We’ve recently teamed up with the Col Collective to produce a series of articles for their new ‘Learn’ section on their recently launched website. For the first article of series one, we take a look at the body’s 3 energy systems. 

As you exercise, your working muscles require a constant supply of energy. In order to satisfy this need the body relies upon three energy systems. These systems seamlessly work together to turn the fuel that we eat into the watts that we produce to turn the pedals. Fundamental to all nutrition and training theory is a clear understanding of these systems.

Therefore, in this article we are going to give you an overview of what these systems are and how they work. The energy systems can be divided into two main categories, aerobic (requiring oxygen) and anaerobic (not requiring oxygen). The anaerobic system can then be further divided into, the ATP-Phosphocreatine system which is alactic (doesn’t produce Lactic Acid) and the Glycolytic or Lactic Acid system (which does produce Lactic Acid).

Anaerobic – Phosphocreatine (PCR) System – Have you ever wondered why when you first start sprinting the first 10-15 seconds feel almost effortless? During these short, high power bursts the body has to rely on a fuel that can provide energy instantaneously, here’s where the Phosphocreatine system comes in. This system breaks down phosphate and creatine stored in the muscles and has the capacity to produce vast quantities of energy very quickly. It is the only system that doesn’t require a blood supply and has no by-products.

Like all good things though, the effortless burst of power that the system produces doesn’t last and as soon as the muscles’ limited stores of PCr are depleted (in around 10-15 seconds), the Lactic Acid system kicks in and you quickly become aware of your efforts! Once you have depleted you PCr stores in a sprint, it can take as long as 5 minutes to restore them to their resting levels, ready to sprint again. The PCr system is the lesser known of the three energy systems, but is still equally as important as the other two since it is called upon at the start of activity and during any fast bursts of speed.

Anaerobic – Lactic Acid System (LA) – This system breaks down carbohydrate (a fuel in limited supply) in the body, to produce medium amounts of power for medium amounts of time. During very high intensity exercise, its by-product, lactic acid, can accumulate in the muscles and creates an acidic environment, which wreaks havoc with the body’s enzymes, causing a great amount of physical pain and resulting in an eventual drop in performance as the muscles’ function is inhibited.

The Anaerobic system’s use of carbohydrate is significant, as the body’s stores (in the muscle and liver), only equate to around 500grams (2000Kcal) worth of carbohydrate, which when excercising at a high intensity can be completely exhausted in as little as 60minutes. Any breathless cycling would be heavily reliant on this system, which would result in rapid use of carbohydrate, a good example of which would be attacking a mountain col. Relying on this system will allow you to go quickly, for as long as the fuel in your body lasts – which is the basis for fuelling during exercise.

The Aerobic System – This is commonly referred to as your body’s diesel engine. It is a system that produces energy through the combustion of carbohydrate and fat in the presence of oxygen. It is only able to produce relatively small amount of energy, so cannot produce enough energy for any sprinting, but can produce power for extended periods of time, making it the predominant system used during any endurance ride.

Due to the requirement of oxygen for the system to work, there is a time lag between starting exercise and the oxygen being delivered to the working muscle to allow the system to function. In some individuals, this can take as long as 120 seconds.

Although some carbohydrate is required to burn fat, the major fuel is fat, which is stored in abundance in the body.

An average person can have between 50,000 – 60,000 calories stored in their body – which is enough to cycle over a thousand miles without having to re-fuel. This, arguably, makes it the most important of the three energy systems for any endurance athlete!

How much power this system produces is dependent upon your own fitness. Typically if a lot of your training is of a long duration, you will have developed a good Aerobic system. For example, when we tested The Col Collective’s Mike Cotty (back in June 2013), he could produce 200 Watts at his Aerobic threshold, which equates to 3.2watts per kilogram of his body weight. Aerobically, Mike can produce more power at his Aerobic threshold than your average club cyclist at their Anaerobic threshold. This may at least partly explain why he is able to ride at a consistent pace over a significant distance – as demonstrated in some of his epic challenges (click here for an example)!

To illustrate how these systems work together, we can look at the contribution of these three systems to the power produced during a 120 second sprint (or a Wingate Test), as it’s known in the sports science laboratories. Individuals are placed on a static bike and asked to sprint as fast as they can for 120 seconds. When the contribution of each of the energy systems is graphed it looks something like this…

As you can see, the first 10-15seconds are fuelled almost entirely by the PCr system, producing a huge burst of power (peaking at 900watts!), but very quickly fatiguing. After around 10 seconds, the Phosphocreatine system is completely exhausted and the lactate acid system starts to kick in. By 30 seconds, the LA system has fully taken over, but rapidly starts to fatigue as lactate acid accumulates. By 40 seconds, the aerobic system has begun to kick in, as oxygen has made it to the working muscle and begins to assist with the aerobic contribution of energy production.

As an athlete, it is important to have all three systems working at their optimum, so that any system can be called upon to produce energy for the working muscle. In the next few articles, we shall cover how to determine the strengths of each of these systems through testing.

If you are looking for some more inspiration, head to The Col Collective website, which will guide you to some of the greatest mountain passes in the World. Also look out for the next article on fitness testing, which will be coming up quite soon.