HIIT, high-intensity interval training, is cardio built from repeated near-maximal efforts, roughly 80 to 95 percent of max heart rate, separated by recovery periods. The recovery is the point: it lets you reach a true high intensity again and again, which one continuous effort can't do. A real HIIT session is short, often 15 to 25 minutes including rest, because genuine near-max work can't be sustained for an hour.
Why it matters
Done right, HIIT improves VO2 max fast, and VO2 max is one of the best predictors of long-term health we have. It's also brutally time-efficient. Four 4-minute hard intervals can move your aerobic ceiling more than an hour of moderate slogging.
But here's the catch. Most classes sold as HIIT are really circuit training: an hour of moderate work with short breaks, heart rate hovering at 70-something percent. Decent exercise. Just not high-intensity intervals, and it won't deliver HIIT's specific adaptations.
How to use it in training
Once or twice a week is plenty. More than that and recovery starts eating your lifting.
Pick a simple modality you can push safely: bike, rower, hill sprints, incline treadmill. Try 4 rounds of 4 minutes hard with 3 minutes easy between, or 8 rounds of 30 seconds hard and 90 easy. "Hard" means you couldn't hold a conversation and you're genuinely relieved when the interval ends. If you finish a session feeling fresh, it was cardio. Fine, but call it that.
Related terms
Go deeper
HIIT or steady state for your goals? We compared the research head to head: HIIT vs steady-state cardio.