People don't exercise for many reasons; even though everyone knows exercise is good for them. But if exercise so good for us, why don't more people do it? Why aren't we exercising everyday? There may be as many reasons for sedentary lifestyles as there are ways to train.

Here are the top-five reasons I hear most why people don't exercise:

• I simply don't have enough time - I need to organize my time better.
• I don't know how - where do I begin?
• I've tried every different cardio, flexibility and strength- training program and I simply don't enjoy it.
• I always get injured so it's better if I just don't exercise.
• I don't have anyone to workout with - it's too hard to do alone.

Social researchers report the primary reason individuals don't regularly exercise (even when they know the vast benefits) is they believe they don't have enough time.

Yet, the benefits of working out to the beat of the heart or the sound of an MP3 player are enormous. Getting and staying fit keeps you from gaining weight and help weight maintenance. It helps to prevent degenerative disease so you can live to see your children's children play. And it gives you more energy and displaces stress.

So, if you're a card-holding member of the "inactives" - the popular new term for sedentary individuals - there's a way to get all of fitness benefits without investing a lot of time. It's called the Training in Zone 5b Program or non-aerobic interval training.

Before I give you the hard sales pitch on this high- intensity training, all of the normal precautions apply. Don't train above threshold heart rate (above the aerobic state) if a medical professional hasn't cleared you.

High intensity training is physically stressful. Training in the high red zones doesn't take much time because it is high stress and high recovery (After all, it's called high intensity interval training) and it extends into the stratosphere of high, hot, hard. And, yes, you burn a lot of calories while you hang out, albeit for five seconds to several minutes, in the Red Zone, 5b.

Where is this wonderful, high-calorie and sweat- burning Zone 5b found? Just refer to the Heart Zones Training chart below you'll find it in the column on the far left adjacent to the word Red:

Zone 5b is one of the three sub-parts to the Red Line Zone. It is 105-110 percent of your threshold heart rate so it is a non-aerobic or above the aerobic effort level (see chart below).

The zone is in the middle with Zone 5c above and Zone 5a below. As the Threshold Zones chart below shows, when you exercise in Zone 5b you can't breathe easily. It's "very, very uncomfortable." The high respiratory rate leads to adaptations, and as a result, you're getting faster as you burn high total calories, mostly carbohydrate calories.

Zone 5b training is sprint-type exercise. By training in heart rate Zone 5b you get many performance benefits - raising your peak oxygen uptake called VO2 peak or pushing to the max the activities of the mitochondrial enzymes in skeletal muscles. Additionally, many benefits of training long slow and seemingly forever also get accomplished with time in Zone 5b in an all-out effort intermittently mixed with recovery periods.

How do you do Zone 5b Training? There many ways, but the basic premise is to do 5-10 repeat efforts of all-out effort followed by recovery, low-intensity efforts. You may be familiar with intervals if you're a weight trainer - they build muscle size called hypertrophy. Zone 5 interval training in cycling, swimming or any cardio activity does not lead to increased skeletal muscle mass.

If you're like others and fail to get the minimum recommended amount of exercise for health benefits, whether it is lack of time or another reason, the Zone 5b Training method is ideal - it cuts the necessary exercise time by well more than 50 percent. But it yields the same, if not more, benefits. The price is hefty in terms of effort. Going all out to near max is very difficult and requires self-motivation and motivating training partners.

Some people may say that Zone 5c interval training is unsafe, impractical, and intolerable. But several prominent researchers' note high-intensity training (also called HIT) stimulates improvements in health and fitness in a range of populations. This low frequency and high-hot-hard approach to training is associated with greater long-term commitment when compared with high frequency and low-intensity training. In short, you train less often in the highest zones and get the same, if not more, benefits than those doing LSD (long slow distance) efforts more frequently.

Martin Gibala, Ph.D., McMaster University, Hamilton, Ontario, Canada reports that only six sessions over two weeks stimulates rapid change. His workout prescriptions are 30- seconds spurts of maximum cycling against a high- braking force on a bicycle ergometer, a stationary bike using braking resistance.

For years, elite cardio-athletes have been training in Zone 5b as a part of their comprehensive training program. Recent research reports in young, healthy persons of average fitness levels - Zone 5b training is a time-efficient strategy to build muscle strength, speed and endurance.

In as little as 15 minutes of very intense sessions with as few as six sessions over two weeks, you can improve your aerobic and non-aerobic fitness enormously. And you only need to do Zone 5b sessions every other day with a day off between. It's great training method if you have minimal time for exercise and particularly if it's not everyday.

Are there are other reasons to do high intensity interval training sessions? Yes. Another benefit is that time passes quickly as you count high-intensity intervals followed by the low-intensity time. It keeps you focused and saving even more time.

Even though your legs maybe feel like "jelly" at the end of the workout, the discomfort is tolerable because the work interval is short - a few seconds to several minutes. Granted, you're not in the "comfort zone" during the work interval. But when your heart rate drops, you can talk again.

Warm-up time is longer during Zone 5b training sessions. Do an extended warm-up for the strenuous training session and adequately finish the workout session with a long cool-down. The active rest time between the work and the recovery interval needs to match your fitness level. Insufficient recovery time between intervals can result in your not being able to reach the required high intensity level because of accumulated fatigue.

Don't worry about missing out on the mythical "fat burning zone" because it doesn't exist - it's a fabricated concept based on misunderstanding of metabolic response to exercise at different intensities. The myth that "it takes 30- minutes to start to burn fat" is not supported by science. Total energy expenditure is extremely high during Zone 5b training. According to Martin Gibala, Ph.D, the aforementioned high- intensity training advocate, "A recent study showed that only six sessions of high-intensity interval training over two weeks increased fat burning during exercise by more than 30 percent."

As you get more experience with Zone 5b training sessions, decrease the recovery interval time to 90 seconds and increase the work interval to 90 seconds - a one-to- one ratio of work to recovery. If you want to reach your fittest, varying your Zone 5.

Finally, beyond threshold intensity training, aerobic training also has many benefits. The best approach to training for fitness is to design a training program that incorporates the different training modalities - flexibility, non-aerobic speed, strength, aerobic endurance and proper nutrition and emotional fitness training. The bottom line: If you're able to endure the discomfort and have the motivation required to do Zone 5b training, try it because you get away with lower training volume and reap huge rewards. Of course, it's only one solution for those who don't have enough time to train but know that they should.