched Google Pixel Watch 2 and Fitbit Charge 6 both feature our most accurate heart rate tracking yet on a smartwatch and tracker. Here’s what’s going on underneath the hood that makes it all possible.

It starts with sensors

If you, like me, thought that smartwatches and fitness trackers measured heart rate by feeling that “bump bump bump” vibration of your pulse on your wrist, you would be wrong. Well, sort of wrong. “When you think of taking someone’s pulse, you imagine putting two fingers on their neck and picking up on the beat or vibration of their heart,” DeCarlos says. “But with heart rate sensors, that’s not quite what they’re doing.” Instead of feeling something, the sensor is seeing something.

Specifically, a PPG sensor — PPG is short for photoplethysmography. A PPG sensor uses light to measure the volume and movement of your blood circulation. It does that by shining a green LED light onto your wrist, which then reflects the movement of your blood back to the sensor. “We pick up that rate of reflection and we’re able to understand how your blood is flowing — aka, your heart rate,” DeCarlos says.

While the original Pixel Watch had a single path PPG sensor, the new Pixel Watch 2 features a brand new multi-path LED sensor. Having multiple LED and photodiodes (a device that detects the amount of light reflected by blood) helps the watch cover more surface area on your skin, DeCarlos says, which gives you more accurate recordings when all sensors are activated.