Interval recommendations have ranged from 15 seconds to 5 minutes (these are known as aerobic intervals). So let's take a look at each interval recommendation and all those in between.

1) 15 Second Intervals

The great thing about 15 second intervals is that you'll be able to work at a very high rate (almost near your maximum power output), as long as you get adequate recovery between work intervals. The downside is that it is very difficult to do 15 second intervals on machines, because it takes a long time to "build up" and "bring down" the machine settings to the correct speed.

Intervals are for advanced fitness levels. Your rest interval should be at least 15 seconds long, and can be as long as 60 seconds. The longer you rest, the harder you will be able to exercise in each interval.

2) 20 seconds on, 10 seconds off

This method is known as the Tabata protocol, after the Japanese scientist that published a study on this routine. This is hardcore. Not for the faint of heart. If you are a beginner, try easy interval trainig.

Clearly, the pro's with this method (as well as the 15 second intervals) is that you'll get your workout done faster (provided you do the same number of intervals as any other workout). Again, it would be very difficult to perform this type of interval training on a machine, due to the time lag as you increase or decrease the settings. And finally, these too should only be performed by above average fitness levels.

3) 30 Second Intervals

The Turbulence Training workouts tend to use a lot of 30 second intervals. Beginners will rest up to 90 seconds between intervals, while advanced fitness levels will rest 30-60 seconds. The longer (relative) rest allows you to work harder in each successive interval (i.e. you'll almost be able to match your performance in the first interval with each following interval). Short rest intervals (as in the Tabata protocol) will lead to a dramatic drop-off in performance with each interval. You can easily do the 3-second intervals on any machine.

4) 45 Second Intervals

These intervals are proven for fat loss, in addition to being effective for many team sports (such as hockey, soccer, basketball, and rugby). I have used 45 second intervals extensively in both areas of training. Not only will these tax your muscles, they will also tax your will to complete each interval (if done at the right intensity). Use 45-90 seconds of recovery between intervals. Do 3-6 intervals per workout. Get ready to be fitter and leaner than ever.

5) 60 second intervals

Similar to the 45 second intervals in benefits and toughness. Rest 1-2 minutes before starting your next interval.

6) 120 second intervals

These are now officially aerobic intervals, and can be used for both fat loss and improving aerobic capacity for sports and running. A great way to achieve two fitness goals at once. Exercise for 2 minutes and then recover for 2 minutes. Repeat 6 times. These workouts take longer (obviously), but can have a role in changing your body and improving your performance.

7) 5 minute intervals

Same strategy as with the two minute intervals. This really increases your workout time, so these are only used with serious endurance athletes.

Beginner vs. Advanced

If you are thinking that these intervals all sound "too intense" for you, please don't worry. Interval training is all relative. You don't have to sprint for your life in each type of interval. Instead, just work at a slightly harder than normal pace. By the end of the interval, you should be getting tired, but you shouldn't be gasping for air. Start conservatively and you will get the hang of it.

For example, if you regularly use level 5 on the stationary bike for 30 minutes continuously, you might try doing a 1 minute interval at level 7. Try that for an interval workout and let me know how it goes.

My favorite intervals for fat loss are between 30-60 seconds. These have been the staple intervals in my Turbulence Training workouts since the first workout was designed back in 2001. But again, I think you will get your best fat loss results if you vary your interval training workouts - just like you must vary your strength training workouts.

Without fat burning intervals, you might never burn belly fat.