Moov made a heart rate sensor you wear on your head
 

When representatives from the fitness tracking company Moov first told me they had made a heart rate sensor that you wear on your head, I thought the idea was ridiculous. But after I saw the Moov HR Sweat headband and swim caps (Moov HR Swim), it all...

https://www.engadget.com/2016/10/19/moov-hr-sweat-swim/