The Biochemistry of Alactic + Aerobic Training - Aerobic Pathway
[This is part 5 of many about the details of Alactic + Aerobic training and why you should be doing it.]
I'll conclude my energy-systems portion of this series with the "aerobic system" - involving the good old mitochondria. These guys are super cool. They're almost a cell-within-a-cell, they have their own DNA and do lots of stuff we don't understand. Basically they take fat (and sugar and lactate and other stuff) and use the energy stored there to create ATP. This is the low-power clean-burning constant-output energy pathway, and what we normally live on (provided that we're breathing oxygen).
Side note about mitochondria: there seems to be lots of compelling evidence that many of today's modern illnesses are related to mitochondrial dysfunction. That is something causes your mitochondria to not work well and then the downstream effects manifest as a illness. I've seen things like Lyme's disease (or really the antibiotics that treat Lyme's disease) leading to mitochondrial dysfunction which causes all of the nasty symptoms. Terry Wahls suggests that MS is really a mitochondrial problem and that targeting the fix there can reverse MS. Lots of other stuff here, but that's not the main point of this series - but do keep in mind that if A+A training is good for your mitochondria, it might be good for you health-wise, not just fitness-wise
So, if you've taken Biology 101 you probably had to memorize the Kreb's Cycle (I was a Chemist, so I got to avoid the Biology wing of the science building). The Krebs Cycle is pretty cool. It takes as input Acetyl CoA (fancy word) as input and through a bunch of reactions it makes ATP. As a good Computer Scientist, I like that the only input to the Krebs Cycle is Acetyl CoA - it makes a good interface. There are other things that turn Fats, Sugars, Lactate, etc into Acetyl CoA and then that goes into the ATP-making machinery.
Where does oxygen come into the mix here? If you look at the pretty picture, you'll see lots of places where NAD+ comes in and NADH comes out. To go from positive(+) to neutral you need an electron. Inside of the mitochondria there's a nebulous idea called the "electron transport chain" that we really don't understand. Other than it's a chain and it transports electrons. Those electrons come from oxygen (in chemistry, oxidation is the loss of an electron - it was originally named that because oxygen was the biggest thing to be involved in oxidation, but anything that gives up electrons is a oxidation agent). So oxygen comes in one side of the chain, electrons come out the other and help to fuel the cycle.
The rate that electrons come through the chain is the rate limiting step. As long as you're breathing, you have sufficient oxygen available that it's not a limit to how fast energy can be produced. There's always a surplus of oxygen, it's just the speed at which the electrons can be ripped off and transported down the chain that controls the speed of the Kreb's cycle. That's what make this ATP generation a "low power" system.
In the context of A+A training, we are primarily using this cycle to replenish our ATP after our bout of short intense work that depletes it. That's why we prescribe lots of rest time. You need to give this cycle time to generate enough ATP to go again. Usually, in a class setting, I prescribe something like 10 swings on the minute. 10 swings takes about 15 seconds, giving you 45 seconds to recover. The better plan would be for you to regulate your own rest time, but I've tried that and it just doesn't work in a class setting - some folks rush it, and other hang out and talk too much :) So the 45 second rest is a great compromise.