Is there anything easier to procrastinate on than exercise? When you have a full schedule of responsibilities, it's often the first thing to cross off the list. Next week, you think to yourself. When I'm not so busy. After all, most of us feel like we're always stretching to make every minute count. Who can regularly find an hour to work out at the gym, not to mention finding the time to get there, shower and change, and get back?

When we think of how else we might allocate that time, it can be easy to forget that exercise is not an "extra," but a vital part of a healthy life. When we work out, we feel better, look better, sleep better, stave off a variety of ailments, and set a good example for our family. What other activity packs that much punch?

OK, so the problem is still a lack of time. That's why 20-minute workouts are back in vogue. By being strategic — alternating high-intensity cardio with multipurpose exercises — you don't have to spend an hour and a half on your workout regime. You can squeeze it in before the kids get up, during a tight lunch hour, or at night without sacrificing your entire evening. So go ahead and cross the excuse off your list — rather than the exercise itself!

Will 20 minutes really matter? 

According to the Physical Activity Guidelines published by the National Institutes of Health, you can get substantial health benefits from just 75 minutes a week of high-intensity physical activity. These benefits include a reduced risk of heart disease, stroke, hypertension, type 2 diabetes, depression and premature death. Exercise can also improve your mood, energy level and mental sharpness, which all translate to better performance at work — so that workout break you take might actually mean you get more accomplished, and feel better doing it.

What's the catch?

"High intensity" means you've got to be pushing yourself hard in activities like running, swimming, playing tennis, jumping rope, kickboxing or spinning. If you aren't ready for something that vigorous, a general rule is that two minutes of moderate-intensity activity (like brisk walking, easy biking, doubles tennis) is equal to one minute of high-intensity exercise. A couple of days each week, alternate your cardio blast with strength-training exercises that target different muscle groups, like those outlined below. If you need more structure, many gyms are offering "express" high-intensity workouts to get you in and out in a half-hour or less. Check to see if your gym offers such a program.

Here are some double-duty toning moves recommended by Fitness magazine. To keep your heart rate up, do a set of 10 of each move and continue circuit-style, with 50 jump ropes (or imaginary jump ropes) between each exercise. Go!

- Lunge shoulder press: Hold 5-pound dumbbells at shoulder height with elbows bent, and lunge with one leg while pressing weights overhead; return and alternate.

- Reverse-grip dead-lift row: Stand with knees slightly bent and a dumbbell in each hand, arms by your sides and palms facing forward. Hinge forward from the hips, pushing your tush out as far as possible. Drive your elbows behind you to bring the dumbbells up by your ribs, palms up, shoulder blades squeezing together. Lower the dumbbells toward the floor, releasing your shoulder blades so that they move away from each other. Repeat.

- Side-crunch hammer curl: Stand with your feet together holding a dumbbell in your right hand, arm by your side, palm facing in, and touch your left fingertips to your left temple. Side bend at your waist to the right, chest facing forward, so that your left elbow points up. Repeat on the other side.

- Triceps extension reverse crunch: Lie face-up with knees bent, feet off the floor, a dumbbell in each hand, arms extended toward the ceiling and palms facing each other; bend your elbows to bring the dumbbells by your ears. Repeat.

- Pullover curl-up: Lie face-up on a mat, knees bent and feet flat, holding the ends of one dumbbell with both hands, arms extended behind your head. Slowly curl up to a crunch position, head to knees. Repeat.

For any fast workout, keep in mind these tips from fitness expert Jennifer Cohen, author of "No Gym Required":

- Warm up for four to five minutes and stretch for five minutes at the end.

- Keep your heart rate between 65 percent and 85 percent of your maximum heart rate the whole time, higher in the range if possible. (Wear a heart rate monitor if you can, or aim for heavy breathing with an inability to carry on a conversation.)

- Do high-intensity intervals, with very short lower-intensity "breaks."

- On your off days, do an easy workout, like a long walk. (And don't forget to high-five yourself for a job well done!)