What Is It? • The repeated alternating of periods of high intensity effort with periods of low intensity effort. • The basic tenets of interval training can be satisfied on a treadmill, stationary bike, elliptical machine, or outside, with the latter ‘probably’ being the most effective. • Normally defined by a ‘work:rest’ ratio (e.g. 1:3), in which the ‘work’ component represents the high intensity/sprint component and the ‘rest’ represents the low intensity/active recovery component. • For example, alternating 20 second fast runs with 60 second brisk walks (or jogs) until the desired time has elapsed. • Typically shorter in duration than traditional low intensity cardio owing to the higher intensity effort. The Benefits of Interval Training Relative to Endurance Cardio 1. Greater Energy Expenditure and Resultant Fat Loss • With increasing exercise intensities, the proportion of energy substrate derived from fat decreases, while the proportion of carbohydrate usage increases. (3) • However, the predominant fuel substrate used during exercise does not play a significant role in fat loss. • Total daily energy expenditure is more important for fat loss than the major fuel used during exercise. (3) • The most notable study comparing interval training to endurance cardio concluded that interval training is the most optimal method for fat loss. (14) o In this study subjects engaged in either an endurance program (4-5 times per week for 30-45 minutes) for 20 weeks or a high-low intensity program for 15 weeks. o Neither group was placed on a diet. o The mean estimated energy cost of the endurance protocol was 120.4 MJ, whereas the mean estimated energy cost of the high-low intensity protocol was only 57.9 MJ. (more than double) o However, the decrease in six subcutaneous skin folds was greater in the high-low intensity group than it was in the endurance group. This is despite the lower energy cost during exercise. o After statistical analysis it was shown that the high-low intensity group experienced nine times the fat loss of the endurance group. o This same study found the high-low intensity protocol to significantly increase the activity of an enzyme which is a marker of the activity of ‘fat burning’ over endurance protocol. • While one burns less overall calories and less fat during interval training (due to the involved energy systems) compared to endurance cardio, when the post-exercise recovery period is factored in, interval training leads to significantly greater energy expenditure and fat loss. • This is due to the effects interval training has on excess post-exercise oxygen consumption (EPOC) o EPOC – post-exercise oxygen consumption above resting values used to restore the body to the pre-exercise condition. o After exercise, oxygen uptake remains above pre-exercise levels for a period of time that varies according to the length and intensity of exercise. • The number of calories burned following interval training is significantly enhanced due to the increased EPOC. • There is much research to show that interval training protocols result in significantly greater post-exercise energy expenditure and fat utilization when compared to low to moderate intensity protocols. (1, 4, 7, 8, 10, 15) • Another study showed that even with no significant difference in total work, groups that exercised at a high intensity lost significant amounts of fat, while no significant changes were found in the lower intensity group. (2) • Another study showed that those who participated in high intensity exercise had lower skin fold measurements and waist-to-hip ratios than those who participated in lower intensity exercise. (13) • Improved VO2max, as a result of interval training, has been associated with increased thermic effect of food (TEF) (6) o TEF – An increase in energy expenditure due to an increase in cellular activity associated with digestion. 2. Improved Cardiovascular Conditioning & Fitness • Interval training has been shown to increase both aerobic and anaerobic capacity whereas endurance cardio only increases aerobic capacity. (9, 11, 12, 14) • It is well established that interval training increases aerobic capacity/VO2 max more than endurance cardio. (5, 12) o 28% increase in aerobic capacity with high-low intensity cardio vs. 14% increase in aerobic capacity with endurance cardio. (12) • Maximal oxygen uptake, or V02max, is generally regarded as the best single measure of aerobic fitness. • Interval training is more conducive to improving the muscle's ability to use fat. The more fit one becomes, the more likely they are to use fat as fuel for any given activity. • Post training, you will burn more fat. Numerous studies show that HIIT can elevate your metabolic rate significantly after the workout – with most of the calories coming from fat. • Fat/calorie burning is elevated afterwards to restore homeostasis (i.e. body temperature, catecholamines, hemoglobin, myoglobin, etc.).And it doesn't matter if you eat carbs right away either. This will not inhibit the oxidation of fat. In fact research actually shows that getting nutrients in immediately after HIIT will actually increase EPOC, reduces muscle protein catabolism, and increases recovery, all while having the wonderful benefit of not inhbiting lipolysis. • HIIT dramatically increases the amount of GH released into the bloodstream. This has some fat burning implications. (muscle building benefits are questionable) • Long periods of low-intensity/steady state cardio tend to convince some fast-twitch fibers to convert to slow-twitch fibers (or at least take on some slow twitch qualities). Conventional slow, long-duration cardio workouts tend to "overtrain" the fast-twitch muscle fibers and to convert the intermediate muscle fibers to slow-twitch suitors. HIIT prevents this from occurring, preserving your muscle growth potential. • High intensity cardiovascular exercise increases oxygen expenditure and forces the body to adapt by becoming more efficient at oxygen transport (increase in VO2 max). That means healthy benefits for the heart, lungs and other components of your cardiovascular and pulmonary systems.