For the most part, energy systems can be divided into two main camps; aerobic energy pathways are those that rely on the presence of oxygen and anaerobic energy pathways are those that operate without the need for oxygen. Today we will be talking about anaerobic energy pathways.
There are two anaerobic energy pathways, the phosphocreatine (PCr) system and anaerobic glycolysis. Both of these pathways can operate in the absence of oxygen but are supported to a great degree by the aerobic system.
When muscles contract they use ATP (adenosine TRIphosphate) and the resultant contraction leaves us with ADP (adenosine DIphosphate) and inorganic phosphate. Muscles have a very limited supply of available ATP and under high intensity activities (think 1-3RM weightlifting or 5-10 second maximal effort sprints) and it can be exhausted quickly. Phosphocreatine is a combination of creatine (a supplement you are likely familiar with) and phosphate. The PCr system is a very quick acting and highly powerful system that synthesizes energy (ATP) by donating a phosphate to ADP to be re-phosphorylated into ATP. Phosphocreatine is like a short term storage battery for phosphates that can be quickly sewn back to ADP to make ATP. The PCr system, though very quick to resynthesize ATP exhausts just as quickly, after only 8-12 seconds. So, when maximal intensity exercise stretches out past 10 seconds, we turn to anaerobic glycolysis for a little more runway.
Anaerobic glycolysis is a step process that yields only 2 ATP per one glucose. Though not very economical, anaerobic glycolysis is fast! When maximal intensity exercise extends beyond 10 seconds, we rely very heavily on anaerobic glycolysis. The downside to this energy pathway is that there are lots of byproducts created. These byproducts can accumulate in our blood and tissues causing fatigue and discomfort.
Although anaerobic systems don’t need oxygen to synthesize ATP, they do rely on the aerobic systems for support. One of the ways the aerobic system contributes to anaerobic metabolism efficiency is by using byproducts such as pyruvate for fuel. Pyruvate is a byproduct of anaerobic energy metabolism that is used by the aerobic system to make more ATP. However, if there is not enough oxygen present for aerobic metabolism or if intensity is too high and we produce too much pyruvate, then pyruvate will ferment into lactate. It is this fermentation into lactate that leads to a common state known as lactic acidosis. Now, lactate isn’t necessarily a bad thing, it’s actually used by the liver and converted back into glucose for more fuel. What causes trouble is the excess hydrogen that accompanies lactate. Excess hydrogen results in a decrease in pH (potential Hydrogen; when we have lots of hydrogen building up in our blood then we have less potential for more). This drop in pH is considered an acidic environment and leads to fatigue and discomfort. ‘Lactic acid’ is the term you would be familiar with, this is a common misunderstanding of the simultaneous presence of lactate and excess hydrogen. Our aerobic systems help out by using up the pyruvate before it gets a chance to ferment into lactate, and by buffering out the excess hydrogen via the electron transport chain.
The PCr and anaerobic glycolysis are very powerful and fast energy producing systems. But these systems are nothing without the support they glean from the aerobic system. Stay tuned for our next post talking about the aerobic system.
