What is HIIT and Why is it so Popular?

by Michael Palevo

Over the recent years, High-Intensity Interval Training, better known as HIIT, has become an increasingly popular mode of exercise primarily due to its effects on exercise capacity, small amount of time requirements, and its exponential payoff in regards to weight loss. Here’s my take on it.

In today’s world, people want to get results in the fastest way possible. Would you rather spend 60 to 90 minutes on a treadmill or 30 to 40 minutes of different intervals? While both options burn the same amount of calories (if not more), HIIT tends to house better focus and engagement on each immediate task. One of the major appeals in HIIT is it’s time-efficient way to accomplish the adaptive goals of exercise training.

But what do we really know about High-Intensity Interval Training? Two things are obvious – it takes less time to complete and we can lose fat at a faster rate than the long, boring, stationary, cardio we have grown accustomed to. However, this article is meant to dig a little deeper and truly explore what adaptations there are with HIIT. To follow, we are addressing three physiological adaptations that can happen with consistent and structured HIIT.

1) Fat Oxidation

Fat Oxidation can be associated with increased mitochondrial (muscle) volume, which can be assessed by citrate synthesis activities, which improve the potential for the muscle to utilize lipids as a substrate for energy. It is also associated with improved insulin sensitivity (the same as intermittent fasting). Improvement of skeletal muscle fatty acid oxidation is a key factor in someone trying to increase fat oxidation during exercise and is important for athletes attempting to spare carbohydrates during competition. This statement is acknowledging that through higher intensity workouts your body can use fat as a direct energy source instead of carbohydrates, which is a primary energy source for muscles and Glycogen Synthesis.

2) V02 Max and Blood Cholesterol


People are very concerned with the changes that they can physically see like added muscle definition and fat loss. However, HIIT will actually alter acute and chronic changes first. For example, an increase in V02 max or the volume of oxygen the body can supply muscles during exercise (up to 13 to 18 percent in some studies). We can also see an increase with HDL (High-Density Lipoproteins, also known as the good cholesterol), Decrease in Triglycerides and body fat. We may not be able to see all of the changes that are happening to our bodies, but that time will come. HIIT conditionings will reshape the body from the inside out to begin with and your internal processes, plus your physicians will thank you later.

3) Aerobic and Anaerobic Capacity Changes

Aerobic capacity is the ability of the heart and lungs to get oxygen to the muscles. An example of aerobic capacity is the body’s ability to take in and use oxygen to improve aerobic performance (think endurance). On the other side, we have anaerobic capacity, which is the total amount of energy from the anaerobic (without oxygen) energy systems that is the combined amount of output for the ATP, phosphocreatine and lactic acid systems (think short bursts of intense activity).

As an individual completes a consistent and structured HIIT program they should see aerobic and anaerobic changes over shorter amounts of time due to the increase in the body’s ability to use oxygen more effectively and efficiently throughout a workout. HIIT combines both forms of aerobic duration and exercise with a variety of anaerobic movements (sprinting, strength, and plyometric movements) in order to structure internal results before ever seeing external changes.

You might also be wondering, “How much is too much?” in terms of HIIT workouts. But just like any other mode of exercise, your body needs time to recover and rest. During a HIIT session, you are working at a much higher energy level. Your muscles will need time to reabsorb nutrients and make necessary adaptations to keep you progressing. With that being said, give yourself 24 to 36 hours of rest or active recovery. Most likely, this will mean two or three true HIIT sessions per week, which will decrease the contingency of injury and increase productivity, allowing for optimum benefits out of each session.

NFM Staff
Author: NFM Staff

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