Welcome to your aerobic countdown to a healthy body! This plan, created by Kevin Lewis, owner of State of the Art Fitness in Woodland Hills, Calif., incorporates two types of aerobic training, interval and steady-state endurance, for maximum results with no chance of boredom. And it's got one more bonus: You'll train indoors two days a week and outdoors the other two.

More important is the science behind the workout. It's based on monitoring your heart rate while exercising at different intensities.

"Doing cardio exercise without monitoring your heart rate is like strength training without knowing how much weight you're lifting or how many reps you're doing," Lewis says.

You can learn how to measure your maximum heart rate in the following pages. Meanwhile, here's why this program, which incorporates outdoor and indoor endurance and interval training, will keep your heart pumping -- and the calories burning -- for the next eight weeks.

"The temperature changes, wind resistance and uneven terrain of the outdoors naturally increase your workload in ways indoor training doesn't," says Lewis. "Indoor training is important, though, because you can control the elements and, therefore, the parameters of your workout."

Lewis suggests you choose indoor cardio exercises that will complement your outdoor activity, such as a Spinning class or recumbent bike, if you plan to cycle, or a treadmill, if you like to walk around your neighborhood.

Whatever your chosen activity, you'll do two types of workouts: steady-state endurance and interval training.

"Endurance training allows you to do any activity for longer," says Lewis. "In contrast, interval training, which alternates moderate aerobic activity with intense activity, strengthens your heart and burns lots of calories, so you lose more fat. Put the two routines together and not only will your heart be in great shape, but so will your physique."

Better cardio workouts
The sure-fire way to an effective cardio workout? A heart-rate monitor.

One proviso: "Monitors that claim to give caloric expenditure are at best questionable," warns Kosich. "They base the calories-expended figure on body weight and heart rate without including the workload factor, such as speed or grade, so the number is likely to be off by a lot of calories."

If you don't have a heart-rate monitor, you can measure your maximum heart rate (MHR) yourself. To measure your MHR as called for throughout this workout, subtract your age from 220. Then, multiply that number by the percentage for your interval level. For example, if you're 30 years old and a beginner doing the Days 1 and 3 workout, multiply 190 by 60 percent, which gives you an MHR of 114 (eg. 190 X .60=114). This means you need to keep your heart rate at 114 beats per minute for 30 minutes.

While exercising you can determine your heart rate by taking your pulse and counting the beats for 10 seconds. Then, multiply by 6 to get your 1-minute heart rate. For example, if you get a 15-beat count at 10 seconds, your MHR is 90, which means you're not working hard enough.

The plan
Choose your level

Beginner: You've been exercising aerobically for less than 3 months. Do 2-4 weeks of the endurance program before you incorporate the intervals.

Intermediate: You've been exercising aerobically at least 3 times a week for 3 months or more.

Advanced: You've been training aerobically 4-5 times per week for at least 6 months.

For all of the workouts, warm up for 5 minutes with low-intensity cardio activity and cool down for the same amount of time.

Choose an intensity that allows you to maintain a steady heart rate for the duration of your workout. For example, select a speed on your indoor bike that keeps your heart beating as fast as it does when you pedal at an equivalent speed on an outdoor straightaway. Every two weeks, beginners and intermediates can increase either the intensity of the workout or the amount of exercise time by 5 percent. However, don't increase both intensity and duration together.

Days 1 and 3
Indoor and outdoor endurance

Beginner
Duration: 30 minutes
Intensity: 60-65% of maximum heart rate

Intermediate
Duration: 45 minutes
Intensity: 65-70% of maximum heart rate

Advanced:
Duration: 60 minutes
Intensity: 70-75% of maximum heart rate

Day 2
Indoor intervals (All workout times include 5 minutes of warm-up and 5 minutes of cool-down.)

Beginner:
Work: 30 seconds at 75% MHR
Active rest: 1:30 minutes at 40% MHR
Intervals: 4
Total time: 18 minutes

Intermediate
Work: 1 minute at 80% MHR
Active rest: 3 minutes at 50% MHR
Intervals: 4
Total time: 26 minutes

Advanced
Work: 2 minutes at 85% MHR
Active rest: 6 minutes at 60% MHR
Intervals: 4
Total time:42 minutes

Day 4
Outdoor intervals (All workout times include 5 minutes of warm-up and 5 minutes of cool-down.)

Beginner
Work: 30 seconds at 70% MHR
Active rest: 2 minutes at 35% MHR
Intervals: 4
Advanced: 20 minutes

Intermediate
Work: 1 minute at 80% MHR
Active rest: 4 minutes at 40% MHR
Intervals: 4
Total time: 30 minutes

Advanced
Work: 1 minute at 85% MHR
Active rest: 4 minutes at 45% MHR
Intervals: 6
Total time: 40 minutes