Maybe one of the most common topics I hear discussed in CrossFit is energy systems. What they are, how to train them, how important they are, which ones are most important… The list goes one. Energy systems is usually a discussion regarding energy metabolism. Energy metabolism is the process of generating energy (ATP) from nutrients. ATP (adenosine triphosphate) is the energy currency for our bodies and is responsible for most actions and interactions within our body.
There are four main energy systems we are concerned with as CrossFitters; Aerobic lipolysis, Aerobic glycolytic, Anaerobic glycolytic, and Phosphocreatine. These four systems are going to be responsible for all of the energy we need to generate for exercise.
You may be familiar with the picture above. It’s commonly shared in the discussion of energy systems. It shows the available power of each pathway balanced against its time to exhaustion. What I find misleading about this graph is that it seems as if the main factor determining which pathway is preferred is time, when in fact energy systems perform on-demand. This means that as power increases and so does our demand for energy, more powerful systems will contribute. What I do like about this graphic is it shows the overlap between pathways. Energy systems are not an on/off switch, but instead, a team with different players having bigger or smaller roles depending on the energy/power demand of the task.
The aerobic lipolysis system is your most heavily relied upon energy pathway. It’s responsible for all of your day to day activities and is incredibly economical. Fat has a very high energy density and this pathway makes use of that high energy density through beta-oxidation. Beta oxidation is a multi-step process that is repeated multiple times to metabolize fat into ATP and it yields a net average of 106 molecules of ATP per 1 molecule of fat. You are primarily using the aerobic oxidative pathway for day to day activities like sleeping, walking, studying, household chores, etc. The capacity of this pathway is indefinite; as long as we provide the fuel source, it will continue to produce energy.
As our demand for energy increases aerobic glycolysis kicks into gear. Aerobic glycolysis uses glucose (sugar) as its substrate. Glucose is derived from carbohydrates in our diet and stored as glycogen in our muscles and liver or as free-floating glucose in our blood. Though, not as efficient as the lipolytic pathway, aerobic glycolysis is still very efficient, netting us on average 30-32 molecules of ATP per 1 molecule of glucose. Aerobic glycolysis begins to contribute more as we ramp up our activity performing moderate activities like hiking or jogging. Aerobic glycolysis is the most heavily relied upon energy pathways in CrossFit.
A further increase in energy demand will fire up the first of our anaerobic systems, the anaerobic glycolytic pathway. Anaerobic meaning in the absence of oxygen. Keep in mind, at this point, both aerobic pathways are still contributing at maximum output but due to their inability to meet energy demands, the body calls upon anaerobic glycolysis. Much less economic but much faster in its ability to produce ATP, anaerobic glycolysis nets us 2 ATP per 1 molecule of glucose. This system fuels you through your max effort rowing and assault bike sprints and exhausts after around 30s of maximum output.
Our second anaerobic system is the fastest producing member of the team, the phosphocreatine system. Phosphocreatine does not rely on fat or glucose as substrates to convert into ATP but instead acts as its own energy substrate. Phosphocreatine is made up of 1 phosphate and 1 creatine molecule, bound together. When ATP is converted to mechanical energy it loses a phosphate becoming ADP (adenosine diphosphate), it is at this stage that phosphocreatine sheds it’s phosphate and delivers it directly to ADP to create ATP again. Incredibly fast when compared to our aerobic systems, the phosphocreatine system quickly helps us generate energy to complete rep maxes of 2-8 and all-out sprints of 8-10 seconds.
The big takeaway when thinking about and understanding energy pathways is that what we are looking at is an on-demand system. Everyone is always contributing, but the energy demand dictates the level at which each pathway contributes. We don’t bring out the big players until it’s really needed as these big anaerobic players have the tendency to fatigue quickly and burn out. More often than not, we are relying on the economy of our aerobic pathways as they are reliable and have great endurance. In the next few posts, I will discuss each pathway in more detail as well as how they apply to CrossFit, specific exercises, and how to target each pathway in training.
