Interval training

You briefly perform vigorous movements such as sprinting, fast pedaling, or jumping, then catch your breath by walking or pedaling lightly before repeating. You can freely adjust the duration, intensity, and work-to-rest ratio—for example, “30 seconds of hard effort, 30 seconds of rest, repeated 8 times.” It can be applied to running, cycling, swimming, jump rope, bodyweight exercises, and more, so it is more a way of structuring an exercise program than a specific sport.

It originally developed from training methods used by middle- and long-distance track athletes. Today, it is also widely used by the general public to maintain basic fitness and manage body weight. Common forms include:

• High-intensity interval training — Alternates high-intensity exercise intervals with short recovery periods.
• Tabata training — A roughly 4-minute format consisting of 20 seconds of exercise and 10 seconds of rest, repeated 8 times. Its name comes from an experiment published by Japanese researchers in 1996.
• Fartlek — Swedish for “speed play,” this involves freely changing your running pace using visible landmarks as cues.

Our bodies produce energy through three pathways during exercise. The ATP-PC system is used primarily for very brief, powerful movements, the glycolytic system for efforts lasting tens of seconds to a few minutes, and the aerobic system for sustained low- to moderate-intensity exercise. These three systems do not switch on and off in sequence; they always work together, with their relative contributions changing according to exercise intensity and duration. You can read more in the Energy systems article.

By moving between high-intensity intervals and recovery periods, interval training has you use these pathways in alternation. Research has continued to show that working through intervals that leave you breathing hard helps improve maximal oxygen uptake (VO2max). After intense exercise, the body's oxygen consumption remains above its usual level for a while. This is called excess post-exercise oxygen consumption. It is a phenomenon in which the body expends additional energy during recovery even after exercise has ended.

• Start with a warm-up. Walking gently for about 5 minutes or thoroughly loosening up your joints before starting helps your body keep up more comfortably during high-intensity intervals. See also Warm-up.
• Start with an easy format. At first, walking briskly for 1 minute and slowly for 2 minutes, repeated 5–6 times, is enough. Once you are familiar with the movements, try gradually lengthening the high-intensity intervals or shortening the rest periods.
• Check your interval intensity by talking. An appropriate level is one where you can barely manage short answers during hard intervals, then recover enough to speak in full sentences during recovery periods. If you would like to check using numbers, you can use rating of perceived exertion or heart rate zones.
• Increase only one thing at a time. Gradually increase just one of the following—the number of repetitions, interval duration, or exercise intensity—to give your body time to adapt. Progressive overload and load management cover this principle.
• Include rest days. High-intensity exercise requires enough recovery time, so it is best to start with about 2–3 sessions per week and avoid exercising on consecutive days. On rest days, loosen up with gentle activities such as a light walk.
• Allow enough recovery time. If you are not used to interval training, try making the recovery period longer than each high-intensity interval.

Longer, easier exercise (low-intensity steady-state exercise) involves maintaining a steady pace, places less stress on the joints, and lets you converse comfortably while exercising. Interval training, by contrast, concentrates a strong stimulus into a short period. Neither is inherently better; the choice depends on your exercise goals and how you are feeling. Many people combine the two, doing intervals on days when time is short and longer, easier exercise when they have more time.

If you feel tightness in your chest during exercise, feel unusually dizzy, or your breathing does not settle even after resting, stop exercising immediately and rest. If the same symptoms recur, it is a good idea to have your condition checked. If you have pronounced soreness the day after exercise, it is likely to be delayed onset muscle soreness, so try lowering the intensity and moving gently as you recover.