It sounds too good to be true: A workout that lasts less than 15 minutes, and you only have to do it a couple times a week.

While the concept may seem like every busy American’s dream, this kind of exercise regimen is far from fantasy – and its effectiveness can be very real according to some experts.

Over the past decade, many trainers have begun advocating for shorter, less-frequent workout regimens – claiming that they are much more efficient for weight loss and muscle building. Routines ranging from the 7-minute workout to a once-a-week 12-minute workout claim to achieve better results than the standard formula of a 30- to 60-minute workout done five to six times a week.

The key to the short workout’s success revolves around a concept known as high-intensity interval training (HIIT).  HIIT is a heightened form of interval training that involves alternating between periods of short, intense physical activity and fixed periods of low activity or rest.  Intervals can include anything from fast squats and pushups to weight lifting and powerful cardio.

But a word of caution: While these workouts may be short – they are also very tough.

How HIIT works

According to many HIIT trainers, the overall goal behind short workouts is optimal efficiency – getting the best results in the proper amount of time and eliminating opportunity costs.  Rather than performing exercise routines at 50 to 75 percent of a person’s maximum heart rate, the intense intervals are meant to utilize 80 to 100 percent of an individual’s heart rate – producing more results per minute of exercise.

“With more traditional workouts, there was a tendency to pace yourself – so holding back for the first 20 to 30 minutes,” Tony Horton, creator of P90X3 and author of the new book, “The Big Picture,” told FoxNews.com. “But when you design them shorter with very few breaks in between…you’re moving multiple body parts over the course of one movement, the heart rate is higher, and it just becomes more efficient.”

Furthermore, some studies suggest that these periods of high intensity exercise produce a unique metabolic response in the body, causing it to continue to burn fat for up to 24 to 48 hours post-workout. However, one fitness expert noted that it’s important to keep these HIIT workouts short, because after a certain period of time, people can wind up damaging their body rather than strengthening it.

“What most studies are showing is if you go over about 40 minutes – not weight training, but interval training or cardio – your body will switch from burning sugar or fat to burning muscle,” Dr. Josh Axe, physician and co-founder of the BurstFIT interval training program, told FoxNews.com.  “It’s right in between 40 to 45 minutes that it starts to have a more negative impact.”

Axe said this concept revolves around an individual’s glycogen storage, which serves as a source of energy for the body.  When individuals eat too many carbohydrates, the body stores the excess as glycogen, which is the first thing burned during a high-intensity workout.  However, the body can only store so much glycogen at a time, so once it is all burned, the body turns to burning muscle instead.

And, not only is it important to work out for shorter periods, but Axe noted it’s also imperative to restore the body’s glycogen once the workout is over.

“If someone goes to work out, you need fruit or quick carbs [after] you work out, because you’re going to restore your body’s glycogen for the next day,” Axe said.  “This is something I say during nutrition consults: Your post-exercise diet is important, because if you want to perform at your highest the next day, you need glycogen.”

Working out just once a week?

Because short HIIT workouts are supposed to burn fat for a couple of days once they’re completed, many trainers claim that they can be performed less frequently than more traditional workouts. Axe maintains that in order to achieve optimal effects from his workouts, they should be performed every other day.

But while that may seem infrequent, one physician has proposed a HIIT workout that requires an even longer break between exercises: one full week.

In his book “Body by Science,” Dr. Doug McGuff utilized hundreds of studies and empirical research to devise the ideal workout.  And according to his findings, that workout is a 12- to 15-minute HIIT routine performed just once a week.

McGuff found that in order for the body to better adapt and reshape its muscles, it must receive the proper exercise stimulus – which involves more focused exercises rather than longer ones.

“In general, the productive component of exercise is not the amount of work that is done. Instead, the stimulus [needed] to make a physiological adaptation is the intensity – in particular, the muscular intensity of the work being done,” McGuff, a partner with Blue Ridge Emergency Physicians, P.A. and weight lifter, told FoxNews.com.  “We found that what really was the key stimulus for triggering adaptation was actually the momentary weakening of the muscle – when you’re rapidly fatiguing the muscle, that was the stimulus that acted upon the body.”

To achieve this proper exercise stimulus, McGuff created a workout he says is so intense that a person cannot tolerate doing it for very long.  But while other HIIT routines often involve intense cardio or fast-paced movements to achieve high intensity, McGuff’s workout advocates the lifting and lowering of weights very slowly – eliminating the need for acceleration and the risks for serious injury.  That way, as individuals proceed through the exercises, they become weaker and weaker until they can no longer move the weights.

And as for that weeklong break, McGuff said it all revolves around not interrupting the body’s response to exercise – which he found typically takes a full week to complete.

“We’ve come to understand that exercise is a stimulus that acts upon the body.  The body receives that stimulus, and it needs time and resources to construct an adaptation to that stimulus,” McGuff said. “...Exercise is viewed as a negative threat to the body, and the body makes an adaptation to the negative thing that is positive….But it’s very important not to apply that stimulus until you’ve completed the adaptation to the first stimulus and fully recovered.”

What’s best for you?

While many fitness experts have flocked to the idea of short, high-intensity workouts, there are still discrepancies on the best way to conduct them.  Though McGuff proposes a once-a-week workout, others ultimately feel that it’s better to do more, if at all possible.

“One of the things I truly believe in is the more you do, the better you get,” Horton said.  “You have to think about, ‘What are [my] goals?’ …I exercise with the main focus that I slow down the aging process, maintain my general state of health and improve physically so I can do interesting things with my body when I get older. I want to train as many times a week as I can so I can ski and mountain climb and bike and swim and hike.  And that’s not going to happen once a week.”

But Axe noted that it’s not always feasible for people with busy schedules to fit in exercise every day.

“I take care of a lot of families who are really busy, and they only have so many hours in the week,” Axe said. “But if you’re trying to exercise every other day, your body will continue to adapt for the next 36 hours.  Most people can fit 20 minutes into their schedule, three times a week.  That is the ideal scenario.”

Additionally, many other experts maintain that moderate exercise and endurance training is the best for getting fit.  In fact, the Centers for Disease Control and Prevention recommends adults perform at least 150 minutes of moderate-intensity aerobic activity at least once week, in addition to muscle-strengthening activities.

Regardless, nearly all experts agree that the best exercise is ultimately the one that gets done, and it’s important for individuals to find the workout that keeps them motivated – and moving.

“Everyone’s always looking for the Holy Grail of exercise, the fountain of youth,” Horton said. “But there isn’t a one-size-fits-all.  A college student’s priorities are different from a mother of five.  You just need to be aware of when [your life] changes, you find a philosophy that works well for you.”