The Energy Systems – Ep. 2 “The Phosphocreatine Pathway”

Last week, we briefly explained the necessity of the energy systems. 

In the coming episodes, we will explain how these systems produce ATP and how they interact with each other in different workout settings. In this episode, we cover the energy system that kicks in first and is the simplest to understand: the PCr pathway. 

The breakdown
All skeletal muscle cells contain a small amount of ATP, which is enough to support  maximal muscular effort for around 2 seconds. Ofcourse that is not enough, therefore, as soon as muscle contractions start, the PCr helps out by regenerating ATP for around 15 seconds. To understand how PCr works, we first need to understand how the energy inside ATP is released. Look at the following formula:
ATP + H20 <—> ADP + Pi + H+ Energy

So, ATP splits into APD + Pi and energy is released. To continue exercise, the muscles need to rebuild ATP, so that it can release energy again. One of the reactions that allow for this is the following:

ADP + ADP + H+ <—> ATP + AMP

So, two molecules of the “rest-product” ADP combine to build one new ATP molecule. 
The PCr reaction also works with the ADP molecule: 
PCr + ADP + H+ <—> ATP + Cr

So, PCr donates a phosphate to an ADP molecule, and ATP is regenerated.


The main function of the PCr pathway in CrossFit is generating energy for heavy movements for a minimal number of reps (1-3), or explosive exercises that require minimal time. Think of heavy deadlifts, a strict muscle up or a sprint on the assault bike. In other words: high power, short time domain.
The system works extremely well. When looking at the molecules inside a muscle cell, the ATP levels stay consistent throughout, whereas the PCr levels decrease fast. 

This system works incredibly fast, and enables high power output, but seen that the phosphocreatine storage is only small, the power output cannot be maintained over longer durations of time.

When you are recovering from training, or when you have a relatively long time in between sets, your muscles are able to replenish the storage by reproducing phosphocreatine from creatine and phosphate (the reaction from above reversed):
ATP + Cr <—> PCr + ADP + H+
The ATP needed for this is produced through other pathways. 


The name creatine probably seems familiar to some of you. And indeed, if you supplement with creatine (or eat a ton of red meat) you can potentially enlarge your storage and improve your power output in these extremely short time domains.  

Training this energy system can be done by tapping into high power and short time: sprinting or heavy lifting. With regards to sprinting, the best results are seen when the sprints are short (<10 seconds) and the rest is relatively long. 

Over the next weeks, we will start looking into the aerobic and anaerobic systems. Stay tuned!