The theory behind interval training is this: By mixing bursts of high intensity work with low intensity periods of recovery, you're overloading both the aerobic and anaerobic systems at the same time, getting the benefits of both aspects of training simultaneously. You can realistically get a complete workout in thirty minutes with interval training.
How is it done?
Start at an easy pace and gradually increase your heart rate for at least five minutes. You can monitor this by taking your pulse for fifteen seconds and multiplying it by four or using a heart rate monitor (here's a cool one that also keeps track of calories!) When you're sufficiently warmed up, you're ready for a burst of high intensity work. If you're on a treadmill, break into a jog or a sprint, depending on what "high intensity" means to you.
During the high intensity periods, you're decreasing your body's ability to exchange oxygen and carbon dioxide. You begin to feel the "burn" as your body eliminates lactic acid (a toxic by-product) and your muscles begin to lose their ability to contract. You wouldn't physically be able to maintain this level of intensity for long.
When you begin to wear your muscles out, decrease the intensity level to something that you could maintain for a longer period. Don't slow down so much that your pulse dips too low, though, or you will lose the aerobic effect completely. Now, you're in the "active recovery period". Your body's ability to exchange oxygen and carbon dioxide increases and it can deliver nutrients to your muscles. The burn goes away and your breathing and heart rate slow down slightly. You have completed one cycle.
Repeat this process of maxing out and recovering your anaerobic system for at least thirty minutes. The high intensity periods should be shorter than the active recovery periods, especially at first. You might walk for five minutes and then run for one when you begin to introduce your body to this type of training. As you become more adept, increase the time you spend in high intensity periods. Forcing yourself to sustain long periods of high intensity activity is dangerous, so do use caution and work yourself up gradually.
Why is it good? Here are four big reasons:
How can it help me lose weight?
By challenging both you aerobic and anaerobic systems simultaneously, you're improving your body's ability to burn calories by leaps and bounds. You're adding new muscle, which speeds up your metabolism of fat in general. You're getting an aerobic workout that burns lots of calories. You're pushing yourself beyond any plateaus that you may have hit doing the same thing over and over again. Your body is becoming a more efficient fat-burning machine.
What activities can I use for interval training?
The possibilities are endless, but the most practical are probably walking/running, other cardiovascular machines like stair steppers, elliptical trainers and stationary bikes, aerobic exercise, water exercise and things like that. You could even incorporate it into jumping rope or a sport like racquetball. If you want to be creative, you can really make fitness fun.
How often should I do it?
If you're a beginner, throw in one session a week, along with your normal routine. If you're more seasoned, two or three times a week is great. This is a demanding form of exercise, so use common sense and listen to your body.