How to Make Your Workout Quick and Sweaty

To get the most from limited exercise time, plan ahead and vary your routine.

The varying intensity of interval training burns more calories than a steady routine.

The varying intensity of interval training burns more calories than a steady routine.

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While researching a book on exercise and the brain, John Ratey learned that intense exercise releases a hormone that burns belly fat and adds muscle fibers. So the Harvard Medical School psychiatrist swapped out 40-minute jogging sessions on the treadmill for something less time consuming but more vigorous: Twice a week, he now jogs for a total of 20 minutes and includes five exhausting sprints of 20 to 30 seconds apiece. After a month, he was amazed at the results. "I didn't change my diet, and I spent less time on the treadmill," he says. "But all of a sudden, I was 10 pounds lighter."

Who wouldn't like to get a greater return on the time invested in exercise? The good news is that it's entirely possible. Science is increasingly pointing the way to the most efficient methods of working out. Meanwhile, trainers and coaches have their own trade secrets for saving time. Which is good, because none of us have any to spare. "Many of the people I work with are time starved," says Todd Durkin, a trainer and conditioning coach in San Diego. "They don't have an hour a day to work out."

Being more productive begins before you even break a sweat. A good workout requires planning. "The biggest mistake people make is that they don't determine what their goal is, both big picture and for today," says Carol Espel, national director of group fitness and Pilates at Equinox Fitness Clubs. Unless you're training for a specific event, your goals are probably to boost your aerobic endurance, strength, or both—and, of course, to burn calories. And the window for doing all that starts to shrink the minute you begin spinning your proverbial wheels. "I saw two girls at the gym walking around saying, 'What am I going to do?' " says Teddy Bass, a Los Angeles-based personal trainer. "Have that in mind before you go to the gym," he says.

So, you know what you're planning to do today, and now you want to warm up. You may be surprised to learn that you can probably skip the stretching. A comprehensive review of previous research published last fall found that muscle stretching doesn't reduce post-workout muscle soreness in healthy adults. Other studies suggest it doesn't prevent injuries, either. "If people eliminated that downtime...and spent 15 more minutes on lifting weights or cardiovascular exercise, there'd be more benefit to their health," says Stacy Ingraham, an exercise physiologist at the University of Minnesota. Stretching is intended to improve your range of motion, she says, but you can do that instead by lifting weights, which also builds strength. (The exception: If you have been very inactive and are starting an exercise routine, you may need to increase your range of motion over time before exercising.)

Start slowly. But just because you're not stretching doesn't mean you can skip a warm-up entirely. "Whenever you do any activity, you need to progressively begin," says Ingraham. "The pace at which you'll be doing the majority of the workout is not the pace you should start at." So pedal, jog, or swim more slowly for three to 10 minutes. Do the same warm-up before strength training.

OK, you're warmed up and ready to go. Let's say it's an aerobic exercise day. (For general fitness, the government recommends either 30 minutes of moderate aerobic exercise most days of the week or 20 minutes of more vigorous exercise three times a week, in addition to strength training twice a week.) Rather than doing a long stretch of steady exercise, you can follow Ratey's example and take advantage of interval training, a term for "alternating periods of high- and low-intensity exercise," says Martin Gibala, a physiologist at McMaster University in Hamilton, Ontario. That may mean bursts of more intensive exercise lasting between 30 seconds and three minutes, followed by recovery periods of as little as a few seconds or as long as 15 minutes, he says. By working at a higher intensity, you're forcing your body to adapt to different conditions, bringing benefits that exercising at a steady pace doesn't provide. And the jumps from minimal to high effort and back down again may improve fitness more than simply plugging along at a medium pace.