The Science of HIIT Cardio

What in the world do these crazy acronyms HIIT and LISS mean? HIIT stands for high intensity interval training, which consists of short sprint intervals coupled with low-moderate intensity work. An example of this would be a 10-30 second sprint followed by a 3-5 minute steady pace walk to cool down and bring your heart rate back to normal and then repeating it.

LISS stands for low intensity steady state cardio, which consists of purely low-moderate intensity work. An example of this would be walking on the treadmill or riding the bike and being able to hold a conversation.
Why testing the lactate threshold (LT) and anaerobic threshold (AT) is a good idea. The AT and LT are extremely powerful predictors of performance in aerobic exercise (cardio). (1) There are 2 ways that muscle can burn glucose (blood sugars) and that is through aerobic work (with air) and anaerobic work (without air).

HIIT produces better changes in exercise capacity as opposed to LISS cardio. High intensity training will hit the AT and LT, that’s what causes the body to make metabolic changes.

When you are doing LISS, you are considered below the AT and LT. A simple test is being able to hold a conversation while doing cardio.
When doing HIIT you are above the AT and LT and when you are above the AT and LT you push for greater improvement in metabolism which thus leads to better fat loss over time.

With lack of time being one of the biggest elements people always identify as “what’s wrong” with their health, HIIT cardio seems to be the most time efficient exercise strategy.

A study by Metcalfe et al. showed if you perform what Metcalfe and colleagues call the “minimal amount of exercise for improving metabolic health” a 3 time per week, 10 minute exercise regimen, with no more than two all-out sprints, everything you’ve got, you will make changes to your metabolic rate.

Moreover, Trapp et al. Showed 20 minutes of HIIT, performed three times per week for 15 weeks compared to the same frequency of 40 min of steady state intensity training was associated with significant reductions in fasting insulin, total body fat, subcutaneous leg fat, and abdominal fat.

Another study by Burgomaster et al. demonstrated that short interval training is a time efficient strategy to induce changes in whole body and skeletal muscle carbohydrate and fat metabolism during exercise that are comparable to changes induced by traditional endurance training.
In a 2008 study, the researchers showed that high-intensity intermittent exercise led to a significant decrease in central abdominal fat, whereas the steady state intensity and control group had non-significant decreases in central abdominal fat.

To support this, research from the University of Lethbridge showed that six weeks of HIIT three times per week reduced both body fat and waist circumference.

Furthermore a study conducted by Wilson et al. From the University of Tampa, shows when you add in LISS you get a temporary boost in weight loss. Subjects lost a couple of pounds the first week and after that they lost nothing. This happened because their metabolism completely adjusted to that and it became their new set point to what they had to do just to maintain.

A study by Trapp et al. showed subjects in the high intensity interval group gained lean mass in their legs compared to the steady state intensity and control group.

Moreover, Burgomaster et al. stated that the very intense nature of sprint interval training might stimulate rapid skeletal muscle remodeling (possibly due to fiber recruitment), whereas adaptations to lower intensity accrue more slowly.

HIIT is quicker and more time efficient, proves to be more effective for fat loss, creates metabolic changes, and helps with muscle retention but not everybody can do HIIT. LISS is safer, but takes twice as long to accomplish similar things and it still has its place for fat loss in moderate amounts, from a pure calorie burning standpoint (meaning only to burn calories & not make changes to your metabolism). Then again, many people enjoy doing LISS cardio for cardiovascular and relaxation purposes and there’s nothing wrong with that.



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