Optical Heart Sensor Vs. ECG: What's Actually Inside Your Smartwatch?

One of the major benefits of wearing a smartwatch is its health and fitness tracking abilities, with devices capable of measuring health metrics such as your step count, calorie burn, stress levels, and sleep quality (though these readings can sometimes be wrong). Key to accurately measuring many of these biometrics is your heart rate, as many smartwatches track your resting heart rate to determine your heart rate variability, stress levels throughout the day, and activity level. What's more, knowing your resting heart rate can offer an insight to your cardiovascular fitness level.

However, not all smartwatches measure your heart rate in the same way. While most smartwatches feature a PPG (photoplethysmography) optical heart rate monitor, which uses a green light to measure how fast blood flows through your veins, others use an ECG (electrocardiogram) heart rate sensor that tracks your heart rate by measuring electrical signals in your blood. Below, we break down more on how these sensors work and which smartwatches you'll find each in.

PPG optical heart rate monitors are the most common monitor found in smartwatches. These monitors use sensors to detect changes in blood volume. PPG monitors shine green LED lights through your skin that are then reflected by the red blood cells in your wrist's blood vessels. When your heart beats faster, it pushes more blood to your wrist and more green light is absorbed; when it beats slowly, less light is absorbed. The sensors in the heart rate monitor detect how much light is reflected over time to determine your heart rate.

ECG heart rate monitors, on the other hand, are often found in chest straps and are commonly used for diagnosing heart conditions, but are utilized in select premium smartwatches, too. These heart monitors track your heart rate by measuring the electrical signals that control your heartbeat. These watches typically require skin-to-skin contact with its sensor for a set period, with the monitor then able to track the timing and strength of your heart rate from the electrical changes on your skin (caused by your heart's contractions). This means these monitors can detect irregularities, such as atrial fibrillation, that PPG monitors may not.

According to experts, ECG heart rate monitors are more accurate than PPG ones, as the accuracy of PPG readings can be negatively affected by factors such as arm tattoos (as the ink may block the light), darker skin (which absorbs more light), or vigorous arm movement. Optical heart rate monitors can be found in many smartwatches, including Garmin's Fenix 5 Plus (which uses Garmin's Elevate optical heart rate sensor technology), the Amazfit Active 2, and the Polar Pacer GPS Sports Watch. However, we're now seeing more premium smartwatches fitted with both PPG and ECG sensors, including the Apple Watch Series 10, Samsung Galaxy Watch 8, and Google Pixel Watch 3.