Interval training
Alternating bouts of harder work with recovery periods, at planned intensities and durations.
Interval training alternates harder efforts with recovery periods rather than holding one steady intensity. The structure is defined by four variables: how hard the work is, how long it lasts, how long the recovery is, and how many repetitions you do.
Those variables change what you are training, which is why the label alone tells you little. Short maximal sprints with long recoveries develop the immediate phosphocreatine system and neuromuscular qualities. Efforts of thirty seconds to two minutes with incomplete recovery develop glycolytic capacity and tolerance. Longer efforts of three to five minutes near maximum aerobic power are the classic VO2max stimulus.
It matters because you can accumulate time at intensities you could never hold continuously. Splitting twenty minutes of very hard work into five four minute pieces is possible; holding it in one go is not, and the total time spent near maximum is what drives the adaptation.
Recovery is a programming variable, not dead time. Shortening it makes the session harder in a metabolic sense; lengthening it preserves the quality of each effort. Deciding which you want before starting is the difference between a session and a beating.
It is expensive to recover from, which is the constraint most people ignore. Two or three genuinely hard sessions a week alongside easy volume is a sustainable ceiling for most, and the polarised evidence suggests the easy volume is what makes the hard work possible rather than a filler between it.
It is not superior for fat loss at matched energy expenditure, despite the marketing. Its real advantages are time efficiency and the aerobic adaptations that only high intensity produces.
In practice, pick the interval length that matches the quality you want, keep the hard parts genuinely hard, and set the bands with our heart rate zone calculator.