Apple has apparently had both since their first watch models, but only uses them for heart rate. https://www.theverge.com/2014/9/9/6126991/apple-watch-four-back

Apple has apparently had both since their first watch models, but only uses them for heart rate. https://www.theverge.com/2014/9/9/6126991/apple-watch-four-back-sensors-detect-activity https://www.theverge.com/2014/9/9/6126991/apple-watch-four-b... EDIT: Apple says it uses green LEDs during workouts and IR for background measurements. Maybe it’s for energy reasons? https://support.apple.com/en-us/HT204666 https://support.apple.com/en-us/HT204666