Understanding Energy Systems

Humans are remarkable creatures, capable of unthinkable feats of endurance (running a sub 2hr marathon) as well as split-second power, on par with nature’s most explosive specimens (like this knockout)

But How?

As humans our capacity to perform across the Energy System Spectrum can be traced to the fact that we have not 1, but 3 different pathways, or energy systems – that power our movement.

Our brain is constantly regulating the contribution of each energy system, in order to deliver the best performance at the lowest cost. Understanding these energy systems is fundamental to achieving our own peak performance.


Each energy system constitutes a pathway to creating energy. Some energy systems have shorter pathways (more readily available energy) and some have high levels of efficiency (more energy at a lower cost) but all energy systems have the same purpose – creating Adenosine TriPhosphate (ATP). ATP is a molecule that fuels all muscle contractions, and is the currency of human life. 

Luckily, our bodies have a way of recycling ATP, which is what makes us capable of being the adventurous creatures we are, able to sustain metabolic work for long periods of time.

Source: Khan Academy

Whether you are squeezing the smallest crimp for 2 seconds, or running a 24hr ultra marathon race – ATP is your body’s usable energy. ATP releases energy through a process called hydrolysis, whereby ATP (TRIphosphate) drops one phosphate molecule to become ADP (DIphosphate).

ATP is like fuel in a car, except that our bodies never run out of ATP because we recycle it. The recycling of ATP requires energy – and so the whole process by which we use water, oxygen and the food we consume to power our work – can be referred to as metabolism.

Our 3 Engines

Our body is powered by 3 different engines, and ATP is the fuel for all of them.

  1. ATP-PC (phosphocreatine) for short explosive bursts (upto about 10-12secs). This is the energy system that fuels our strength and power work.
  2. Glycolytic (Sometimes referred to as Lactate Energy) useful for sustained bursts of high intensity exercise (lasting 30-60secs)
  3. Aerobic (Oxidative energy production) Lasting longer than about 1min.

Source: BioForce Conditioning

As with all things in physiology, there are trade offs to using one energy system, over another.

With increased output, comes lower efficiency. With greater conversion speed (how quickly we can convert the fuel into energy) comes shorter windows of energy production.

  1. ATP PC.
  • Power Output – Used for explosive bouts of hard work. 3-5seconds of ATP + another 5-7sec of phosphocreatine (PC) contribution.
  • Availability – ATP is stored in the actin/myosin filaments, making it immediately available. PC is easily broken down, further powering our explosive bursts.
  • Efficiency – We store a very limited amount of fuel in our muscle cells, making its usefulness to sports, important but short lived. 
  • Work:Rest Ratio is approx 1:10-12. Meaning that for every second of work, 10-12 seconds of recovery is needed in order to replenish the fuel.
  1. Anaerobic Glycolysis
  • Power Output: Used for intense efforts lasting approximately 15-90seconds.
  • Availability: Relies on glucose conversion, which can occur upto 100x faster than fat metabolism. However glycogen is in short supply (about 2000 calories) in the body. 
  • Efficiency – We get 2 ATP molecules (plus 1 Pyruvate molecule) for every glycogen molecule, making it far less efficient than the aerobic engine, but more long lasting than the ATP PC engine.
  • Work:Rest Ratio is approx 1:6. Meaning that for every second of work, 6 seconds of recovery is needed in order to replenish the fuel.
  • Lactate is a byproduct of this energy system. Lactate can either contribute to further work, or lead to metabolic acidosis, and an unsustainable pH within the muscle cell.
  1. Aerobic Metabolism
  • Power Output: Used for continuous and sustained efforts. Low power, long duration.
  • Efficiency – We get approximately 38 ATP molecules out of 1 glycogen molecule this way, making it highly efficient. Many steps, in 3 distinct stages are required for this process, making it slow, but efficient.
  • Work:Rest Ratio is approx 1:1-2. Meaning that for every second of work, 1-2 seconds of recovery is needed in order to replenish the fuel. Because the Aerobic System spans the gamut from 2mins, to several hours, the work:rest range extends accordingly.
  • Aerobic metabolism relies heavily on fat as a fuel source. Humans store vast amounts of fat – making us capable of extremely long durations of low intensity work.

It is important to remember that all 3 pathways to energy production contribute at any one time, so it is not like we switch from one system to another, but rather that contributions from each energy system can pick up and drop off, according to our needs.

Your body will naturally default to the energy system best suited to the demands you place on it. When you’re out trail running and realize you are late for dinner, you pick up the pace, and increase the energy demands, which naturally results in the need for faster ATP recycling, and a greater contribution of a higher performing (but less efficient) energy system. 

What is ATP?
Khan Academy ATP

Aerobic Metabolism https://www.ptdirect.com/training-design/anatomy-and-physiology/the-aerobic-system 

Anaerobic Glycolysis