The Science Behind High Intensity Interval Training

By Level One Training and Fitness

There have many discussions and more than a few research projects regarding the efficacy of high intensity interval training (HIIT) and the "afterburn effect" it purportedly results in. One can reasonably infer from the questions posed that there is indeed something to pushing yourself harder than you would with regular cardio.
 
How well do anaerobic workouts and high intensity interval training (HIIT) beat steady-state cardio? Why do many consider it the holy grail of weight loss and fat loss? Is it really "the miracle cure" so many people have been looking for? Does the science behind it in fact measure up? Are HIIT workouts something anyone can do?
 
Let's just say that hard work as part of a thoughtful strategy aligned with clearly defined goals produces results. In other words, if you are in the clear to do HIIT workouts and you know what else is required of you, particularly when it comes to nutrition, and you know what end result you are passionate about striving for, then HIIT may be the answer for you.

Research
Studies have shown that hill running (sprints) can burn fat at seven times the rate of regular cardio. Can you imagine having to do only one round of hill sprints versus 7 bouts of standard cardio to achieve the same desired effect? How's that for motivation! Of course, the hard part is that running up hills is, well... difficult! The rewards, however, are just that - quite rewarding.

​In a study by Dr. Christopher Scott and the University of Southern Maine, the total calorie burn of low intensity exercise vs. high intensity exercise was examined. A low intensity exercise group cycled at a steady rate of 3.5 minutes. The higher intensity exercise group required three 15 second sprints as fast as the subjects could run.
What was the difference in calorie burn? Quite substantial.
 
The cycling group burned 29 calories vs. 4 calories for the sprinting group during the exercise. But when you take into account the calories burned after exercise, or the afterburn effect, the numbers look much different – 39 calories burned for the cycling group vs. 65 calories burned for the sprinting group. A surprising 95% of the total calorie burn occurred after the sprinting exercise! (Also) keep in mind the cycling group exercised for almost 5x longer than the sprint group (3.5 minutes vs. 45 seconds).(5)
~ Marc Perry, CSCS, CPT

This is why hill sprints and HIIT classes can work so well at burning off those unwanted calories! It's very challenging though because you're having to work very intensely for short periods of time. The key to most HIIT classes is to workout like you're sprinting! 

Burn, burn, burn
Regardless of whether you're sitting at your desk working, watching TV, sleeping, or driving to school, your body is expending energy. Your body, right down to the cellular level, requires energy for you to do everything you do, even when it seems like you’re doing nothing at all.

The concept of anaerobic/HIIT workouts to induce the "afterburn effect" revolves around this premise: the more intensely you make your body work, the more work it needs to put in to repair itself over the next day or two. This repair "work" the body does, just like any other activity, burns calories. We'll lay out the data from Marc's quote above for visual ease:

Virtually identical total calorie expenditure despite the fact that one group exercised for 210 seconds and the other for only 45 seconds!
 
Rachel Cosgrove a well-respected trainer and a Certified Strength and Conditioning Specialist (CSCS) has the following to say:
 
Fat loss is all about caloric expenditure. You must burn off more calories than you consume in order to lose body fat. Even among the proliferation of diets—low carb, low fat, high protein—this simple rule remains.
The key to achieving this is not aerobic training, which will burn calories only while you are doing it. It is anaerobic training, which burns calories while you are working out and increases the calories burned for hours afterwards. In the case of weight training, building muscle burns calories as long as that muscle is retained—even during sleep.(1)
 
Need more proof? Dr. Len Kravitz, program coordinator of exercise science and a researcher at the University of New Mexico at Albuquerque provides this synopsis from the body of HIIT work:
 
Perry et al. (2008) showed that fat oxidation, or fat burning was significantly higher and carbohydrate oxidation (burning) significantly lower after 6 weeks of interval training. Similarly, but in as little as two weeks Talanian et al. (2007) showed a significant shift in fatty acid oxidation with HIIT. In their research review, Horowitz and Klein (2000) summarize that an increase in fatty acid oxidation is a noteworthy adaptation observed with continuous endurance exercise.(9)
 
Even Angelo Tremblay and his team at the University of Colorado get in on the action with a study of 27 men and women, aged 18 to 32 years, who were then divided into two groups, one endurance training and the other high intensity interval training. What happened when all was said and done? "...the results of the present study show that for a given level of energy expenditure, a high intensity training program induces a greater loss of subcutaneous fat compared with a training program of moderate intensity."(8)
​
So what's the catch?The catch to all of this HIIT talk is that in order for HIIT to be effective at burning your fat away, you must perform your work interval at a level greater than 80% of your maximum effort, even approaching 100% of your maximum effort.
 
It's no wonder that despite its effectiveness, more people have not jumped onboard with HIIT: it's a tough pill to swallow! And for others? They're just not in that big of a hurry. It will take a certain type of individual to do the necessary gut check and then double-down to make it happen.
 
The beauty of it all is that everyone is capable of performing that gut check and everyone is capable of doubling-down; they just need to be at the right time and place in their lives in order to make it happen for them. Did you notice the statement didn't say "at the perfect time?" Did you notice the statement DID say "make it happen?"
 
For individuals who are up for a challenge or want to arrive at "point b" a bit sooner than everyone else, HIIT is a potentially great way to do just that - make it happen. These individuals will be tired of remaining the same and will be ready for the next step with minimal investment in both finance and time.
 
HIIT also provides great variability with little complexity. In other words, it's always interesting!
 
In the end, it's about who you are, what you’re capable of doing, and what you're looking for.
 
Freddy Saluna is the Founder of Level One Training and Fitness. Together with Brittany Johnson, a Registered Dietitian and Charlie Satter, a Certified Strength and Conditioning Specialist, they work holistically at helping people effectively and efficiently achieve their health and fitness goals by utilizing a "back-to-basics" approach. They are "Saving the world, one person at a time" through one-on-one and group personal training sessions, outdoor HIIT (high intensity interval training) "bootcamp" classes, and Online/Virtual Training for clientele throughout the United States.