HIRT, Hypertrophy and the Afterburn Effect: What the Science Says

There’s a good chance you’ve never heard of HIRT.

It’s a form of resistance training that’s proven to be very effective at raising your metabolic rate in the hours after exercise.

In fact, a recent study shows that HIRT (short for high-intensity interval resistance training) had a much bigger afterburn effect than a workout lasting twice as long.

Here’s a closer look at the research and what it all means for you.

Researchers from Italy’s University of Padova put a group of 17 trained men through a couple of different workouts.

They measured the men’s metabolic rate on two separate occasions – once before the workout and then again 22 hours after it was finished.

In the first workout, dubbed Traditional Resistance Training (TT), the men performed 4 sets of 8 different exercises (bench press, leg press, leg curl, seated row, military press, biceps curl, triceps extension, sit-up).

They were told to perform as many repetitions as possible in each set, which ended up being somewhere between 8 and 12. Each set was separated by 1-2 minutes of rest.

In total, the workout involved 32 sets and lasted approximately 52 minutes (excluding the warm up).

The HIRT workout was much shorter, and involved the use of heavier weights and lower reps.

The HIRT Workout

With HIRT, you select a weight that you can lift 6 times before hitting muscular failure. Then you put the weight down and rest for 20 seconds. Pick the weight up and crank out 2-3 more reps until you reach failure again. Rest for another 20 seconds. Do another 2-3 reps. All of that counts as one set.

Essentially, you’re taking a weight that you could normally only lift six times and lifting it 10-12 times instead.

The men rested 2.5 minutes between each set. They did 3 sets on the leg press and 2 sets on the bench press and seated row. Excluding the warm up, the length of the HIRT workout was approximately 22 minutes, which was less than half that of the TT workout.

HIRT is just another name for rest-pause training, which has been used for years by bodybuilders and strength athletes to gain both size and strength. So the method itself is nothing new. But its effect on the metabolic rate is not one that I’ve seen tested before.

HIRT and the Afterburn Effect

What impact did each workout have on post-exercise metabolism?

When it was measured 22 hours after TT, resting energy expenditure was about 100 calories higher than normal, going from 1901 to 1999 calories per day.

But the increase seen after HIRT was much greater, where resting metabolism rose by over 450 calories to an average of 2362 calories per day.

The figure below shows resting energy expenditure (REE) before and 22 hours after both HIRT and TT.

The researchers also measured how much fat the men were burning. Again, HIRT increased the rate at which fat was being oxidized to a greater extent than TT.

In short, the rest-pause workout led to a rise in post-exercise metabolism that was over four times higher than the traditional workout, despite the fact that it lasted only half the time. And much of this rise was accounted for by an increase in fat metabolism.

All research has limitations, and this study is certainly no different.

The main problem is that the researchers used something called an ergospirometer to estimate how many calories were being burned.

This involves measuring the amount of air that you breathe out over a period of 15–30 minutes, analysing the composition of that air, and then using the data to estimate how many calories you’re burning. It’s quick and relatively cheap, but not particularly accurate.

A better (albeit more expensive) approach would have been to use something called a metabolic chamber. This is basically a small room that allows for energy expenditure to be measured continually, which makes it a lot more accurate.

In addition, the men taking part in the study were very muscular, weighing an average of 185 pounds with just 8.5% body fat. At a height of just under 5 foot 9 inches, that gives them a fat-free mass index (FFMI) of 25, which is more muscle than most people can expect to gain naturally.

Why does this matter?

We know that the more muscle you have, the more calories you’ll burn after an intense workout.

When exercise ends, it takes time for everything to get back to normal. Depleted energy stores need to be refilled. Damaged muscle cells need to be repaired. All of this requires energy.

The more muscle you have, the more rebuilding work has to be done. And the more rebuilding work that needs to be done, the more calories you burn.

The upshot of which is that this type of training is not going to raise post-exercise metabolism in the average woman to the same extent as it does in the average man.

I should also point out that the men were given a diet designed to match the number of calories they were burning off each day.

But if you’re in a calorie deficit (which you’ll need to be if you want to lose fat), the afterburn effect is going to be lower than it otherwise would be. In fact, just two days of dietary restriction has been shown to lower the post-exercise calorie burn by 40-50%.

In other words, we’d have seen a much smaller rise in post-exercise metabolism if this study had used dieting subjects.

So, the extent to which the rise in post-exercise metabolism contributes to weight loss is going to differ from person to person, and will be heavily influenced by the amount of muscle you’re carrying around and how much food you’re eating.

Finally, I don’t want to create the impression that you should immediately drop everything you’re doing and start doing HIRT in every workout.

This is far from being the last word on strength training and fat loss, and the effect that a given workout has on post-exercise calorie burn is not the only or even most important criteria by which to judge its effectiveness.

While the study shows that HIRT did a better job of raising the metabolic rate in the hours after exercise, you would have burned more calories during the traditional workout itself, mainly because it lasted twice as long.

So the net calorie burn (calories burned both during and after exercise) with the traditional workout would likely have been greater than it was with HIRT.

HIRT for Hypertrophy

HIRT has also been shown to stimulate muscle hypertrophy in a lot less time than conventional training.

In one study, a team of Brazilian researchers found that muscle was built just as quickly with one rest-pause set as it was with three normal sets.

For the study, lifters were assigned to one of two groups: traditional or rest-pause training. Both groups trained four days a week, hitting the chest, shoulders and triceps on Monday and Wednesday, and the legs, back and biceps on Tuesday and Thursday.

The traditional group completed 3 sets of 6 reps with a weight that was 80 percent of their maximum, resting for two minutes between each set.

Lifters in the rest-pause group did the same exercises, also using a weight that was 80 percent of their maximum. However, after completing the first set, they rested for 20 seconds, did a few more reps, rested 20 seconds, and so on. Once they’d completed a total of 18 reps, they stopped.

After six weeks, the researchers reported “no significant differences in strength gains and body composition changes between groups.” Put differently, the results from rest-pause training were equal to those achieved with traditional training.

Crucially, the rest-pause group made their gains with less gym time. In fact, their workouts lasted just 35 minutes—roughly half the length of the hour-long workouts performed by the traditional group.

Final Thoughts

Lifting weights has a number of benefits besides simply making you strong. Not only does it help to maintain muscle mass while you’re in a calorie deficit, the rise in post-exercise metabolism after a bout of heavy lifting appears to be a lot bigger than previously thought.

All of which means that the type of resistance training that makes you bigger and stronger is much the same as the type of resistance training to do when you want to drop fat.