What is HIIT?
HIIT is an acronym for High Intensity Interval Training. Just as the name implies, the trainee works at a high intensity for short periods of time, followed by a lower intensity, and then back to higher intensity again. An example would be going for a jog, followed by a short sprint, then jogging, and so forth. The cycle repeats until the training session is complete (often no more than 20 or 30 minutes total per session).
What are the benefits of HIIT?
The benefits of HIIT are numerous. Perhaps the most obvious advantage is that it saves time. By working harder, you'll also end up working more - in less time. Who couldn't use a few extra minutes in a day? Aside from the time savings, the actual physical benefits of HIIT are significant as well and not to be overlooked. HIIT helps boost your metabolism and is very effective at burning fat.
How does HIIT help?
HIIT works because it forces your body to adapt to a more intense stimulus. Steady-state cardio, such as going for a jog or a slow trot on the treadmill, never really brings your body to its limit. Without increased intensity your body will simply become used to the stimulus and the effects will diminish. HIIT shocks your body into adaptation.
How does HIIT burn fat?
HIIT helps to burn fat in two ways. First, your body will obviously be burning fat as fuel during the actual training session. Secondly, and perhaps more significantly, HIIT will help boost your resting metabolism for up to 24 hours after the training session through a process known as EPOC (excess-post exercise oxygen consumption). If you're currently the steady-state cardio type, try HIIT sometime and compare your appetite post-training. There's a good chance you'll feel hungrier throughout the day after HIIT. As long as you're disciplined and able to refrain from compensating (by eating more) then your body will end up burning more fat instead.
Sample HIIT Routine
Jogging, 20 seconds
Sprint, 10 seconds
Simply repeat as many times as possible before your sprint speed becomes noticeably slower and almost matches your jogging speed. Notice there is a 2:1 work ratio (twice as much low intensity vs. high intensity). The ratio can be adjusted in favor of high intensity as you become more advanced and in need of a greater challenge.
Some supplements you may want to consider while training with HIIT are whey protein, BCAAs, caffeine, yohimbine HCL, and N-Acetyl L-Cysteine (NAC). Whey protein or BCAAs should be taken just prior to your workout to help conserve lean muscle tissue. Caffeine or yohimbine could be taken prior to the training session as well. These may help promote the use of fat as fuel during your workout. N-Acetyl-L-Cysteine (NAC) is a powerful antioxidant that can aid in recovery from training.
HIIT will probably make you hungry. This can be combated to some degree by eating a diet heavy in fiber, green veggies, and dense meat protein sources. These foods take a bit longer to digest and may suppress your appetite, which can help to keep overall calories to a minimum. Have a look at the cutting diet article and cutting routine for more ideas in your quest for definition!