How Your Body Uses Energy As You Continue to Train
When you first start working out, your body will go through a number of changes. These changes are designed to provide you with more energy and your ability to recruit that energy as and when you need it, is one of the biggest factors in determining how fit you are and how well you’re able to train. It’s also what controls the amount of fat you’re able to lose in any given amount of time!
Let’s see how this works and what you can do to improve it… 
The ATP-CP System
Essentially, the body finds its energy using three separate systems and it goes through these three systems in order.
When you first start exercising, you will begin using what is known as the ‘ATP-CP’ system. This stands for adenosine triphosphate creatine phosphate system. Catchy!
Adenosine triphosphate is the most fundamental form of energy known to biology. This is what glucose must be broken down into in order to be used by the body and a fair amount of it exists inside your muscles at all times. As soon as you start exercising, you utilize that ATP and this provides up to a couple of seconds of power. CP – creatine phosphate – allows us to recycle ATP and is also stored in the muscles ready to use.
The ATP-CP system is the most energy efficient and provides us with energy without making us gasp or feel unwell.
The Glycogen Lactic Acid System
The glycogen lactic acid system works by using glycogen stored in the muscles. This is the second most energy efficient source but has the unwanted side effect of producing lactate and other metabolites as a by-product. Lactate makes us feel unwell if we keep pushing ourselves and correlates with other metabolites that can make the muscle feel like it’s burning.
We can usually use this system of energy for a couple of minutes before the lactate becomes too much or the burn becomes too much. It is possible to improve your tolerance to this however, with training.
The Aerobic System
Finally, we switch to the aerobic system. This is the energy system most of us are most familiar with and it works by utilizing oxygen in the blood in order to burn fat stores for energy. This is why it makes us start breathing more heavily and increases our heart rate.
The aerobic system is the least energy efficient and it takes time for the energy to be delivered. Thus we are forced to slow down once we reach this state. However, it is also the type of energy system that we are able to sustain the longest. In fact, we can continue with this kind of exercise indefinitely or until we collapse from a complete lack of body fat!
HIIT
HIIT (high intensity interval training) is so effective because it involves switching from the first two types of energy (anaerobic energy) to the last kind. This enables you to deplete both your glycogen and your fat stores and thus does both more effectively.
