Why does H.I.T.T. work?

1. HIIT - High Intensity Interval Training 
2. What is the  precise mechanism that intervals (HITT) use to help people get fit so quickly. 
3. The  process can be explained, for example, as though a switch is being activated. A switch that signals to the body that it must change itself, and these changes enable the body to get fit. These changes include things like making the heart a better, stronger pump, making the blood vessels more flexible like a hose pipe so they can carry more blood, and adding mitochondria to muscle cells helping them move faster and longer. 
4. For a long time, we thought there was only one way to activate this performance adaptation switch to exercise for long periods of time. 
5. But then sports specialists discovered the way intense passive exercise could activate the same performance adaptations in a fraction of the time. 
6. Traditional endurance exercise flips the exercise switch by depleting the amount of fuel available in muscle tissue. So for continuous exercise, the longer the exercise bout, the greater the fuel depletion, and the larger the adaptive response. The closer you get to emptying the tank, the bigger the performance adaptation. 
7. The situation is a lot different with intervals however. With a few short sprints of 20 or 30 seconds, the total amount of fuel depletion is modest, especially, compared to what can happen over a prolonged bout of endurance exercise. 
8. However, the fuel available to the muscle is being used at a much greater rate, and that's what we think is important. 
9. It all has to do with muscle tissue. Muscle fiber generally is grouped into two broad categories: Smaller type one fibers, also called slow twitch fibers, usually, comprise about half of the overall muscle tissue. These tend to be recruited for relatively easy movements that don't require a lot of force. These fibers are also the ones used mainly during moderate-intensity endurance exercise. 
10. Type two muscle fibers, also known as fast-twitch fibers, tend to be recruited for fast powerful movements that require a lot of force. 
11. The effort demanding during high intensity interval exercise requires both types of muscle fibers, type one and the larger type two fibers. 
12. Sprinting is hard work and it takes all of the muscle fibers to do it. Because interval training recruits the entirety of the muscle, the muscle uses up fuel at a much faster rate. That's why a few short hard intervals can activate molecular signaling pathways to the same extent as much longer bouts of traditional endurance training. 
13. The traditional way to flip the switch was to exercise for a long period of time to deplete the fuel stores. But it turns out that another way exists to activate this switch. This way flips the switch by depleting the energy stores quickly. 
14. If you activate the switch this way, what matters is the rate at which you deplete your energy rather than the absolute energy you exhaust. 
15. The faster you deplete the energy stores, the better. You make the energy stores go down really fast, then you get a lot of exercise results. This is  the most time effective way to get exercise effects. 
16. Using this method, you exercise as hard as you're able, and the results show it's best to repeat this a few times in a row, that is, do a couple of intervals. 
17. To flip the switch this way, it matters less how long you exercise. What's more important is that when you do exercise, you go hard. Exercise hard enough, you deplete your energy stores fast enough, and you can get remarkable benefits, 50 times as much benefit as some as long slow steady exercise in some cases.