DALLAS — Think about your day. Can you unearth a spare 20 minutes?

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.

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 recovery had significant health and fitness effects on unfit volunteers, cardiac patients and, in later research, diabetics.

But how to spend those 20 minutes?

“We can make a simple answer, but it’s not always simple,” says Vingren. “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 “super-set 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 a rowing 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. Here are two more ideas:

Swimming

The expert: Bobby Patten, head coach of Dallas Aquatic Masters

The advice: “Play for effort over distance: short distance fast, short easy.”

He suggests “pyramid sets” — 25 yards fast, then 25 yards easy. Increase distance: 50 fast, 50 easy; 100 fast, 100 easy, resting 10 to 20 seconds between each set.

“You’re increasing your metabolism more that way than by just going at 70 percent for a longer time,” Patten says. “Any interval I think is beneficial: heart rate high, steady aerobics, then zap it up again. In 20 minutes, you can kill yourself, really feel spent.”

Every mile of swimming is equal to about four miles of running, he says. People run two or three miles and think it’s “a good enough workout. Swimmers get a mind-set of ‘only 1,000 meters’ being not any good. But if you do it as intervals, that’s a good workout.”

Calisthenics

The expert: Kristin Moses, co-owner of the Body Bar fitness studio. She’s raising three boys, so “totally understands” the no-time-to-exercise crunch.

The advice: Start with a three- to four-minute warm-up by doing 30 seconds of jumping jacks, 30 seconds of high knees, 30 of glute kicks and 30 of jumping jacks.

From there, go into a series of squats and lunges: 15 squats at moderate tempo, 15 lunges with your right leg and 15 with your left.

Then “come to the floor and do 20 to 25 full or modified push-ups,” Moses says. “You’re moving from your legs, big-muscle groups, to upper-body big muscles and chest, which builds your heart rate and puts you into fat-burning zones.”

Next, turn over and sit, hands on the floor behind you. Raise your hips off the floor for 20 to 25 triceps dips. Repeat the sequence, and this time “pick up your pace, get your butt moving,” Moses says. “Do the leg sequence again. You’re building stamina.”

The third go-round, make it a sprint: “Do jumping jacks as fast as you can, high knees as fast. You’re starting to spike.”

Finish with ab work, she says. Come to the floor and do a plank for 30 seconds to a minute. From there, turn onto your right side and do a side plank. Repeat with the left side.

“Voila!” Moses says. “You’re done, and you’re in amazing shape.”