In the same discussion thread, TI Coach Eric Desanto  raised two fascinating topics that are seldom discussed and rarely understood. The first was "nervous system fatigue."

I have been tentative to add my two cents here, because I have never swum your speeds and only coached one person who reached your speeds. But here are a couple things I believe relevant.

In conversations I have had with Coach Matt Kredich (University of Tennessee women), I learned that it can take as much as 6 weeks to recover if you train the nervous system to "failure." The muscles recovered long before that, but the nervous system recovers more slowly. To avoid this, they did short sprints and focused regularly on holding technique as tempo increased. Because this could be so stressful, they stopped these forms of training six weeks prior to their championship. This fits in with what Terry wrote about the importance of nervous system training.

I’m still uncertain of the answers to several questions such as:

1. Why does it take the nervous system so long to recover?
2. What exactly is nervous system fatigue? Using up neurotransmitters? Messing with Na/K levels? Messing with Ca levels?
3. Could part of the aerobic base concept be training the nervous system to recover faster?
4. How can you recognize nervous system fatigue from muscle fatigue?

Nervous System Training is probably the least understood – by both coaches and swimmers – of all aspects of swim training. Few appreciate how critical it is to performance, nor how much more quickly it can respond to the right kind of training — or how slow it is to recover from the wrong kind — compared to the aerobic system, which has been far more heavily studied and is therefore better understood.

Because quality neural imprinting is more critical to sprinting success than any other events — because high-skilled movements must be performed at exceedingly high rates and under heavy loads — and because the potential for overstimulus and thus fatigue is so great — because of increased reliance on intensive training — it’s far more challenging to get it right when training sprinters.

From Eric:
Do you have any suggested resources to study more on nervous system training? Kredich suggested Tudor Bompa’s book on periodization. It has some interesting ideas. I still haven’t found comfortable answers to a few questions such as:

1. Why does it take the nervous system so long to recover?
2. What exactly is nervous system fatigue? Using up neurotransmitters? Messing with Na/K levels? Messing with Ca levels?
3. Could part of the aerobic base concept be training the nervous system to recover faster?
4. How can you recognize nervous system fatigue from muscle fatigue? (my guess is that nervous system fatigue would prevent you from ever reaching max speed where muscle fatigue would prevent you from maintaining it. But I have not found any reading that supports that idea.)
5. What is different about cats and dogs that makes them different?

Eric
You seem to have studied this subject with admirable depth. When I heard the term "nervous system fatigue" I was deeply intrigued, and sensed it might be incredibly important, but — typical of my right-brain makeup — was more inclined toward practical experiments in coaching application, rather than an academic study delving into considerations like neurotransmitters, Na/K and Ca levels — the last two of which I can only respond to with a blank look.

What form did these practical experiments take? While coaching sprinters at West Point , I figured trial-and-error would be the only way to understand how much nervous system stimulus would be "about right" and how much would qualify as "overstimulus." Being a "cat coach" by inclination and feeling that overstimulus could be a far greater danger with sprinters, I was naturally cautious.

As well, I intuited that nervous system stimulus would need to be more finely calibrated during taper than in mid-season and at meets than in workouts.

Here’s how I handled it: In mid-season training, I came into practice with a general plan, but regularly adjusted it during practice. When I saw the sprinters lacked "snap" or were struggling to execute a task they could normally do with controlled effort (whether because of muscular, neural — or even mental — fatigue) I’d initially modify the task by shortening repeats, increasing rest intervals, etc. If that didn’t work, I’d simply replace all the intense stuff with restorative, easy, technique work (fistgloves a favorite).

In mid-season meets, conscious that they were always dealing with some level of fatigue — often due more to the routine demands of cadet life, than to anything I’d asked them to do — I was particularly careful to avoid "oversprinting" in warmup.

I often saw sprinters on opposing teams doing repeated max-effort 25s from a dive start, with their coaches timing each one. In contrast, I gave our sprinters these instructions:
1) My stopwatch stays in my pocket until the race. Do your speedwork only for feel.
2) The feel you are seeking is what you hope to experience during the race.
3) "Rehearse" in short crisp segments:
- Start, underwater, breakout, 2-3 cycles at race speed, then ease off,
- Build into a turn, then turn, breakout and 2-3 cycles at race speed, then ease off. Etc
4) Give your nervous system "advance notice" of the task you’ll ask it to execute by swimming a few repeats of just 3-4 cycles the way you’d like to swim during the race. Never sustain this to fatigue.

This warmup plan also took into consideration that most would swim three races, requiring three repetitions of some part of this warmup. I reminded them to be mindful of this during the initial warmup.

The upshot was that they usually seemed reasonably sharp — considering it was midseason — during races and could sustain a consistent performance level to their last event. PS: Restorative warmdowns after intense races were just as critical, guessing that the nervous system, as much as the muscles, could benefit from a "cooling-off" period after intense activity.

If I’d been coaching the distance swimmers I would not have felt the need to be nearly so meticulous in planning to accommodate stress or overstimulus.