In addition to both UA books, I recommend these two Science of Ultra podcasts that cover motor unit recruitment, fatigue and training:

Interval Fundamentals
Show notes worth reading too. I found this episode aligned 100% with UA advice in TFTUA and in the Hill Sprint progression in the Big Vert plan.

Neuromuscular Control and Fatigue with Roger Enoka, PhD

From the show notes in the first podcast:

First, a reminder about motor unit recruitment, or a primer if you didn’t listen to my discussion with Dr. Roger Enoka in episode 59. As you progress from running easy to running faster and faster, you engage more and more of the muscle cells (often called muscle fibers) in the muscles you use and you do so in groups of muscle fibers called motor units (that’s called motor unit recruitment). So, running easy – some motor units used, running faster – more motor units recruited. And there’s complementary increases in breathing depth and rate, in heart rate cardiac stroke volume and therefore cardiac output, among others. When you run an ultra-marathon, you aren’t recruiting all your muscle motor units because you’re not running very hard or fast. But, as you fatigue, to maintain your pace, you need to recruit more motor units. The motor units you recruit for very easy contractions are pretty economical but the ones you recruit for faster paces, greater force, or to maintain pace in endurance are progressively less economical. To make those motor units recruited when you’re fatigued in an ultra marathon more economical and less fatiguable, you have to recruit them in training. There are two ways to do this. Option 1 is to run at slower paces until you require them due to fatigue – that’s exhausting and hard to recover from. Or you can run fast, which requires them to engage.

Great conversation here. Thanks.

If you accept the size principle theory of muscle fiber recruitment (which you probably should as it seems to have held up well over the decades) then you can understand that the only fibers that get a training effect are those that get recruited to provide the needed force. Jog along at a AeT or below pace and you are recruiting only the slow twitch fibers that already have a high level of endurance. To give those ST fibers a bigger training stimulus you must increase the duration of the training. If you increase the intensity (increased force of contraction) you will be recruiting some faster twitch, higher force fibers that are not so well endurance trained.

Hopefully you are still with me to here.

So what high intensity (what we’d call Z3-4 all the way up to very short max efforts) do is to cause these higher force fibers to get recruited. Remember that these fibers exist along an endurance/strength spectrum from the lowest power ST fibers you might engage when you are walking your dog which can do their thing for hours upon hours, all the way to the highest force FT fibers that you engage in maximal efforts, which have almost no endurance and fatigue in a few seconds.

By recruiting those fibers for a sustained effort that are right at the frontier of your endurance (see Frontier Fiber Model in TftUA p241) you cause endurance adaptations in those fibers. The reason those fibers are poorly endurance trained to begin with is that the low intensity training you’ve been doing to improve your aerobic base does not engage these FT fibers. It takes higher intensities to get them off the bench and into the game.

This higher intensity work can come in many forms. At one extreme we have short maximal effort hill sprints to engage those super FT fibers that have almost no endurance and get your brain to figure out how to coordinate their engagement for maximum power. Another effect way to do this is by doing repetition or interval training. Repeated hard work bouts separated by rest intervals (hence the name). A way that we’ve found to be super effective is through Local Muscular Endurance training. For this we use special methods (explained in great detail elsewhere on the site and in the books) to force the recruitment of these frontier fibers without the overall global training stress that comes with conventional high intensity aerobic interval training.

I hope this helps.
Scott