If you are trying to burn fat, you may have been advised to work out at medium intensity for an extended period of time (e.g. 60 minutes on the stairmaster), as opposed to having a short, intense workout (e.g. 10 minutes of windsprints). This post will discuss the science supporting this advice.
Exercise metabolism focusses on the metabolism of muscle, which is the primary tissue involved in exercise. Muscle requires ATP (energy) to function and stores enough ATP to last 5 to 10 seconds. Next, muscle transforms another chemical, creatine phosphate, into enough ATP to last 10 to 20 seconds. After that, the muscle uses glucose, which is broken down from the muscle's glycogen stores, to generate ATP. The muscle's glycogen stores can last about 5-10 minutes.
After the muscle's glycogen stores are depleted, it creates ATP from either blood glucose or fat. Oxygen is the deciding factor that causes the muscle to either use blood glucose or fat. If the muscle is not receiving enough oxygen, it will take in blood glucose, which probably originated from the liver, and create ATP. Thus, during short, intense exercise, muscle cells are burning glucose/glycogen stores. However, during medium intensity workouts, muscles decide to burn fat because they are receiving oxygen.
Thus, if you want to burn fat, then exercise metabolism seems to support that longer, medium intensity workouts make sense. Remember, you will need to make sure you can sustain the exercise for more than 5-10 minutes straight because you will not be burning any fat until that point.
Remember that your body is very complex and I don't think burning fat is as simple as exercising for more than 5-10 minutes. Diet, genetics, basal metabolism all come into play too. Also, I think you should focus on burning calories, not burning fat, because in the long run it doesn't really matter WHERE you burn the calories from, as long as they are burned. For example, if you burn 100 calories of glycogen, the next time you have a meal, the first hundred calories of sugar you eat will be turned into glycogen and NOT fat. However, if you burn 100 calories of fat, then the next time you have a meal, the first hundred calories of sugar you eat will be turned into fat because your glycogen stores will be full as they have not been burned. So, my advice in the end...do whatever exercise you enjoy enough to do often, oh, and watch what you eat too.
In the next post, I will talk about the feed/fast cycle.