Optical heart rate sensors (PPG) are susceptible to motion artifacts, which reduce accuracy during movement, higher heart rates, and with darker skin tones or tattoos.

“the biggest problem with PPG sensors Optical sensors as a whole is they get what are called motion artifacts and any kind of movement starts introducing noise into the signal because again we're not getting electrical signal we're just getting this blood flow going beneath the surface that we're using LEDs to detect right for heart rate when you start moving around you get lots and lots of emotion artifacts and it just becomes much more difficult for Those sensors to detect it accurately particularly in like as cyclical mov movements anything where your arm is moving around at random higher heart rates um darker skin colors lots of things throw off PPG sensors so my darker skin is obviously a disadvantage presumably just in general tattoos you know all of these things so Optical censers in particular struggle with higher intensities higher movements higher heart rates all those sorts of things”