If you plan to increase training workload over a season, you can't just ratchet it up every week and expect to go uninjured. Thus, the idea of incorporating easy weeks as a safety valve was created.
In the popular training schedules of Brad Hudson, he keeps mileage constantly "moderately high." His method of nonlinear periodization depends upon a couple of beliefs: 1) The standard method of linear periodization leads to injury from doing too much of one type of training and then abruptly shifting to too much of another. 2) If one runs high mileage, the hard miles one does is a relatively small percentage of the total, so increasing their intensity is not a big deal. This method works best for those who have a keen sense of their abilities and who don't push themselves to their limits in workouts.
By contrast, the other currently popular schedules, those of Jack Daniels, have a repeated three week cycle. While the numbers vary, in general it looks like 100% maximum mileage, 90%, 70% and then repeat, with the low mileage week also having fewer hard miles. He gives no rationale for his weekly mileage variations.
The biggest proponent of easy weeks is Jeff Galloway. In his first book, he recommended a four week cycle of 100%, 70%, 100%, 50%, with the 50% week having little or no speedwork. He gives no reason for these particular percentages (and in other places says "find what works for you") and probably came up with them empirically. Some would argue that his easy week is too easy, but it is better to err on the side of caution. One source of inspiration for his easy weeks may have been Dr. Costill's idea for increasing mileage every other week, with intervening easy weeks, published in the 1970's.
If you search the Web for "crash training," you'll find Peter Snell's plan for increasing mileage. His cycle looks like: 100%, 50%, 75%, 25% and then increasing each number by 25 percentage points each cycle, leading one to double one's mileage in 5 months. This might be possible for beginners, especially if they're young, but can't be recommended. I always advocate beginners getting the most they can out of as little mileage as possible to begin with, racing often for experience and then increasing mileage by 10% per week, but only once or twice per year.
Next: Combining all the ideas into a whole (but there's probably going to be a race report before that).
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