IMMEDIATE ENERGY SYSTEM
Your Immediate energy system can be thought of as your V8 engine. It is extremely powerful, by far the most powerful of your energy systems - creating energy quicker than any other system. The problem with this system, is that the energy it can produce is extremely short lived.
At maximum intensity, your Immediate system will only deliver you around 10-15 seconds of energy. The first few seconds are supplied by the breakdown of Adenosine Triphosphate (ATP) already stored within your muscles.
After this, the breakdown of Phosphocreatine (PC) provides energy for approximately 5-8 seconds. The energy released at this stage allows for ATP to reform. This ATP can then once again be broken down to release more energy.
After the immediate energy stores are spent, it will take at least 2 minutes of rest in order to replenish them.
Whilst it is correct to associate this energy system with maximum effort sprinting, it is important to appreciate that regardless of intensity, ATP already stored within your muscles, will provide almost all of your energy in the first few seconds of movement.
Whether, you are just setting off for an easy session, accelerating during a workout, or simply changing direction, it is your stored ATP which will be initially called upon to supply your energy. After about 30 seconds of sustained sub-maximal activity, it is the turn of your Anaerobic System to take over as the primary energy supplier.