It is important to keep your workouts short and intense. For anaerobic exercise the repetitions don't have to be many, because as you increase intensity, adding lots of repetitions will also trigger the adrenal stress hormone cortisol and defeat the purpose.

To use exercise to trigger fat-burning hormones, it is important to discuss the variables of exercise:

1) Intensity (the power behind the exercise) and difficulty
2) Frequency
3) Duration (total time of exercise)
4) Type
5) Rest (The goal is to use exercise strategically to maximize and keep your body in fat burning and minimize fat-making stress hormones.)

Today we will discuss the first two: Intensity and Frequency.

Intensity and Difficulty

Intensity is a primary factor; it is the most powerful stimulus for the fat-burning hormones. Growth hormone is the best example of this. If the exercise is not intense enough, you won’t be able to stimulate growth hormone.

At the beginning of this exercise program you will stimulate these hormones to burn fat. The body will repair itself and get used to it. The more you get used to the difficulty, the less the fat-burning hormones are affected. So, with time, there will need to be a progressive increase of intensity to create the same effects on fat-burning hormones. You have to keep the difficulty level high in order to keep the intensity high enough. Therefore, you need to keep challenging yourself.

I personally started out biking around my neighborhood on flat pavement. I used to ride my bike through some trails and initially could barely make it up a steep hill this was intense. Several years later, I am going up very steep hills, which I never thought I could achieve, and it’s not even a challenge. So I’ve had to set my gears lower to make it very difficult going up the hill in order to keep the intensity high. This way I can stay in fat-burning mode. The body adapts and you need to keep challenging it.

Frequency

There are two different frequencies we are talking about. The first is the frequency of repetitions of exercise during one session. The second is how many days per week one exercises.

It is important to keep your workouts short and intense. For anaerobic exercise the repetitions don’t have to be many, because as you increase intensity, adding lots of repetitions will also trigger the adrenal stress hormone cortisol and defeat the purpose. If you are doing weight training, instead of increasing repetitions add more weight to increase intensity, but keep the duration short. In the beginning, you’ll start out with five repetitions and progress from there. The thing to not do is use light weights with a high number of repetitions, as there’s not enough intensity to trigger fat-burning hormones. Anaerobic exercise should be done every other day not daily unless you are still sore, in which case you should wait till most of the soreness goes away before working out again. Take more rest between exercises and keep soreness at the low end.

From The 7 Principles of Fat Burning