DALLAS -- Think about your day. Can you unearth a spare 20 minutes? They may be masquerading as Internet-surfing or lurking within the commercials-skipped sitcom you record and watch every evening.
This search comes with a caveat: When you find those wayward 1,200 seconds, you lose your no-time-to-exercise excuse. Because though most health recommendations are for a half-hour workout daily, a concentrated 20 minutes can suffice quite nicely.
"Do as much as you can in that 20 minutes," says Jakob Vingren, assistant professor in the department of kinesiology, health promotion and recreation at the University of North Texas, in Denton. "Get as much work done as possible in the allotted time."
At McMaster University, in Hamilton, Ontario, researchers studied the effect that intervals -- short bursts of intense exercise -- had on various groups of people. They found that a 20-minute workout consisting of one minute of strenuous activity alternated with a minute of easy recovery, had significant health and fitness effects on unfit volunteers, cardiac patients and, in later research, diabetics.
"There's a lot of benefit in 20 minutes if done the right way," says Bobby Patten, co-founder and head coach of Dallas Aquatic Masters. "If you go for a leisurely walk for 20 minutes, that's better than sitting. If you upped the intensity, it's better than a stroll. If you walked up and down hills, that's even better."
The brevity is good from a convenience standpoint, though, and breaking the time into pieces helps it pass even more quickly, says Craig Leverette, academic chair professor of physical education at Collin College, in Plano.
"Interval training is about fluctuation of heart rate within a target zone," he says. "Anything you do, a bunch of little milestones along the way goes by faster than staring at the exact same tree out the exact same window on the exact same treadmill."
The first part of most workouts tends to involve burning carbohydrates as an energy source, he says. Most people have a goal of burning fat, which comes later in a workout. The concentrated intensity of a 20-minute workout helps speed the process a little.
"Everybody has a target zone" for their heart rate, Leverette says, "based on age, resting heart rate, things like that. You want it to be as high -- moderate to vigorous -- as you can be. It will be hard to breathe and talk, not that you would want to."
Additionally, Vingren says, "The higher the intensity, the more muscle fibers you activate." Plus, working hard in bursts of intensity helps improve VO2 max, which is a measure for cardiovascular endurance.
How to spend those 20 minutes?
"We can make a simple answer, but it's not always simple," says Vingren, who has a doctorate in exercise physiology. "It would be like going to the doctor and saying, 'I'm sick. I need to take a pill.' It depends on your goal, current level of training, risk factors. It should all be done on an individual basis."
He recommends a "superset workout" -- moving from one strength-training exercise to the next without stopping, then repeating at least one more time. For example, do a set of squats (lower body, abs and core) followed by bench pressing (arms and chest) and row exercise (back).
"Do larger muscle exercises before smaller muscle exercises, and alternate muscle groups," Vingren says.
One of the Ontario researchers suggested a 20-minute workout that alternates one minute of running or cycling hard with one minute of decreased intensity, done a couple of times a week, after clearing with your doctor, of course. Here are two more ideas:
The expert: Bobby Patten, head coach of Dallas Aquatic Masters
The advice: "Play for effort over distance: short distance fast, short easy."