HIITHigh-Intensity Interval Training
Alternating short, hard efforts with recovery periods.
High intensity interval training alternates hard efforts with recovery periods. The definition is loose enough to cover very different things: thirty second sprints with three minutes rest, four minute efforts at near maximum with equal recovery, and the twenty second bursts of the original Tabata protocol are all called HIIT.
It matters because it is time efficient. Interval training improves VO2max and several metabolic markers with less total training time than steady state work, which is a genuine advantage for people whose real constraint is hours rather than motivation.
What it is not is a superior fat loss method. Studies comparing interval and continuous training at matched energy expenditure find broadly similar fat loss, and the afterburn effect that gets quoted, EPOC, is real but small in absolute calories. Fat loss still runs on the energy ledger.
It also carries the highest recovery cost per minute of anything you can do. Two or three genuinely hard sessions a week is plenty for most people alongside other training, and the common failure mode is doing five, which turns every session into a moderate rut that is too hard to recover from and too easy to drive adaptation.
That is why the polarised model exists: mostly easy work, a small amount of genuinely hard work, and relatively little in the middle. A 2024 meta-analysis found a modest VO2peak advantage for polarised distributions, and only in shorter interventions and in well trained athletes.
Intensity has to be real for it to count. If you can hold a conversation during the hard part, it is not the hard part, and the interval structure is doing nothing for you.
In practice, keep it to a couple of sessions a week, make them properly hard, and let easy aerobic volume carry the rest. Our heart rate zone calculator sets both ends.