High-intensity interval training (HIIT) is an exercise strategy that is intended to improve performance with short training sessions.
A HIIT session involves a warmup period, several short, maximum-intensity efforts separated by moderate recovery intervals, and a cooldown period. The period of alternating effort and recovery intervals typically lasts a total of 15 minutes.
Studies by Tabata, Tremblay and others have shown this method to be more effective at burning fat and maintaining, or building, muscle mass than high-volume, lower intensity aerobic work-outs. According to a study by King  , HIIT increases the resting metabolic rate (RMR) for the following 24 hours due to excess post-exercise oxygen consumption, and may improve maximal oxygen consumption (VO2 max) more effectively than doing only traditional, long aerobic workouts. Long aerobic workouts have been promoted as the best method to reduce fat, as fatty acid utilization usually occurs after at least 30 minutes of training. HIIT is somewhat counter intuitive in this regard, but has nonetheless been shown to burn fat more effectively. There may be a number of factors that contribute to this, including an increase in RMR, and possibly other physiological effects.
- HIIT and Run from Muscle Media
- Impact of Exercise Intensity on Body Fatness and Skeletal Muscle Metabolism from ExRx
- Getting started with HIIT
- Effects of High-intensity Training on Performance and Physiology of Endurance Athletes (.doc format)