Quote Originally Posted by GLC1968 View Post
There have been numerous studies about how your body can 'adjust' (that's not the word I really want, but it'll do for now) to certain activities, become more efficient and therefore get less of a benefit from those activities. This is why weight lifters have to change up their routines every 16 weeks or so and why there is such a huge trend towards periodization in so many sports.

I don't know that there are any cycling specific studies (there may be), but I would bet that you could find lots of examples for other sports if you searched under the words 'fitness plateau' and/or 'training plateau'.
Yep, this is a fact. I can't find my personal trainer manual to give you an exact quote--but the body adapts. The heart becomes more efficient, the muscles become more efficient, the lungs become more efficient. Over time it takes less exertion to do the same activity. Therefore, to continue to increase your fitness, you have to change up your workouts (pedal faster, find a more challenging route or increase workout time).

This is true of walking, weight lifting, running or cycling.