Matt/Jeffrey/Stuart,
I don't lecture on this topic regularly and I don't have a physiology text on hand, so this is from memory...
All compounds are converted into ATP which is the substrate that the cell actually uses to do work, so what we refer to as fuel, sugar fat, etc., needs to be converted into ATP. It is generally accepted that 3 systems supply this fuel: alactic, or ATP-PCr; glycolytic, or lactic, and the oxidative, or fat. (The names change as you travel the world.) The first 2 are anaerobic, so they can function without oxygen, the latter is aerobic only, so there must be O2 present (this is important for our discussion). All 3 systems are always "on" but, depending on what you are doing, one can be dominant.. and this is what we mean when we say an activity is glycolytic, or aerobic - which fuel system is dominating ATP conversion?
Each cell has free ATP floating around that it is using. The fuel systems combine to resupply the ATP pool.
Free ATP can be resynthesized in a circular fashion providing energy for about 3s.
Alactic fueling provides ATP up to about 10-15s in untrained individuals.
Glycolytic fueling provides ATP out to about 90s-2min.
Oxidative fueling can last for days and weeks.
The intensity and duration of the work primarily dictate which system will dominate supply. So,
Time now is zero. Assuming that you have been doing nothing other than reading over your copy of S&S, looking for ways to change that program, you are running on predominantly on fat: fatty acids.
If you begin walking at leisurely pace, you will generally supply ATP through your oxidative system, even if you walk at an easy pace for the next 3 days.
At any time that the intensity of movement increases such that ATP conversion from fat “outruns” the ability for oxygen to be present (and useful) in the cell, anaerobic fueling, by definition, must occur… or, you can’t do more intense work (i.e., hitting the wall). But, it will only occur out to those durations listed, assuming that, in our scenario, they are full up in the first place when the work turned more intense (we were already moving, remember?). Then, intensity must decrease and “wait up for”, or match, O2 supply… we are now aerobic again.
Alactic fueling will cover short, max power spurts of intensity up to 10s, then need to recover; glycolytic fueling will cover this out to 2min, until it needs to recover. Then, the intensity must decrease until one of these two systems have recovered. So you can sprint, jog, sprint, jog, etc… the quick energy systems are recovering during the lower-intensity movement.
Make sense?
Ok, let’s begin back at time zero and go all out on the track: after 10-15s, even though you are trying not too, you will slow down; this will happen again at about 90s… these two systems are out of gas. If you keep running as hard as you can, theoretically, you are using the oxidative system and the quicker systems recover some, so you will get short bursts of increased speed if you still have the accelerator to the floor. Picture a CF metcon now; this is exactly what those cats are trying to do w/r to time and intensity.
Finally, the above descriptions are chemical reactions, resulting in substrates as well as energy: ATP, the thing that we want. Simply put, it is anaerobic glycolysis (there is also aerobic glycolysis, but it is not important here) that results in substrates that are harmful to the cell, if allowed to accumulate, like when you push yourself through a bootcamp session. It’s not a single bout of excessive accumulation that is very harmful, but months and years of it. So, systemically, those fat-burning and superior training programs (“bastardized” HITT) are now leading to fatigue and lethargy… if you keep it up, and adrenal burnout will occur. Locally, the cell undergoes its own nuclear fallout in this scenario.
Pathway targeted training changes, to some extent, the ability of each system to adapt in a way that allows them to cover different intensities and durations. The oxidative system can be trained to “turn on” quicker, and cover higher intensity… so, you get to be aerobic running at a faster speed than when you were untrained. Also, you are aerobic earlier into your session.
The alactic system can be trained to last longer… to 35s, I believe I’ve read somewhere; and have more depth: to do more work within that 35s time period.
The same goes for the glycolytic system.
I’ll try not to speak for SF as a whole here, but the belief of how to condition favored by many within this community is to train the alactic system and the aerobic system, leaving the glycolytic system alone. It seems that the glycolytic system stays pretty close to previous levels as long as you are doing repeated alactic work. Not true the other way around, however, + you have that harmful physiologic-nuclear fallout going on.
How? Sets of short, high-power work, repeated, with adequate recovery, for increasing durations. “Some” pure LSD aerobic work. I refer you to my last article.
Now, to peak for a season, or event (not powerlifting), in addition to your year-round alactic+oxidative program, hit that turbo-charger and train your glycolytic pathway for 6-8 weeks, or so (I think Lydiard favored 10). Leave it alone otherwise…