The smartwatch's Blood Oxygen app lets you measure the oxygen level of your blood on-demand. It's been MIA for more than a year thanks to a patent spat, but it's back in action now.

Apple Watch Users Get a Major Upgrade That Could Transform Your Training Forever originally appeared on Men's Fitness. Apple Watch users are in for a serious upgrade. Apple is updating the Blood Oxygen feature for select Apple Watch models, giving fitness-minded users more ways to track their health.