The ATP-PC System
As the name suggests the ATP-PC system consists of adenosine triphosphate (ATP) and phosphocreatine (PC).
This energy system provides immediate energy through the breakdown of these stored high energy phosphates. If this energy system is 'fully stocked' it will provide energy for maximal intensity, short duration exercise for between10-15 seconds before it fatigues.
Think of the ATP-PC system as the V8 of your energy systems – it provides you with the most 'power' because it produces ATP more quickly than any other system and because of this it fuels all very high intensity activities. It's downfall however is that it burns out very quickly.
How does the ATP-PC system work?
There are only a few steps involved in the ATP-PC which is why it provides energy so quickly.
Steps of the ATP-PC system:
1. Initially ATP stored in the myosin cross-bridges (microscopic contractile parts of muscle) is broken down to release energy for muscle contraction. This leaves the by-products of ATP breakdown: adenosine diphosphate (ADP) and one single phosphate (Pi) all on its own.
2. Phosphocreatine (PC) is then broken down by the enzyme creatine kinase into Creatine and Pi
3. The energy released in the breakdown of PC allows ADP and Pi to rejoin forming more ATP. This newly formed ATP can now be broken down to release energy to fuel activity.
ATPase in this case assists the synthesis of new ATP rather than the breakdown. We see how this works in the diagram below.
During the first few seconds of exercise regardless of intensity, the ATP-PC system is relied on almost exclusively, with energy coming from the breakdown of the ATP stores within the muscles.
These ATP stores last only a few seconds after which the breakdown of PC provides energy for another 5-8 seconds of activity.
Combined, the ATP-PC system can sustain all-out exercise for up to 10-15 seconds and it is during this time that the potential rate for power output is at its greatest.
If activity continues beyond this immediate period, the body must rely on other energy systems to produce ATP as the limited stores of both ATP and PC will be exhausted and will need time to replenish.
These stores are replenished after about two minutes rest.
If activity continues at a high intensity these stores may only partially replenish as there will not be enough energy available for creatine and Pi to reform PC and the rate of ATP breakdown through other energy systems will impede the replenishment of ATP stores in the muscle.
Training the ATP-PC Energy System
To develop this energy system, sessions involving repeats of up to 10-15 seconds of maximum intensity activity/work are required, with approximately two minutes rest between repeats to allow the system to replenish.
There is a more scientific formula for rest periods called the ‘work to rest ratio’. For the ATP-PC system the rest ratio is 1:10/12. This means that for every second of ‘work’ you need to allow 10 to 12 seconds for recovery.
- Lifting the heaviest weight you possibly can for one or two repetitions.
- Sprinting as fast as you can for 50 – 100 metres with 2-3 minute recovery intervals before repeating.
- Punching a boxing bag as hard as you possibly can for 2 – 3 punches.
- Getting up out of your chair to go and make a coffee (alright it’s not really ‘training’ but as it requires immediate energy for movement the energy comes exclusively from the ATP-PC system).
Note that when you design training to condition the ATP-PC system you must build in adequate rest and stop the session if the quality of the movements or their power decreases significantly.
When this happens you are starting to work on power endurance (as fatigue is evident) and that is counter productive if your goal is purely to increase the ATP-PC system's capacity.
For example, if you were training to increase your explosive leaping ability (say for basketball) by jumping as high as you could you would notice that after two or three leaps the next leap may not get you the same height.
You would then stop and rest as your ATP-PC system is depleted. If you continue you will be starting to train endurance which will be counterproductive to developing explosive leaping power.