Energy  is required by each & every cell in the body to function, whether that be muscle contractions for movement, exercises, regulation of body temperature, sleep, breath or any other basic functioning.

All energy is produced from the breakdown of the molecule ATP (adenosine triphosphate ). ATP is found in all cells of the body - but due to it being a large molecule there is a limited number of ATP in cells. So, when it comes to physical movement and exercise, ATP gets used up very quickely and needs to be replenished to continue the exercise.

We have three major energy systems that can restore ATP: ATP-PC System (AKA Alactic System), Glycolytic System (AKA Lactate System) and Oxidative System (AKA Aerobic System). 

The ATP-PC and the glycolytic system are anerobic, means they don't require oxygen to produce ATP. On other hand, the Oxidative System is aerobic, because it uses oxygen to produce ATP.

So far, so good?

The ATP-PC System lasts for 6-10 seconds and is used maximal pkwer output exercises such as all-out effort sprints, jumps, 1RMs etc. It's the simplest energy production process, as it enables a fast muscle contraction. There is also no lactic acid production as by-productm which usually causes fatigue.

However, this energy system is only in abundance for first 6-10 seconds before it starts to move into deceleration zone, where you start to notice your performance going dowm. The ATP-CP System rebounds to over 85% in about 3 minutes and is completely replenished after 10 minutes. That's why it's important to take rest periods of at least 3 minutes in between these maximal output exercises.

The Glycolytic System relies massively on the rapid breakdown of carbs in the form of glucose that is constantly circulating in the blood stream, and as well as this, glucose is stored in the muscles and liver in the form of glycogen.

When exercising at maximal effort for period of 10-90 seconds - the glucose and glycogen rapidly go through a process known as 'glycolysis' [ means breakdown of glucose] to produce ATP. This why it is important to have carbs pre and post training.

During this process lactate and hydrogen ions are produced. Interesting fact - it is in the presence of the hydrogen ions in the blood and muscles that make it mor3 acidic and leads to fatigue and thr burining sensation you love hahaha. Poor old lactate gets an unnecessarily bad rep for being the cause!

The 3rd Energy System, Oxidative System involves the use of oxygen to produce ATP - during this system two processes are used. The Kerbs cycle and Electron Transport Chain. So, these two processes can't produce ATP as quick as the first two energy syatems, but it can produce ATP continously for a far longer period of time! This system can use Carbs, Fats and Protein to fuel the energy system. Fats and protein how  ever take far longer to breakdown in order to produce ATP.