What is HIIT?

HIIT, which is short for high-intensity interval training, is an alternative approach to cardiovascular training.  It employs short durations of high-intensity exercises like sprints, where you’re performing very close to your maximum effort. 

In between each all-out rep, a recovery period of either lower-intensity activity or rest is generally performed for 4-6x the rep period.  This basic process is usually repeated over the course of somewhere between 10 – 30 minutes in total time.      

As you can see, one of the key differences between HIIT and steady-state cardio (SSC) is that instead of working at a consistent pace, with HIIT you’re shifting between periods of high intensity and rest, rather than just trying to maintain the same moderate speed on say a treadmill.    

Another important difference between high-intensity interval training and SSC lies in how they affect your heart rate.  In order for what you’re doing to be considered a high-intensity exercise, you need to be pushing your heart rate to 90 – 95% of your max. (1)  On the other hand, with SSC your heart rate usually won’t get much above 70%, which is why you can ultimately keep at it for longer.   

An easy way to estimate your average max heart rate is simply by subtracting your age from the number 220.  So for instance, if you’re 30 years old, using this calculation you’d wind up with a max heart rate of 190 beats per minute (BPM).  That means for a high-intensity workout (90 – 95% MHR) you’d be shooting to maintain a heart rate of between 171 – 181 BPM during the exercise.  

From there it’s as easy as monitoring your heart rate while you’re exercising so you know when you’re where you need to be — nowadays, there are sorts of apps and devices out there that make tracking your heart rate incredibly easy.  

Benefits of HIIT

Okay, so we know that HIIT is a different approach altogether when it comes to aerobic endurance training but does it actually work and if so, what are some of the benefits?

1. Increase Your Maximal Oxygen Uptake

Numerous studies have illustrated that just like with steady-state cardio, HIIT also helps to improve your muscles’ maximal oxygen uptake (MOU), which is perhaps the most important factor of all when it comes to building your endurance. (2)(3) 

Your muscles need a continuous supply of oxygen when you’re working out and your heart plays a central role in the delivery process.  Fresh oxygen is delivered to your muscles via the blood-stream and old, nutrient-deprived blood is, in turn, circulated back to the heart for replenishment. 

As the exercise you’re doing becomes more demanding, however, your heart has to work harder and harder to supply your muscles with the oxygen they need.  When your heart isn’t able to fulfill your muscles’ oxygen demands, it leads to diminishing results in performance until exhaustion sets in completely.  

Not surprisingly, numerous studies have explored the effects of steady-state exercises like long-distance running and cycling on maximal oxygen uptake.  While researchers have found that traditional exercise like these can be effective for building endurance, they’ve also found that well-trained individuals often have trouble making further improvements with long-distance training alone.       

High Intensity Interval Training

Some research even suggests that HIIT may actually be the better of the two methods when it comes to building cardiovascular endurance.(4)(5) For example, a 2007 study exploring maximal oxygen uptake in moderately-trained individuals compared the effects of aerobic endurance training across a variety of different intensities.(6) 

Study participants were placed in one of 4 groups, each tasked with the same work-load, but spread across different amounts of time.  Groups 1 and 2 were tasked with lower-intensity, longer-distance exercises at either at 70% or 85% of their max heart rate.  Groups 3 and 4 were given 1 of 2 different high-intensity training protocols.

The 1st HIIT group (3) did 15 seconds of sprints at 90 – 95% max heart rate followed by a 15 second rest period.  The second HIIT group (4) was tasked with running for 4 minutes at 90 – 95% max heart rate followed by a 4 minute rest period.   

Participants performed the same training protocol 3 days a week for 8 weeks.  Ultimately, the researchers found that those in the HIIT groups on average saw significantly greater improvements in MOU compared to participants in the lower-intensity, longer-distance groups.  

When looking specifically at the HIIT groups, the researchers found that the group who ran for 4 minutes followed by 4 minutes of rest on average saw the best results, however, only by the smallest of margin — 7.2% increase vs 5.5% for the other group.  Long-story-short, both approaches to high-intensity interval training appear to be effective when it comes to improving your oxygen uptake.  

2. Burn Calories Quicker

Burning calories is another reason many people become interested in endurance training.   But the thing is, just maintaining a constant speed at lower intensities isn’t all that efficient when we’re talking about burning calories.  For example, if you want to burn 500 calories, you’d have to run for 45 minutes if you’re running at about 10 minutes per mile.   

The reason it’s pretty inefficient all has to do with intensity.  When you’re working at lower intensities, you’re not really taxing your muscles all that much, which also means you’re not really pushing your heart rate.  That all ultimately means it can take a long time running on the treadmill if you’re looking to burn off hundreds of calories.    

But with high intensity-interval training, you’re working at intensities ranging from 90 – 95% of your maximal heart rate, which greatly increases your body’s calorie expenditure.  That means, the exercises might be more challenging, but you’ll spend far less time doing them, which is one of the biggest benefits.  

For instance, 12-15 minutes of HIIT is the equivalent from a conditioning and health standpoint of somewhere closer to 30-40 minutes of steady-state cardio.  For those who are not particularly keen on cardio, those are some pretty big-time savings. 

3. HIIT for Weight Loss

Not only does HIIT burn calories quicker, but it also keeps your metabolism elevated for longer after your workout.   Several studies comparing HIIT and steady-state training protocols have found that those partaking in higher-intensity workouts on average have higher metabolisms in the hours following training.(7)(8)

Raising your metabolism ultimately increases your fat-burning potential because it heightens your body’s caloric needs.  If you’re eating in a caloric deficit, that ultimately means your body is going to turn to its fat reserves in order to make up for the calories that are missing from your diet. 

The higher your metabolism and the greater your caloric deficit, the more fat your body is going to have to burn up in order to meet it’s energy needs.    


4. You Can Do It Anywhere

Another benefit to HIIT is that you can do it anywhere.  There are a number of HIIT exercise and even entire training protocols that require no special equipment or training facilities at all — you don’t need a treadmill, bicycle or track.  There are dozens of different HIIT exercises you can literally do just about anywhere. Here are a few popular and effective ones:

HIIT Exercises You Can Do Anywhere

HIIT Exercises (Some Equipment Required)

How to Make Your Own HIIT Workout

You don’t need to do HIIT every day.  In fact, most research shows that 2-3 days of training a week is more than enough to build endurance and burn some serious calories. (9)(10)   Because you’re working at such high-intensities, your body also needs an ample amount of time to recover, so you don’t want to overdo it with the amount of session you throw into your weekly schedule.  

In terms of what exercises you should do in your HIIT routine, it really depends on what you like and what you’re comfortable with.  There are hundreds of different exercises that can all potentially be effective when used correctly, so its really more about finding activities that are challenging but still enjoyable.  The key is staying consistent, so finding ways the get back into the gym time and again is really the most important aspect of choosing your own routine.

There can also be just about as much variation in terms of how you structure your routine, regardless of what exercise you choose to do.  As we saw earlier, you can burn plenty of calories and improve your endurance no matter whether you do intervals of 4 minute or 15-second at a time.  So, it again comes down to figuring out what your preferences are and setting up a routine that suits them.  

You also don’t have to stop at doing just one exercise in your HIIT training.  Building your routine around a circuit of different exercise can break up the monotony even further.  So for example, you could do 20 seconds of squat jumps at 90 – 95% of your max heart rate, after that, the same thing for burpees and mountain climbers, then 1.5 – 2 minute of rest, then you repeat the whole process.