Cardio interval training, also known as high-intensity interval training, is intense cardiovascular exercise intended to burn fat and build lean muscle. In normal cardio exercise, an individual engages in some form of generally low-intensity exercise, such as jogging or biking, for an extended period of time with few, if any, breaks. Cardio interval training involves a different approach: short, high-intensity cardio exercises punctuated by short breaks. Intense, varied workouts are of particular importance—the body's natural inclination is to try to conserve energy and burn as few calories as possible, but it can not conserve energy if it can not adapt itself to a particular unfamiliar workout.
One of the main benefits of cardio interval training comes from a phenomenon called excess post-exercise oxygen consumption, or EPOC. After exercise, the body continues to burn calories in an attempt to completely return to a healthy post-exercise resting equilibrium. Hormones must be returned to proper balance, cells must be repaired, energy stores must be replenished, and metabolism must be returned to a normal resting level. This process does occur after normal cardiovascular or aerobic exercises, but the effects are much stronger and longer-lasting after short, intense exercises. Essentially, this means that, for up to a day and a half after engaging in varied, high-intensity workouts, the body will continue to burn calories at a slightly increased rate.
Muscles grow and change based on the particular types of work they are doing. Long, constant exercises require endurance and energy conservation more than muscular strength, so the body will adapt accordingly by burning calories slowly and developing lean muscles optimized for endurance. On the other hand, cardio interval training emphasizes shorter, more intense workouts that require much greater energy expenditure in a short period of time. Muscles will grow and be optimized for fast action, and calories will be burned quickly to facilitate fast and powerful muscle work.
Different kinds of workouts are better for different people. For those who want to run marathons, swim, or engage in other sports that place a strong emphasis on endurance, simple aerobic exercises, such as jogging or working on exercise bikes for extended periods of time, are probably best. For those who engage in fast-paced, strength or speed-based sports, however, cardio interval training can provide a much more appropriate exercise. Cardio interval training also tends to be more effective for those who are trying to lose weight and build muscle.