With High Intensity Interval Training (HIIT) you become a metabolically efficient, fat-burning machine in far less time than you could through long aerobic sessions (1).
But these benefits go above and beyond what you burn during your actual workout session. For example, many endurance athletes are also interested in weight loss. The good news for those of us who want a better power-to-weight ratio is that, in addition to the body burning more fat during exercise, HIIT also increases excess postexercise oxygen consumption *EPOC* and thus the total amount of calories you burn after the workout remains elevated as physiological and metabolic factors in your cells are restored to their pre-exercise levels.
A review article of studies on HIIT found significantly higher EPOC values with HIIT training than with continuous aerobic training - which translates into higher and longer calorie burning long after your workout is over.
Month after month, this *bonus* calorie burn can significantly add up !
HIIT - YOUR SKELETAL MUSCLE
How about your skeletal muscle ? When your muscles contract, they propel blood back to the heart, which increases the amount of blood filing your heart and the heart's subsequent stroke volume. Within just a day or two of HIIT training, tiny blood vessels changes begin to take place in your skeletal muscle that improve the flow of oxygen in and out of the muscle and better *match* oxygen delivery to oxygen utilization (2). The microvascular adaptations are accompanied by an increase in the strength of the skeletal muscle fibers themselves, which allows for even more forceful pumping potential.
Plus, HIIT significantly lowers insulin resistance and results in a number of skeletal muscle adaptation that cause enhanced muscular fat oxidation and improved glucose tolerance (3) - yet another way HIIT can turn you into a fat-burning machine.
(1) Laursen, P. (2004). The influence of high-intensity interval training on endurance performance in well trained cyclists. In D. D. Jenkins (Ed.), PhD Thesis (pp. 01-309). The University of Queensland: School of Human Movement Studies.
(2) 17. McKay BR, Paterson DH, Kowalchuk JM. (2009). Effect of short-term high-intensity interval training vs. continuous training on O2 uptake kinetics, muscle deoxygenation, and exercise performance. Journal of Applied Physiology. 107(1):128-38
(3) Laursen, P. (2004). The influence of high-intensity interval training on endurance performance in well trained cyclists. In D. D. Jenkins (Ed.), PhD Thesis (pp. 01-309). The University of Queensland: School of Human Movement Studies.