I have a couple questions/thoughts about energy system utilization and how S&S fits in. Please bare with me, because it has been a couple years since I've studied these topics, I do not have a textbook handy, and I always struggled as soon as I reached the cellular level.
Caveat: studying energy systems is really, really hard. Difficult to measure, lots of competing variables, tons of theories. What we do not know outweighs what we do.
Also, I've only skimmed through this thread, so my apologies if I repeat anything. Al and Mike have both made some very well thought out, articulate posts. Everyone should reread those.
First of all, in my mind it is silly to say that one type of training solely utilizes one type of energy system. Al touched on this on the first page. Yes, you have the extreme ends of the spectrum (olympic lifting and marathon running) which may use 99% of their energy from one system, but almost everything else is going to be a mix. The energy systems do not just "turn on" sequentially, they act in tandem.
Now, my (admittedly limited) knowledge about the alactic energy system is that it is for brief 10-15 second all out efforts, and ATP is provided by the donation of a phosphate from phosphocreatine (PCr) to ADP. However, we only have a limited supply of PCr in our muscles, and it takes a little while to completely replenish the stores; 3-5 minutes. However, a single set would likely not completely deplete stores, so it is likely that after a short rest a second set could be done primarily using the remaining PCr for fuel. What I do not think could be possible is that the alactic energy system would be the primary energy system during 10 sets of 10 rep swings with limited rest.
I know that it is not recommended to push the density in S&S, so I can hypothetically see that a well trained subject using maximal effort with long rest periods could primarily use alactic system. But, when the rest periods drop to below a minute I believe that you are going to see a drop in power output and/or an increase in the use of the glycolytic pathways.
Here is an interesting and, I hope, applicable study: <a title="Creatine repletion" href="http://www.ncbi.nlm.nih.gov/pubmed/9241025">"Muscle phosphocreatine repletion following single and repeated short sprint efforts."</a>
It looked at the effect of single sprints and repeated sprints on PCr repletion. The two groups either performed 1x6second maximal sprints on a bicycle ergometer, or 5x6s maximal sprints departing every 30 seconds (sound familiar?). For the single sprint group muscle biopsies immediately following the sprint (well, 10 seconds after) found that PCr levels were 55% of the pre-exercise value. By 3 minutes the values were back to 90%. The researches found that for the repeated effort group PCr values 10 seconds after completion were 27% of their initial pre-exercise levels. After 3 minutes of rest PCr was back up to 84%.
Unfortunately, I cannot access the full study so I only have the abstract to work with. I cannot comment on how "trained" the subjects are, and I do not know how much peak power dropped during the repeated sprints.
But, I do have a few take away points. What this study tells me is that you cannot maintain peak power through the alactic system without longer rests. When the group performed a single sprint they, on average, depleted 45% of their PCr stores. However, when they performed 5 sprints they depleted 73%. Despite completing 4 more sprints they only used up 28% more PCr. What I believe happened (and what has happened in<a title="Similar Studies" href="http://www.zone5endurance.com/wp-content/uploads/2011/09/Gaitanos-1993.pdf"> similar studies</a>) is that after the first sprint the subjects still used maximal effort, but that their absolute intensity (likely wattage for this study) steadily decreased and their utilization of glycolytic and aerobic pathways steadily increased.
A study on energy contribution during KB ballistics would be ideal, but this will have to do instead. First of all, the study I referenced had 6 second efforts, much lower in duration than the length of time it takes to perform 10 swings. However, cycling is very different as one leg is always working, so it is hard to directly compare. Also, the group only performed 5 sprints, not 10 like in S&S. So it is definitely a bit of apples to oranges, but I think the message is still pretty clear. The modality may be different, but it is still the same human body.
Based on the results of this study, and on what I know about the different energy systems, here is what I imagine happens during S&S. The very first set is likely to be predominantly alactic based. I do not doubt that. Depending on the length of rest the second and third may be too. But, unless you are resting for 2-3 minutes you are going to see the glycolytic and aerobic systems becoming increasing utilized. This is echoed in a <a title="Energy Systems Review" href="http://www.zone5endurance.com/wp-content/uploads/2011/09/Gastin-2001.pdf">REVIEW</a> article that states "It now seems evident that all 3 energy systems make a contribution to the energy supply during sprinting, even during efforts as short as 6 seconds."
Now, this is all assuming you are not pushing the density at all. But, in order to achieve the simple standard you need to explosively swing a kettlebell 10 times every 30 seconds for 50 minutes. And, it has to be relatively comfortable, you have to "own" the weight. I don't think someone can accomplish this goal without heavily using the glycolytic system. Mike Perry touched on this earlier, if you are going to train power than you need LONG rests. 30s-1min is not going to cut it. One of the reasons that something like S&S works so well is that you are training ALL of the energy systems, alactic, glycolytic, and aerobic, during the swings.
My recommendation: do not worry about what systems you are stressing. Train smart, train hard, train progressively, and train for your goals. Use a program that fits these goals, is reasonable/realistic, and that you enjoy doing. Push it some of the time, back off at others.
However, if you want to do further reading on the current thought about energy systems, please read the review article I linked earlier. It's fairly comprehensive, if slightly dated.
Also, I would love it if someone dove into kettlebell ballistics and energy systems; VO2max, lactate and HR values, muscle biopsies for PCr levels. All of that stuff would be tremendous. Sadly I doubt we will be getting it. If Pavel knows of any studies performed in Russia I would love to hear about them.
Now, I would love to hear feedback on what I have gotten wrong.
One final thought, an interesting S&S variant would be alternating swings and get ups. The TGU places very different demands than the swing, and could almost work as an active recovery. Something like 10 Swings L, 10s fast and loose, 1 Get Up L, 10 Swings R, 10s Fast and Loose, 1 Get Up R x 5 could be a very effective method. Apologies if someone has already thought this up.