The 3 Energy Systems

In this blog, I’ll try and describe the 3 energy systems and when they are used by our bodies. I did make a reference to the energy systems in my kettlebells blog.

Before we begin on the three energy systems, we need to understand what Adenosine Triphosphate (ATP) is and where it comes from. ATP is our source of energy and it is produced with the energy provided by the macronutrients (carbohydrates, fats and proteins) we eat. ATP is stored within our own muscles.


Creatine phosphate system: immediate energy

For high intensity, low duration activities, such as sprinting, long jump, or shot putting, energy for muscular contraction is required quickly. This primarily supplied by intramuscular (within the muscle) stores (ATP) and creatine phosphate (CP). ATP stores are extremely limited and may only last for the first few seconds of exercise. Once these have been depleted, they can be almost immediately regenerated by the breakdown of creatine phosphate. This compound, like ATP, has a high energy bond, which when broken down will release enough energy to yield an ATP molecule. This chemical reaction is very rapid and like the ATP stores, CP stores are also very limited, thus exercise will only last for a very short period of time, approximately 5 – 8 seconds. In fact, it is noted that during a 100m sprint, lasting approximately 10 seconds, runners are usually slowing down in the final few seconds; unless he’s a freak like Usain Bolt!

As this system is derived exclusively from chemical energy stored within the muscles, the process requires no oxygen (anaerobic) and places no immediate demands on fat or carbohydrate stores. Depending on the intensity and duration of activity, the recovery period for this system ranges from 30 seconds to 4 minutes.


Lactate system

The lactate system can essentially bridge the gap between the aerobic and CP systems. It allows rapid ATP production to continue beyond the few seconds of the CP system, and at a rate significantly greater than the aerobic system can achieve. It can sustain exercise activity for between 60 – 180 seconds e.g. 400m on the track or 100m in the pool.

At some point, the build-up of fatiguing waste products associated with the lactate system reaches levels sufficient to bring about the familiar decline in performance and burning sensations associated with this type of intensity. Recovery from this type of activity can vary from 20 minutes to 2 hours depending on intensity and duration.


Aerobic system

Aerobic simply means ‘with oxygen’, and refers to the energy system that produces ATP from the complete breakdown of carbohydrate (CHO) and fat, in the presence of oxygen. The aerobic energy system is dominant during lower intensity activities when ATP demands are low and oxygen is relatively plentiful.

The aerobic system produces carbon dioxide, water and heat as by-products of the breakdown of CHO and fat. With an abundance of these nutrients in the body, there are almost no limits on the amounts of ATP that can be produced. There are, however, limits on the rate of aerobic ATP production. The individuals aerobic fitness now comes into play; the ability to take in, transport and utilise oxygen.

Assuming the absence of any overuse injury, the recovery time from this type of exercise will be the time taken to eat, drink and replenish fuel stores.

At rest or during very low intensity activity most aerobic energy is supplied by fat. As exercise demands increase and ATP is required more quickly, CHO will begin to contribute more to the activity.



Hopefully, this helps us understand why marathon runners or ultra-triathletes can keep going for hours on end but 100m sprinters can only last 10 seconds. I hope this blog wasn’t too bogged down with scientific terminology and the basic points about the three energy systems came through.