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By Melissa Dahl

Next year, your workout is going to get a lot more intense. Once the domain of fitness fanatics, high-intensity interval training is poised to hit the mainstream in 2014, according to a new survey from the American College of Sports Medicine.

“We’ve never seen something be introduced to the market and catch on so fast,” says Walt Thompson, regents’ professor of kinesiology at Georgia State University and lead author of the ACSM report. He’s talking about home workout DVDs like P90X or Insanity, or gyms like CrossFit and the rapidly expanding Orangetheory Fitness

Last year, high intensity interval training programs didn't even crack the top 20 on the ACSM's annual list predicting fitness trends for the upcoming year. This year, it's at the top. 

High intensity interval training, or HIIT, refers to workouts in which bouts of intense exercise are alternated with bouts of either rest of low-intensity activity. HIIT results in more fat loss than long, slow exercises like jogging, some studies have shown, and it's beneficial for your heart, too. But the best part: It's the most efficient workout around. You can knock out one of these routines in 20 minutes.

Here’s Jessica Matthews, an exercise physiologist for the American Council on Exercise, with an example of one of those 20-minute, fat-burning workouts.  

For the last couple of years, these programs -- especially Insanity, CrossFit and P90X -- have mostly been popular among people in their 20s and 30s, usually those who are already very active, Thompson says. "Exercise can be boring, and this is especially attractive for young people looking for an alternative," he says. 

Now, this new survey of 28,924 fitness professionals indicates that HIIT is growing up, and that older people and non-fitness-freaks are likely to ——start trying intense interval workouts next year. All of this has some experts worried that some sedentary people will, in a well-intentioned New Year’s resolution haze, hit the treadmill and try to run for 30 seconds at a 6-minute-mile pace – and then badly hurt themselves.

“I’m a little concerned, I think, is probably a good way of putting it,” Thompson says. He says this year, as HIIT workouts have soared in popularity, he's seen more orthopedic injuries - mostly joints, knees, hips and ankles -- and cardiovascular complications, like heart attacks or strokes. 

But high-intensity doesn't necessarily have to mean high-impact -- jumping exercises like plyometrics, or sprinting on the treadmill, are not appropriate for everyone, especially older folks or people prone to joint problems or heart issues, fitness experts caution.

“The interesting thing to me is that it’s name, the emphasis, that puts people off – it’s not called low-intensity training. But it could be,” says Michael Mantell, a behavioral scientist with ACE. 

Because the recovery or rest period is just as important as the period of intensity, he explains. It helps your heart become stronger, "because you have a rapid heart rate and then dropping it creates ventricular remodeling – it helps your heart with faster cardiac output," Mantell says. 

He says that if you're interested in trying interval training, but you haven't been regularly exercising, keep it slow to start. You can even do this on a walk around your neighborhood: Speed-walk for half a block, and then walk slowly again for a block. Keep going for 20 minutes -- you just did high-intensity intervals! How 2014 of you.

But if you'd like to try something a little more involved than a walk around the block, here's that ACE-approved workout. (If you need reminders on how to do any of the exercise moves here, watch the video above.)

Round 1 (4 minutes): 2 sets of high knees, 2 sets of plank punches, 2 sets of jumping jacks, 2 sets of side skatersRound 2 (4 minutes) 2 sets of jump rope 2 sets of high/low boat 2 sets of line jumps 2 sets of push-upsRound 3 (4 minutes) 2 sets of burpees 2 sets of Russian twists 2 sets of squats 2 sets of lungesRound 4 (4 minutes) 2 sets of mountain climbers 2 sets of push-ups 2 sets of split squats 2 sets of box jumps.