4-minute workout

What it is: Four minutes of continuous running, swimming, cycling or other exercise — performed at 90 percent of your maximum heart rate. Those who did this workout three times a week for 10 weeks improved their blood sugar and blood pressure levels and had more endurance, a study by Norwegian researchers found.

What the experts say: Seven minutes is probably the shortest of the "legitimate workouts," said Dr. Tom Kottke, a cardiologist with HealthPartners. He warned that high-intensity exercise has been linked to sudden death. For anyone who is middle-aged and has not been active, he recommends having a supervised exercise test before attempting such a short, intense workout.

7-minute workout

What it is: Research-based workout created by the American College of Sports Medicine. Features 12 high-intensity exercises, including jumping jacks, wall sit, push-ups, crunches, squats and triceps dip. Perform each exercise as many times as you can for 30 seconds.

What the experts say: "It will increase your aerobic performance a little bit and it will increase your strength," said Mark Blegen, head of the Department of Nutrition and Exercise Sciences at St. Catherine University. "Are you going to get results? Sure. Are you going to break any world records? No. But I think there is something valuable to it."

20-minute workout

What it is: Short version of the popular Insanity workout. Circuit of six exercises (including souped-up versions of jumping jacks, side lunges, squats and sitting leg kicks), done for one minute each. Repeat circuit three times, resting between circuits for a minute. Do this two days in a row. Take a day off, repeat.

What the experts say: "For people who are fit and can do it, it's a reasonable alternative," Kottke said. "Most people who are unfit will find this to be extremely unpleasant if they can do it at all." He also said 20 minutes is not long enough to burn much fat or to build muscle.

Allie Shah