Better cardio workout routines: The sure-fire way to an effective cardio workout? Use 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 maximum heart rate throughout this cardio 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 maximum heart rate is 90, which means you're not working hard enough.

The plan: Choose your level of workout routines.

• Beginner: You've been exercising aerobically for less than 3 months. Do 2-4 weeks of the endurance training 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 cardio workout plans, 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 routines.

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.

Read on to discover how to put interval training and endurance training together for maximum results.