If you want to gain extra aerobic fitness fast, there is no better way than to do interval training. What is interval training? Aerobic (cardiovascular) exercise done in high-intensity bursts. Often known as HIIT or High Intensity Interval Training. It allows a fitness increase over a shorter time frame compared to less intense but longer training.

Who Is It For?

Some people say this is the "only" way to train but there isn't scientific consensus for that belief.

In spite of the hype surrounding HIIT, it isn't necessarily better for weight loss than other types of exercise. In fact, a longer less-strenuous endurance type exercise ("cardio" or "aerobic") will burn more overall calories. It's true that the after-exercise calorie burn is more elevated after a HIIT session than lower-intensity cardio, but not enough to overcome the fact that the workout isn't long enough to burn significant calories.

So HIIT is mainly a technique for increasing fitness for performance purposes (including boosting VO2 max or maximal aerobic capacity).

Anyone can do HIIT to some degree, but take care — it should be added into a workout only after you've been doing a regular routine for awhile and are ready to kick it up a notch. It's best as an addendum to a plan, as a way to "sharpen the saw".

Finally, remember that doing HIIT all the time as your main workout technique is not recommended for beginners because of the greater chance of injury. Constant HIIT training is only for advanced athletes.

How To Do It

The basic idea is just like it sounds: Increase the intensity of your workout for a short burst, then back off until you recover. Repeat.

You can do this on your daily walk, on a treadmill, an elliptical, or on a run.

How long is the high-intensity burst? Anywhere from 10 seconds to 3 minutes or so. Most commonly it is 30 to 90 seconds.

How hard? Usually, it should be "all out". The burst is so short that you really can go as hard as you can for that brief time. You will be breathing hard when you're done. It should feel like hard work.

How long is the recovery? Typically about the same time as the burst. You can usually recover that fast; if not, then give yourself more time.

How many times? It's up to you. Try doing it 2 or 3 times during your regular workout. Or if you want to do your whole workout with HIIT, you can. As you get more fit, you can gradually increase the number of times you do it.

How often? It depends on the number of bursts and intensity. You can do a few short bursts in every workout if you want. If you do an entire, long workout as HIIT, try once a week. If it's a very intense and long workout, you might only want to do it every 2 weeks.

Examples:

A few times during your workout, bump up effort for 10 - 60 seconds
30 seconds intense, 30 seconds recovery. Repeat as many times as you want.
A pyramid: 30:30, 45:45, 60:60, 90:90. Repeat the series 2 or 3 times.

Always warm up and cool down. Take at least 3 - 5 minutes to warm up before you start any high-intensity bursts. When you are done with the last one, take 3 - 5 minutes to cool down.