Switch up your workout routines to keep them fresh and to improve your overall endurance and calorie burning.

It's a common cardio workout conundrum: You sweat it out on the treadmill for 40 minutes three to four times per week with the goal of getting fit and shedding weight, but neither the scale nor your fitness is budging. You wonder whether you should be putting in more time at the gym, but what you really need is a change of pace and focus.

Most of us know the importance of varying our fitness workouts. So why are we still jumping on the same cardio machines or going for the same three-mile jogs around the neighborhood? "We love routines," says Robert Sherman, a master trainer for Reebok University. But workout routines are eventual dead ends: Results peter out along with our interest. And muscles used the same way day in and day out can lead to injury. With that in mind, Sherman created "Cardio Double Cross" -- an eight-week cardio workout program that changes regularly.

If your body gets used to doing the same fitness workouts all the time, it can cause you to plateau.

So says Sherman, president of F.I.T., a personal-training, yoga and fitness studio in Bethesda, Md. If you have a program that keeps surprising and taxing your body, you can avoid those plateaus -- not to mention keep your fitness workouts from getting stale and boring.

There's more to it than switching from the treadmill to, say, a rowing machine or a stair climber. With Double-Cross training, you vary your activities and your intensity, your environment and the timing of your cardio workout. "It's a step beyond cross training," Sherman explains. "Your body is constantly adapting to new equipment, terrain and schedules, and different energy systems." And as you get fitter, you'll actually increase the total work you do. That, he adds, is the best way to improve your overall conditioning and burn sufficient calories.

Sound complicated? Don't worry. On the following pages, you'll find specific fitness workouts and set cardio workout schedules, so there's no guesswork involved.

Says Sherman: "It's an easy and proactive way of changing your training program and your body." Concentrate on eating healthfully and working out consistently using this effective program, and you'll achieve a healthy weight.

Get the most out of the amazing Double-Cross cardio workout program that includes both interval training and endurance training.

How your cardio workout routines will work In this Double-Cross Plan, you do four cardio workouts a week for eight weeks. There are three main variables for change:

• Activity (running or cycling)
• Type of workout (interval or endurance)
• Location (indoor or outdoor)

Over the next eight weeks, you'll also be rotating the days of the week that you do these workouts. End result: No two training sessions are the same.

How you can adapt these fitness workouts While our program includes indoor and outdoor running and cycling workouts, you can make modifications according to your personal preferences and environment.

For example, if you don't have access to a treadmill or stationary bike, you can stick to outdoor workouts. If it's raining, you can take your outdoor cardio workout routines inside.

Or, if you don't like running or cycling, you can choose another activity. For instance, you can hit the elliptical trainer indoors and in-line skate outdoors. One caveat: Over a week, you're better off pairing a higher-impact activity (such as running or stair climbing) with a lower-impact activity (such as cycling or elliptical training). If you plan to alter the timing (i.e., switch a Monday workout to Tuesday), that's OK -- but since interval training workouts are intense, don't do them back to back.

How to monitor your intensity during your fitness workouts You'll be doing two types of workouts: interval training and endurance training. Both are important for burning calories and overall conditioning. Use the Rate of Perceived Exertion to gauge your effort level or intensity.

How to warm up/cool down after your workout routines Five-minute warm-ups and cool-downs are built into your fitness workouts. At the end of each training session, you should also do 20- to 30-second stretches for all your major muscle groups.

Take a look at the following page to learn more about the interval training and endurance training workout routines.

Interval Training & Endurance Training: The Double-Cross Cardio Workout Routines

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Interval training

Interval training essentially means alternating high and low intensity in measured spurts of exercise. The goal is to get your heart rate up, up, up, then briefly bring it down so your body can recover before you raise it again.

This type of training lets you burn more calories in less time and more effectively boosts your aerobic capacity. Since you'll be pushing your body hard during the high-intensity spurts, the low-intensity "recovery" periods are essential.

Use the Rate of Perceived Exertion (RPE) chart to gauge how hard you're working. If you feel like you can't complete an interval at the suggested RPE, take the intensity down a notch.

• Total workout time
  40 min.
• Warm-up
  5 min. at RPE 4-5
• Workout
  1 min. at RPE 7-8 followed by 4-min. "recovery" at RPE 3-5
  1 min. at RPE 7-8 followed by 3-min. "recovery" at RPE 3-5
  30 seconds at RPE 8-9 followed by 90-second "recovery" at RPE 3-5 (repeat for a total of 6 times)
  1 min. at RPE 7-8 followed by 3-min. "recovery" at RPE 3-5
  1 min. at RPE 7-8 followed by 4-min. "recovery" at RPE 3-5
• Cool-down
  5 min. at RPE 2-3

Endurance training

This moderate-intensity workout is designed to build stamina, so you can do longer runs and rides without running out of steam. It will also help prepare you to push hard during your interval workouts.

Unlike interval training, your pace remains relatively steady throughout the entire endurance training session. You improve your aerobic fitness, but more slowly than when you're interval training. If you get tired, reduce your intensity or RPE. When you feel better, you can increase your intensity again.

• Total workout time
  50-60 min.
• Warm-up
  5 min. at RPE 4
• Workout
  40-50 min. at RPE 5-6
• Cool-down
  5 min. at RPE 4

Here’s your weekly cardio workout routines:

Week 1

• Monday indoor interval cycling
• Wednesday outdoor endurance walk/run
• Friday indoor interval walk/run
• Saturday outdoor endurance cycling

Week 2

• Monday outdoor interval walk/run
• Wednesday indoor endurance cycling
• Friday indoor interval cycling
• Saturday outdoor endurance walk/run

Week 3

• Monday indoor endurance cycling
• Wednesday outdoor interval walk/run
• Friday indoor endurance walk/run
• Saturday outdoor interval cycling

Week 4

• Monday indoor endurance walk/run
• Wednesday outdoor interval cycling
• Friday indoor endurance cycling
• Saturday outdoor interval walk/run

Weeks 5-8

• Repeat Weeks 1-4, switching all indoor workouts to outdoors and vice versa. For example: In Week 5, on Monday do outdoor interval cycling, and on Wednesday do an indoor endurance walk/run.