I just read this off of the Total Immersion blog and had to comment immediately:
How long can you sustain Fast Swimming?
I found it to be extremely interesting in its discussion on adaptation and about neurological training. He states regarding adaptation:

These days, the coaches of elite swimmers are far more likely to give a moderate training load, let the swimmer adapt to it, then give a slightly more demanding load, adapt to that, etc. Rather than one major peak per season, they’re looking to produce a prolonged series of carefully-calibrated smaller advances in capacity and performance.

I’ve always thought that shocking the system as in the past was only going to wear somebody down and I’ve adjusted my own training to reflect an approach that is very similar to what he describes. I up my training load, then spend about 3 weeks to cement that adaptation into my system before raising it again.
The other important point is here:

The seldom-acknowledged weakness in this approach is that, while it may work reasonably well for the metabolic systems (aerobic capacity, muscle strength, etc.), neurological capacity was poorly served. A swimmer who is barely surviving workouts, because of prolonged intensity or volume, is far more likely to “practice struggle” in their movements, hurting the neuromuscular imprint needed to swim fast.

Lately, I’ve really come to realize the importance of neurological training. This is not only practicing and imprinting proper form in swimming, but also in the way my legs move in running, and also getting my nerves to fire faster so that my legs are more comfortable in cycling fast, even when tired.
Driving your system to exhaustion so that you can’t even focus on form is just dumb. I’ve discovered this as well where I would get to a point of tiredness and can’t even maintain form while running. The result is that I start stomping more, heel striking, my legs start moving slower and slower: this is all bad not only for racing fast, but increasing the likelihood of injury.
This is why I am mentally extra focused on maintaining form in all 3 sports. It’s super important to practice this, especially when you get into tired states because the body just gets lazy as you focus on “keep moving” versus proper form. The worst thing that can happen, as the TI blog entry suggests, is that when you get tired, you start imprinting improper form or you never gain the ability to imprint proper form because you’re always tired and you can’t.
Love this blog entry and love how it validates some of my own personal discoveries.