comma wrote:
Well, you do adapt to training, after a certain period of training and adaptation you need to change the stimulus to continue to progress.
If you train all the systems all the time without periodization you will become stale and not progress. Variety is not only the spice of life.
You have some good answers here, some that answer a similar question very well, and a few that I disagree with.
Most people who contributed have brought up progression which is an immensely important part of training and many mentioned either how to progress by making workouts more difficult or by saying what things you might see from one period to another. But since the original question was, "why do we periodize our training," I think that comma here is right.
We periodize things for progress, yes, but even before that we periodize because we get used to a stimulus and no longer receive as much benefit from it to justify doing it. Lets use a very simplified example of a person who has never run a day in their life and today decided that they want to improve their mile as much as they can. So what do we do?
lets say we make them run 1 mile a day. For the first month this might work great for them, they'll probably shoot up like a weed but after running a mile a day for 3 months do you think they're still going to improve in a 1 mile race? its probably unlikely. So again, what do we do? Lets make them run 2 miles a day. There they go shooting up like a weed again as they are now running longer than their race distance and are getting more comfortable. After 3 months? they probably haven't improved for a while, we waited too long to adjust the stimulus again. Lets do 4 miles every other day and 2 miles on the days between... This goes on for a while.
We are more clever than this and don't simply add one more mile a day but instead modulate workouts by type, distance, rest, pace, etc. making us much more dynamic and balanced runners.
But to be thorough lets think about the converse example (ie. no periodization). Here we progress just as many people have mentioned but we take no time to let our bodies adjust to a workload, we simply progress. So week 1 our never-run-a-day-in-his-life runner does 1 mile a day, week 2 he does 2 miles a day, week 3..., week 10 he does 100 miles a week. So which week do you think we injured this guy? I'm guessing week 4 he regretted his decision to let us coach him but week 5 he switched to Hokas and week 6 may or may not have tested a new god each morning as he tied said Hokas. My point is that we didn't let him adjust to anything, we just progressed him and assumed that the benefits were immediate; they aren't.
So yes, I wholeheartedly agree with a lot of the progressions I saw here, but to answer the question as to "why" we periodize, its because we take time to adjust to a particular stress (or we don't get used to it and either over train or get injured) but we need to move onto a new "step" that progresses us forward and again gives us some time to get used to that new stress. So always keep in mind that a stress needs to be provided and a period of adjustment needs to be allowed.