Three Energy Systems
There are three energy systems in the body, all using different substrates to synthesize adenosine triphosphate (ATP) which is the energy currency of the cell. Without ATP our muscles would not be able to contract making locomotion impossible. Briefly, the three energy systems are:
1) the Phosphagen system which uses phosphocreatine (PCr) and can generally be thought of as our high-octane rocket fuel, due to its ability to synthesize ATP the fastest.
2) Glycolytic, which uses glucose from the blood or from stored muscle glycogen and is our intermediate fuel source.
3) The last energy system is Oxidative Phosphorylation (Oxphos) which can use intermediates from carbohydrate, fat or protein to synthesize ATP, Oxphos is our bodies long term energy source, being the predominant source of fuel for anything over 3-minutes and requires oxygen to create ATP.
Aerobic and Anaerobic
These energy systems are typically broken into two categories: aerobic (oxphos) and anaerobic (phosphagen and glycolytic).
Aerobic meaning that oxygen is required for a pathway to function.
Anaerobic meaning that oxygen is not required for that pathway to work correctly.
These systems are thought to work semi-independently, meaning that one system is always providing the predominant amount of ATP based on intensity of exercise. While the differential contribution of different energy systems to the maintenance of ATP stores is dependent on intensity, it is still thought that high intensity exercise (think sprinting or strength training) results in an anaerobic environment. A simple example of why this is wrong, is the immediate drop in SmO2, seen during the start of high intensity exercise, which never reaches zero. If a truly anaerobic environment, where there was no need for oxygen was being created, then the first 3-15s of high intensity exercise would result in no change in oxygen use.
Interplay between the Energy Systems
This conundrum is explained by a study done in 1999 by Haseler et al. which shows the direct dependence of PCr stores, measured by phosphorus magnetic resonance spectroscopy, P-MRS, on the availability of oxygen in the cell. As the amount of available oxygen goes up, the percent of PCr stores goes up as well. This study shows that the high-octane fuel we need for high intensity movements like sprinting, or strength and power is directly related to our bodies ability to keep the muscle in an oxygen rich environment.
Since the bodies’ ability to resynthesize PCr is directly related to oxygen availability the next step would be to try and monitor PCr recovery via oxygen recovery kinetics which can be directly monitored via NIRS. A study done by Ryan et al. 2013 compared recovery of P-MRS and oxygen recovery via NIRS and showed a direct correlation between oxygen and PCr recovery kinetics, confirming the notion that NIRS can be used to monitor PCr recovery.
Putting it altogether, all of the energy systems are tied together and deeply affect one another. Meaning that the bodies’ ability to provide oxygen to the working muscle allows for more availability of high-octane fuel and in turn, higher intensities of exercise to be reached. And using NIRS to monitor oxygen recovery, can give athletes and coaches an idea of how recovered they are, (when SmO2 reaches a plateau), giving us a very good idea of when to start new intervals, rest a player, or stop a workout/game when a player is no longer able to recover like they once were. If you’re interested in more information about the relationship between oxygen and PCr. Andri Feldmann put together a nice video on youtube explaining the nuances: https://www.youtube.com/watch?v=m8u1FGOg6sw.