Everyone knows about high intensity interval training—better known as HIIT. But unless you've been hanging around some high-performance training labs, you probably haven't heard of its even more intense relative: supramaximal interval training (SMIT). Both training strategies involve short bursts of intense exercise, followed by periods of recovery, then a repetition of that cycle that ultimately lands you with faster neuromuscular and cardiorespiratory adaptations and some serious fat burn.

The main difference between the two? HIIT is training done near your maximum capacity—at 90%, maybe—while SMIT is performed above 100% of your maximum fitness capacity, says Michele Olson, Ph.D., professor of exercise physiology at Auburn University.

Wait: Above 100%? How is that even possible? Read on.

What is SMIT?

With supramaximal interval training, intervals are performed at an all-out effort—as hard as you can possibly go, says Richard Metcalfe, Ph.D., lecturer in exercise and health at Ulster University in the UK who has studied SMIT intensely.

“Because you are exercising 'all out,' you are working at intensities much higher than that which would elicit your VO2max (maximal oxygen uptake), hence the term ‘supramaximal,’” he explains.

Compared to HIIT, your intervals are shorter and recovery periods longer because your muscle metabolism needs to recover, as do your cardiovascular and neuromuscular systems.

Okay, but how exactly do you go above 100% max effort? Metcalfe explains a scenario based on the most commonly studied form of SMIT, cycling sprints on a specialized bike where you can adjust the resistance instantaneously. It works like this: You start by cycling as fast as you can against little or no resistance. Then, when you're at your top speed, you drop on some resistance (usually 7.5% of your body weight), and keep cycling as fast as you can for 20 to 30 seconds. Then, you rest for 3–4 minutes. Ultimately, you'll repeat that interval 4–6 times in a session.

“Imagine freewheeling down a steep hill and then immediately cycling as hard as you can up a steep hill for 20 to 30 seconds, and you get the idea,” he adds. “SMIT is not for the faint of heart and few can do it properly,” Olsen says. “What most people end up doing is HIIT—which is fine. But to truly do SMIT, you have to do more than your maximum effort, which means you need equipment that ensures you push harder than your max.” Employing equipment like treadmills and specially-braked bikes force your legs to move faster than they can on their own, she explains.

That means that you can’t apply SMIT to all workouts, though. You can run sprints on a treadmill or do cycling sprints on a specialized stationary bike, or you can leverage squat jumps and lunge jumps if you can push yourself beyond exhaustion, Olsen says. On the other hand, calisthenic classics like burpees, push-ups, and crunches don’t employ enough muscle mass for you to push yourself as necessary.

What’s the advantage of SMIT?

Just as with HIIT, SMIT offers efficient conditioning—more progress in a shorter time—compared to steady-state cardio training. Sprint interval training (another name for SMIT) has been shown to increase aerobic capacity in healthy adults, without taking up nearly as much of their time as training via continuous exercise, according to a 2014 study analysis in Sports Medicine.

And just as with HIIT, SMIT trains both your cardiovascular and nervous system efficiently and effectively. A 2013 study in the European Journal of Sport Science pitted SMIT against both HIIT and continuous running. Researchers had sprinters train three times per week for six weeks either via SMIT (30 seconds on at 130% running velocity with 150 seconds recovery between the 7 to 12 sets), HIIT (4 minutes on at 100% running velocity with 4 minutes recovery in between the 4 to 6 sets), and continuous running (30 minutes at 75% running velocity). The results: In a 3,000 meter time trial, the group who trained via SMIT ran faster than those who did continuous running.

SMIT may even have an advantage over HIIT. In that same EJSS study, the SMIT-trained sprinters were faster in both the 40-meter sprint and repeated sprints (6x40 meters) than the HITT trainees.

SMIT's also more efficient than HIIT. Doing a few supramaximal cycling sprints lead to better improvements in VO2max—the gold standard measurement of your body's ability to use oxygen and general fitness status—rather than doing more, according to a study Metcalfe published January in Medicine & Science in Sports & Exercise. In fact, after fit folks performed two supramaximal cycle sprints, each subsequent sprint actually reduced participants' improvements by about 5%, Metcalfe’s study found.

It is important to note, though, that while SMIT can improve speed and aspects of performance in some sports, it's not always much better than HIIT. In fact, while both training methods outperformed control groups, neither HIIT nor SMIT led to improvements in collegiate rowers’ performance during time trials, peak power output, or VO2 peak, according to a study in the Journal of Strength and Conditioning Research.

You probably won’t gain more muscle, Metcalfe says. But Olson adds that you will burn slightly more fat with SMIT, since intense exercise increases production of the neurohormone  epinephrine, which causes more fat to be released from the fat cells. Furthermore, excess post-exercise oxygen consumption (EPOC), or “the afterburn,” is higher with SMIT than HIIT. Even if you don’t necessarily burn more calories during the workout, the higher intensity boosts your metabolism for a longer period of time after you leave the gym, she explains.

How long do you need to do SMIT to see results?

This depends on if we’re talking clinically or realistically. Metcalfe points out that most studies use a strategy of 30 seconds on, 3 to 4 minutes off, repeated four to six times. But his newest study shows that when it comes to cycling SMIT, you only need to get after two or three sets of 20-second sprints to see max gains and optimal performance.

Olsen says most folks should aim for 10-second intervals of all-out effort, since it’s a helluva lot harder than you think and most studies are using highly trained athletes who are already absurdly fit.

You also need to devote more time and effort to the post-workout plan. “You will need extra recovery. Extra rest, more water, and more post-exercise carbs are all essential,” Olsen says. You are sending you body over the edge, after all—so you definitely need to give it the time and tools to recover.

Okay, so how do you know if you're ready to try SMIT?

If you’re still training at 50%, 60%, even 75% of your max, it’s probably not time to take this on yet. “It’s important to build up slowly with a period of lower-intensity cardiovascular training,” Metcalfe says.

If you’re seasoned at HIIT—a veteran CrossFit type, or an experienced sprinter—then start using your heart rate monitor to first see if you can work during intervals of 90–100% of your max heart rate, Olsen advises. Once you’re not struggling here, you’re ready to take SMIT on.

During the first session or two, though, you may feel nauseous or light-headed during the recovery periods in between, Metcalfe warns. Be sure to eat something a couple hours before and avoid any heavy meals within the hour preceding a workout, he advises.

There’s no gadget or tool that can tell you you’re actually hitting SMIT and not just HIIT. All you can gauge with is whether your level of perceived exertion is at 100%, Metcalfe says. “Then, by extension, what we can measure physiologically, like heart rate, lactate acid, and VO2max, will then depend on the length of the intervals and the number of repetitions you do,” he explains.

For example, if you did one 10-second ‘all-out’ sprint, your heart rate might only reach 80–85% of max and you might rate it as a 7 out of 10 for exertion. “If you increased the sprint to 20 seconds, then you might reach 90–95% of your max HR, and rate it as a 9 out of 10 for exertion. Similarly, if you did repeated 10-second sprints, then with each additional repetition your HR and perceived exertion will likely increase,” he explains.

Translation: The exertion will build over time, so all you can do is give the sprint *all* of your effort, and hope you’re doing SMIT. Even then, the worse-case scenario is that you’re hitting HIIT territory, and, as we all know, that’s certainly not a bad place to be.

If you survive one bout and want to jump on board the SMIT train, cap the training regimen at once or twice a week tops, Olsen says, considering it’s so intense.

But if you never reach that point where you feel confident pushing beyond 100%, that’s totally okay. Because while SMIT may burn slightly more fat, send you into more of an afterburn, and save some time on training, you also need specialized equipment and, most of the time, the workout is incredibly uncomfortable. Considering it doesn’t deliver eons better results than HIIT, you shouldn’t stress yourself out just to achieve the ego trophy of saying you can handle the top-level burn.