HIIT is a type of cardio where a cycle of short (30-60 seconds) intense work intervals followed by a recovery interval is used. Work intervals are at maximum intensity. For example, 30 seconds sprinting followed by 30 seconds of light jogging. This work/rest cycle is usually repeated for ~15-20 minutes. HIIT is very intense and can only be carried out for short periods of time (15-20 min. is usually considered optimal). Because HIIT sessions are shorter than traditional cardio sessions, the energy expenditure during a HIIT session is considerably less than that of a longer moderate intensity cardio session. The benefits of HIIT are reaped after the session is over, not during it.
HIIT is a unique form of cardio in that it increases post-exercise energy expenditure (it increases your metabolism). The only other way that basal metabolic rate (BMR) can be increased is by increasing muscle mass (eg weight training).
HIIT is a very effective way of reducing body fat. When compared to traditional longer duration moderate intensity cardio, fat loss in subjects using HIIT was 9 fold greater. Also, the value of (change in subcutaneous skinfolds)/(calories burned during exercise) is much higher for subjects using HIIT. In short, this means that you are burning more fat doing less cardio.
The effect of HIIT on energy metabolism is quite similar to that of many thermogenic supplements (ephedrine, caffeine, capasaicin, and Yoshioka). The thermogenic effect of these supplements is due to stimulation of the sympathetic nervous system. Thermogenic supplements affect every cell in the body, and thus have some undesirable side effects such as increased heart rate. There is evidence that HIIT increases the number of B-adrenergic receptors in skeletal muscle cells while having the opposite effect in cardiac muscle. This makes the skeletal muscle cells more sensitive to sympathetic stimulation while not affecting cardiac muscle. Therefore, the thermogenic effect of HIIT occurs in skeletal muscle and is due to the upregulation of B-adrenergic in skeletal muscles.
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