HIIT is High Intensity Interval Training.
HIIT is NOT interval training.
HIIT was originally developed by a Japanese scientist (Tabata) who wanted to mess around with athletes. He gave them the hardest workout they had ever had - and it was only FOUR minutes.
(Skip this quote if you aren't sciency)
"Seven students, also young and physically active, exercised five days per week using a training program similar to the Japanese speed skaters. After a 10-minute warm-up, the subjects did seven to eight sets of 20 seconds at 170% of V02max, with a 10 second rest between each bout. Pedaling speed was 90-rpm and sets were terminated when rpms dropped below 85. When subjects could complete more than 9 sets, exercise intensity was increased by 11 watts. The training protocol was altered one day per week. On that day, the students exercised for 30 minutes at 70% of V02max before doing 4 sets of 20 second intervals at 170% of V02max. This latter session was not continued to exhaustion. Again, V02max and anaerobic capacity was determined before, during and after the training.
"In some respects the results were no surprise, but in others they may be ground breaking. The moderate-intensity endurance training program [Note: Control group not doing HIIT] produced a significant increase in V02max (about 10%), but had no effect on anaerobic capacity. The high-intensity intermittent protocol improved V02max by about 14%; anaerobic capacity increased by a whopping 28%.
"Dr. Tabata and his colleagues believe this is the first study to demonstrate an increase in both aerobic and anaerobic power. What's more, in an e-mail response to Dick Winett, Dr. Tabata said, "The fact is that the rate of increase in V02max [14% for the high-intensity protocol - in only 6 weeks] is one of the highest ever reported in exercise science." (Note, the students participating in this study were members of varsity table tennis, baseball, basketball, soccer and swimming teams and already had relatively high aerobic capacities.)" (http://www.cbass.com/FATBURN.HTM
(end of quote)
The surprising finding was that even with highly conditioned athletes who should have already been at peak condition, their VO2 max was increased. In other words, they got in much better shape much more quickly than any other method ever before.
In subsequent studies, it was found that HIIT training, when done for just 6 minutes a day, three times a week *for only two weeks* was the equivalent of doing hours of traditional aerobics per week for weeks on end in terms of increases in endurance. What that means is this: People were able to increase their ability to do aerobic exercise from 26 minutes to 51 minutes in only two weeks. That's impossible with traditional measures.
In other studies, those participants who did HIIT were able to burn actual fat (not water weight, muscle, etc) at NINE TIMES the rate of those who did traditional aerobics, even in cases where they burned less calories total, because this type of exercise if more efficient at burning fat and raises your metabolism.
HIIT also has an anabolic effect - that is, instead of catabolic, muscle breaking endurance exercise, HIIT supports muscle growth - as long as you eat enough.
Now you are wondering - I'm doing interval training, more than 4 minutes, but I haven't seen the results! Here's why: you're not doing HIIT. Sorry.
Keep this in mind - if you read the original study, the college athletes, who were already in good shape - were vomiting after 4 minutes. They mentioned that in the actual published report. They were not "going hard" for a few seconds. They were going AS HARD AS THEY COULD. (170% VO2 max!) Not pacing. Not anything.
If you can do HIIT more than three times a week (which you should not) and you are not already a conditioned athlete, you are probably not doing HIIT. It should take all the energy you have to get through a workout that only takes up to 4 minutes or so. That isn't to say that what you are doing isn't beneficial, won't work, etc. That's just to say - you're doing interval training, not HIIT, so you shouldn't misrepresent HIIT to people who are learning.
Here's the basic workout. Can be done running, biking, etc.
10 minute warmup (especially if you don't want to hurt yourself)
Then do this 7-8 times:
20 seconds AS HARD AS YOU CAN - imagine you're running for your life.
10 second rest
Repeat. Should only take 4 minutes when you are first starting.
Cool down - otherwise you'll just die. Stretch, or you'll feel terrible.
Keep this in mind - as it says above, once the athletes could do more than 9 sets, they made them go harder - in other words, once you can do it 9 times, you're not running/biking/whatever hard enough. You should only be able to do this a few times.
Warning: this can be dangerous in certain populations. Enjoy!