Swimming Fast Repeatedly
It may be there has been research on how the body responds to increases in work or rest. But, historically, I think that there's been an overemphasis and over-reliance on the idea of needing to peak physically to swim fast.
That idea was based on a traditional approach to training which was to intentionally bring a swimmer close to failure through heavy training, with the idea that the swimmer would "super-adapt" when given rest. This was my experience as a college swimmer 35 years ago, and it was pretty much how everyone trained back then -- and probably until earlier this decade.
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.
As well, swimmers are likely to get stale mentally during prolonged periods of being kept on the brink of exhaustion.
These days, the coaches of elite swimmers are far more likely to give a moderate training load, rest briefly to let the swimmer adapt to it, then give a slightly more demanding load, etc. Rather than one major peak per season, they're looking to produce a prolonged series of smaller advances in capacity and performance.
You can see this reflected in the season-long performances of college swimmers from the most successful NCAA programs, like Texas, Arizona, Stanford, Auburn, Georgia, etc. They swim remarkably fast from November straight through to February, then even faster in March -- in this case with the additional aid of high tech suits.
Also, if you observe the performances of international elite swimmers in FINA Grand Prix meets, they swim breathtakingly fast for a period of two months or more. There are two Grand Prix circuits -- a short-course meters circuit from mid-October through early Dec, and a long-course meters circuit from late spring to mid-summer. In the fall, the elite swimmers on this circuit go from city to city -- often many time zones apart -- swimming a meet each weekend for perhaps 8 consecutive weeks. Their performances hold up with impressive consistency, at a very high level, the entire time.
I think this reflects the increasing tendency for top level coaches and swimmers to train in a far more sophisticated way than in years past. They definitely focus more on neural training (i.e. tweaking and imprinting technique and exacting combinations of Stroke Length and Stroke Rate), and rely less heavily on aerobic training. Neural function should be able to remain steady at a high level for extended periods. Not so physiological function.
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