You know what is cool and unique about the human body? Now matter how you exercise the body has a response for it. The heart, the lungs, the mind, etc vary in response to exercise. The Fuel system works the same way. No oxygen for fuel? Bet, we will use carbs. We out of carbs? No problem. We can throw fat and protein at the problem. We got oxygen. Cool now we can make more ATP. It is the epitome of the video of Jocko saying "Good" over and over.
How does the body do this? Much like everything about the human body. It depends on what you're doing and how long you're doing it. In other words, it depends on Intensity and duration.
Short, quick, high intensity exercises such as sprinting, resistance training and plyometrics utilize a bioenergetics system called the Phosphagen system. The Phosphagen system provides ATP for roughly 5-10 seconds of work. The energy is derived from the hydrolysis of ATP and the breakdown of another high energy molecule called creatine phosphate (CP) or also known as phosphocreatine (PCr). If creatine sound familiar it is in fact the ergogenic aid creatine. If I may go on a tangent for a few sentences, creatine is the most widely studied supplement on the market and is totally safe to take. It should assist with increasing performance through the phosphagen system (as long as it is trained). Taking creatine and training aerobically has minimal effects. The body stores more creatine than ATP, however it burns quickly and is slower to replenish than ATP. In fact, the body stores roughly 80g to 100g of ATP at any given time while the body hold four to six times more creatine. Since the intent of the phosphagen system is short, quick, powerful bursts it takes time to recuperate the ATP and creatine usage. Ideally, you would want a 1:12 to 1:20 work rest ratio. Meaning that if I conduct broad jumps that take roughly 5 seconds then I would wait at least 60 seconds before my next set (5x12) or 1 min and 40 seconds (5x20). This is done because after a single bout 50%-60% of ATP is lost.
Fast Glycolysis (FG) is another biological energy system that typically lasts 15-30 seconds per bout. Since FG lasts a bit longer than the phosphagen system another process is utilized. That process is called the Cori cycle. In order to replenish the FG system a work/rest cycle would be 1:3 to 1:5. A 30 second sprint for example would require 2 and a half minutes of rest (30x5).
Slow Glycolysis (SG)/oxidative starts off the same way as the Cori cycle with the exception of the direction of pyruvate. Instead of sarcoplasm (which then turns into lactate which turns back into glycogen. Think of a turbo in a car). SG utilizes the mitochondria and oxygen within a cell to form ATP. This is a longer process (costs two ATP) but yields larger amounts of ATP. In total the Krebs cycle produces 40 ATP. Since it takes 2 ATP for the Krebs cycle the net yield is 38 ATP. Typically, training the SG/oxidative system lands in the 1-3 min periods with work to rest ratios of 1:3-1:4. An example is a 400m run. Let's say it takes 2 minutes to cover 400m; a proper rest ratio would fall between 6 to 8 minutes. Now, the intensity of this training is much less then maxed out effort of the phosphagen system hence the shorter rest periods.
Utilizing the oxidative system also uses the Krebs cycle with the addition of oxidation of carbohydrates, fat and in a pinch protein. Carbs are your best source of energy yielding macros as the body can readily use them instead of breaking down fats and proteins first then using the end result for the Krebs cycle. Any training over 3 min uses the oxidation process. The recommended work to rest ratios for this type of training is 1:1 to 1:3. For example, mile repeats conducted at 8 min mile pace requires a range of 8 min to 24 min.
During exercise not one energy system is used. It blends a variety of systems in order to ensure the body has enough fuel to sustain exercise. Intensity and duration dictate how much of each system is used. These systems can also be trained to be better. Training reflects increases in a particular energy system. Think about it. If I want to get better at a two-mile run, then I should train the Oxidative system into order to get better at a longer distance. Inversely, if I am attempting to get better at the 100m sprint then I should train the phosphagen system more.
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