HIIT is commonly performed at about 80 percent of peak heart rate, while sprint intervals are more of an “all-out” effort designed to get you to 100 percent of your VO2 max, the researchers noted.

Though it might feel great to finish a workout and see "calories burned" pop up on your smartwatch, that number is often surprisingly inaccurate, with estimated error rates of 30%–80%. The watch's ...