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A system of training for aerobic fitness that alternates spurts of intensive exertion (work interval) with periods of lower intensity activity (relief period) in one exercise session. By carefully spacing the periods of exertion and relief, more total work can be accomplished than with continuous training. High intensity exercise can also be achieved with less stress and strain than in one continuous session.

Each session is usually described in terms of an interval training prescription indicating the number of sets or repetitions, training distance, training time, and relief time. Thus, a running session may have the following prescription: one set of six repetitions of 200 metres each run at a pace of about thirty seconds, with a relief period between each run of 1 minute 10 seconds. This can be abbreviated as:

Set 1 6 × 200 at 0:30 (1:10)

Interval training for elite athletes was pioneered by the German coach, Gerschler. It is now used by many people as an effective way of becoming fit. Often, the intensity of the work intervals and relief periods are strictly controlled.

A typical session for a well-conditioned 20-year-old 1500 metre runner might consist of stretching and other warm-up exercises to raise the pulse rate to about 120. Then running 400-metre repetitions at a pace sufficient to raise the pulse to between 170-180 beats per minute, with the recovery between each repetition consisting of a walk or jog continued until the pulse rate goes down to between 120-140. Often 20 repetitions are planned for a session, but it should be terminated with cool-down exercises when recovery takes longer than 90 seconds.

Interval training is infinitely variable and can be adapted to the needs of the total beginner as well as to the prospective world champion. As fitness improves, the length and intensity of each session can be increased gradually. Of course, the principles of interval training can be applied equally well to cycling, swimming, and exercising on machines.

A system of training that alternates short to moderate bouts of intense activity (the work interval) with short to moderate periods of rest or reduced activity. By optimally spacing the periods of work and relief, a person can accomplish more total work than would be possible in a continuous training session. Interval training can be used in almost any sport, but it is most often used by track athletes, cross-country runners, and swimmers. Interval training can be adapted to fit individual requirements by adjusting the following: rate and duration of the work interval (load and duration of resistance training); number of repetitions and sets during each training session: frequency of training per week; duration of rest (recovery) interval; and type of activity during the rest interval. Gerschler interval work, devised for runners by the famous German coach Woldemar Gerschler, consists of a large number of repetitions run at a short distance (typically, 200-400 m) with a relatively long rest period (2-3 min). Each repetition is run at or above race pace in order to develop a sense of race pace. In the controlled interval method, work and rest periods are related precisely to a physiological measure of the athletes condition (e.g. pulse rate). A typical programme of a runner may consist of a warm-up raising the pulse rate to about 120 beats min⁻¹ (bpm), a set of repetition runs which raises the pulse to about 170 bpm, and a rest of a jog or walk between each run, which allows the pulse to return to about 120 bpm. The session stops when the recovery takes more than 90 s.

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