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If you happen to be one of those people who hits the gym or goes running for an hour, three times a week like clockwork, then congratulations — you're within the minority of Americans who gets the recommended amount of physical activity for healthy adults.

For the rest of us, exercise tends to be yet another chore that can easily fall by the wayside. Amid the countless responsibilities of our work, family and social lives, there doesn't seem to be enough hours in the day to spend one of them working out.

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According to its current guidelines, the American College of Sports Medicine recommends that healthy adults should be getting at least 150 minutes of moderate or 75 minutes of vigorous physical activity throughout the week. Yet less than half of U.S. adults reach these benchmarks, with not enough time reported as a common barrier to regular exercise.

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But being “too busy” is starting to sound more like an excuse with the recent rise in popularity of a time-efficient exercise strategy called high-intensity interval training (HIIT). In terms of physiological benefits, HIIT compresses of an hour or more of traditional exercise into a few minutes of high-intensity training per session. Studies have shown improvements in markers of cardiovascular health, metabolic capacity and aerobic fitness that often exceed those seen in continuous moderate-intensity exercise — and all using workout routines that are relatively short.

“The benefits [of HIIT] are many, from reduced body composition to increased aerobic capacity, but one of the best benefits is the short amount of time required,” said Yuri Feito, an assistant professor of Exercise Science at Kennesaw State University. “Considering the vast majority of individuals typically cite 'lack of time' as the primary reason why they do not exercise, HIIT provides a great alternative to those 'long and boring' workouts that most people dread.”

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A typical HIIT routine is characterized by alternating bursts of intense, heart-pumping activity followed by short periods of recovery, which can either be complete rest or low-intensity effort. As long as you stick with this pattern, the actual exercise used can be whatever gets you moving — cycling at the gym, sprinting outside, climbing stairs, or even using your own body weight at home through burpees and jump squats.

A single workout often lasts less than a half-hour and can be as short as 10 minutes per session. But the overall time spent doing vigorous physical activity — minus the warm up, cool down and recovery periods — adds up to only minutes per week.

A 2014 study found that only three minutes of high-intensity exercise per week was enough to improve cardiometabolic health in overweight individuals. The protocol had subjects cycle on a stationary bike for 20 seconds at an all-out, sprinting pace, followed by 3 minutes of cool down, and repeated three times. After six weeks doing three sessions per week, they showed an increase in peak oxygen uptake and lowered mean arterial pressure.

A mere 20 seconds of exercise at a time sounds easy enough, but many HIIT routines use a grueling, all-out pace that can be incredibly taxing on the body. Traditional moderate exercise — think jogging — aims for a target heart rate that is 50 to 69 percent of maximum heart rate[heart.org], whereas the 20-second bursts in the study protocol had subjects reaching an agonizing 90-plus percent.

“You can get away with a small total dose of exercise if it's performed in a very extreme manner, which can be very efficient but not necessarily widely applicable, and it can be very uncomfortable,” said study author Martin Gibala[science.mcmaster.ca], the chair of the Department of Kinesiology at McMaster University and one of the leading researchers of HIIT. “What it really comes down to is that there's no free lunch, and you have this intensity-duration trade-off.”

Also, when comparing the same amount of exercise based on total calorie expenditure or total workload, HIIT often yields greater benefits over continuous exercise. A number of studies using certain HIIT protocols have shown superior improvements in peak oxygen uptake, muscle mitochondrial content, and glucose control in various populations.

What is it about short bursts of intense activity that can lead to such profound changes in the body? The exact mechanisms are not yet known, but some experts think that performing HIIT exercises recruits a population of muscle fibers that otherwise lies dormant for the majority of our lives.

“Broadly speaking, there are two types of muscle fibers in the body: slow-twitch fibers are what we call upon for most of our daily living, while fast-twitch fiber are used for fast, explosive-type movements,” Gibala said. “Most people rarely call upon these muscle fibers, but when you do these HIIT exercises, these fast-twitch fibers respond, adapt and remodel.”

He notes that there might also be something about the on-off pattern that is characteristic of interval training. The multiple transitions from activity to rest in a single HIIT workout session could help trigger faster changes in the muscle fibers and add up to a greater overall effect, as opposed to continuous exercise which only has a single transition.

“We think many people can perform and benefit from an interval training-based approach to fitness,” Gibala said. “Some people think that interval training only happens at a 'sprint-to-save-your-child' pace, but that's only one type of interval training. There are many forms of interval training that can be tailored to different starting levels of fitness.”

For instance, elderly individuals can reap the benefits of HIIT through interval walking instead of sprinting. A 2007 study of middle-aged and older people [sciencedirect.com] found that subjects who trained in interval walking at least four days per week had a reduction in blood pressure, increase in thigh muscle strength and improved peak aerobic capacity after the 5-month study period. These positive changes seen after interval walking also exceeded those recorded in subjects who performed moderate-intensity continuous walking, suggesting that HIIT may be more helpful in protecting against age-related declines.

Gibala recommends that beginners can start with the easy-to-remember 10x1 routine: one minute of high-intensity exercise, one minute of recovery, and repeat 10 times. A single training session consists of 10 minutes of hard work out of 25 total minutes, including a recommended 2.5 minutes each of warm up and cool down.

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