HIIT Cardio Breakdown

Lose more fat in less time with high-intensity interval cardio.


It’s no secret that cardio is more boring than algebra class. Unfortunately, it’s an integral part of the get-ripped equation. The good news is that there’s a formula designed to cut cardio time by a significant amount while boosting your body’s ability to torch fat. But in order to gain entry to this fat-incinerating state, you’ll have to ditch the old-school, slow-and-steady mentality and study up on high-intensity interval training.

Of course, as with any cardio program, HIIT only works when it’s paired with the right diet and training program. After a few sweat-drenched workouts and a payoff well worth the grueling task, you’ll be asking yourself, Why the hell haven’t I been using HIIT all along? Good question. What follows is a breakdown on HIIT and cardio in general – including machines to use and to avoid – with information from FLEX Senior Science Editor Jim Stoppani, PhD, and superstar trainer/nutritionist Hany Rambod, to help you get ripped and ready.


Although the exact definition varies depending on whom you ask, HIIT mixes periods of high-intensity training with low-intensity training or, at times, inactivity. For example, instead of walking on a treadmill for 40 minutes at a consistent rate — say 60%-70% of your maximum heart rate — you integrate periods of high intensity, when you run at 90% MHR, followed by intervals of low-intensity, slower-paced cardio. To find your MHR, subtract your age from 220.


In a 2008 study, data showed that subjects following a 20-minute HIIT program lost nearly six times more bodyfat than those who followed a 40-minute cardio program performed with constant intensity of 60% of their maximum heart rate. 

“Your metabolic rate remains higher for longer periods of time after HIIT,” Stoppani says. “Greater calorie burn, or EPOC [excess postexercise oxygen consumption], occurs postworkout, so while you’re at home sitting on the couch, your body is still burning fat.”

A 2001 study from East Tennessee State University (Johnson City) showed that those who implemented HIIT over an eight-week period lost 2% bodyfat compared to no bodyfat lost for those who trained without using HIIT. A separate study presented at the 2007 annual meeting of the American College of Sports Medicine by Florida State University (Tallahassee) confirmed that subjects who performed HIIT burned roughly 10% more calories in the 24 hours following the workout compared to those who performed a steady-state exercise, despite the fact that the total calories burned were the same for each at the end of the workout.

“If you’ve been doing cardio consistently and are aiming to break fat-loss plateaus, this is the way to do it,” Rambod concludes. “HIIT is a good way to attack some deep-seated fat.”

The numbers don’t lie. HIIT flips the postworkout switch and enhances the metabolic machinery in muscle cells that promote fat burning and restrict fat production. Both Rambod and Stoppani agree that 30- to 40-minute sessions are all the HIIT you need.


The only time, perhaps, that you shouldn’t utilize HIIT is if you are nursing an injury. Exerting more energy and adding more stress to an ailment during high-intensity periods could exacerbate the damage and affect your training. Put it this way: if your ankle’s throbbing, don’t hop on a treadmill and gear up for a HIIT session. Instead, find an alternative — such as a recumbent bike — and apply the HIIT principles there.


It’s a legit concern. Would HIIT impede your chances at winning a show? The answer is no, but there are exceptions. For example, when you’re dieting down, it’s important to be mindful of the amount of energy you can exert.

“Always know where you are in terms of your diet,” Rambod warns. “I’ve seen people pass out. They’re going at it and — boom! — they hit the floor. That’s a good way to create an injury or prolong a current one.”

Aside from exhaustion, HIIT can fulfill your cardio needs until it’s time to hit the stage. “Depending on your goals, you can utilize this method up to six days per week,” Stoppani says. “Start with a minimum of three, and — depending on how badly you’re looking to burn fat — boost the frequency and time.”


Not every aspect of HIIT was harmonious for Rambod and Stoppani. Sure, they both agreed on its usefulness, but preference also crept into the equation.


RAMBOD: “Always do cardio first thing in the morning, because you have low, fasting blood sugar, so you’ll be more likely to burn higher amounts of fat.”

STOPPANI: “The best time to do cardio is after you train, whether that’s in the morning or evening. Research shows that after you hit the weights, you burn more bodyfat. Of course, the main thing is to make sure it fits somewhere — anywhere — into your schedule.”


RAMBOD: “Go from 30 seconds to 2 minutes max, or 60%-70% of your MHR, and then go to 80%-90% MHR during high-intensity portions. I don’t allow any of my clients to do more than that. Or, 3-5 minutes at a lower rate and 30 seconds to 1 or 2 minutes at a higher rate.”

STOPPANI: “If you’re just starting, go with 15 minutes and slowly increase the duration. Use a 2:1 ratio. For example, a beginner would go with 30 seconds high intensity to 15 seconds low intensity for 15 minutes. Increase your high-intensity duration from there, but work up to a full minute, and then rest for 30 seconds.”


RAMBOD: “You might, if you don’t watch it. Exerting too much force for too long can lead to overtraining and muscle breakdown. So don’t overdo it.”

STOPPANI: “HIIT can actually help you gain muscle. The shorter, more intense bouts can stimulate muscle growth, while longer cardio at a steady pace is more likely a culprit for burning muscle. A recent study from the University of Oklahoma (Norman) found that subjects performing 15 minutes of HIIT three times per week for three weeks — while supplementing with beta-alanine — gained almost three pounds of lean muscle without picking up a weight!”


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