High Intensity Interval Training is a form of interval workouts that involves doing several repetitions of maximum intensity with periods of low intensity in between. An example of that would be sprinting for 15-20 seconds, then walking for a minute, and repeating the whole process several times.
High Intensity Interval Training is also known as HIIT. There are many ways to do it, for example cycling, jumping rope and running (my favorite method).
This form of exercise has a lot of advantages compared to it's lower intensity counterpart, research shows that HIIT elevates growth hormone secretion and fat burning for up to 24 hours, or longer, after a workout. These kind of workouts are also usually shorter and 15-20 minutes are plenty to get a really good workout.
For a HIIT workout to be effective, you need to exceed the "lactate threshold", because lactate in the muscles is a stimulus to enhance growth hormone secretion. When your muscles need to release an intense amount of energy in a short time and the cardiovascular system can not keep up feeding oxygen to the muscles, they start turning glucose into lactate as a form of short term energy release. This is more commonly known as anaerobic exercise, which you may have heard of.
In a regular high intensity interval workout, this threshold is exceeded multiple times, with short recovery periods in between.
Sprinters are very muscular and long distance runners are not, that is a common sight.
Interval workouts cause a greater growth hormone release than low intensity exercises. According to one study, low intensity training did not even raise growth hormone above baseline. Growth hormone has many positive effects in the human body, including enhanced fat burning.
Interval workouts greatly increase EPOC (Excess Post-Exercise Oxygen Consumption), which suggests that metabolism is elevated for a while after training. One study suggests that after high intensity weight lifting, this effect can last for up to 38 hours, or even longer.
Research comparing one group performing interval workouts and another one performing an endurance workout, found that the interval group lost three times as much subcutaneous fat as the endurance group, despite spending less energy during the actual workout. This suggests that interval workouts are way more effective than endurance training for fat loss.
There are many research studies showing how interval workouts are way superior to endurance workouts with regards to fat burning, in addition to positive effects on physique and health. This kind of workout can also be done in a much shorter time period than low intensity cardio. People who have any health issues may want to take this slowly in the beginning, or talk to a doctor before incorporating this into their program.
It is a bad idea to do this many times a week, once or twice being an absolute maximum. This is very taxing on the body and may lead to severe fatigue if proper rest is not taken between workouts. It's not recommended to do high intensity interval workouts at the same day you do another high intensity workout.
Some sports, for example crossfit, gymnastics and soccer, automatically include some form of this into their programs.