Following up on yesterday’s post, I’m going to delve into even a more hackneyed complaint — the “10% rule.”
I wrote about this for Running Times years ago, but there’s considerably less editorial oversight on this blog, so I’ll give it a whirl here, too. For the non-runners who have read all the way to this sentence, the 10% rule is supposed to be a guideline for how aggressively you should ramp up your training mileage from some established baseline. It’s meant to keep people from being too ambitious and placing themselves at injury.
The problem is that no one knows what the fuck it means. Sure, some Americans understand the concept of percents and a few can even do simple calculations, but when it comes to this, even people who invoke it in mantra-like fashion can’t tell you what they’re saying.
The discussions usually go like this:
“How fast should I increase my weekly mileage?”
“Use the 10% rule so you don’t get injured or burn out.”
“I’ve been averaging 30 miles a week, and my highest ever was 40. Does that mean I can’t go over 44 next week?”
“No, silly. It applies to how much you can safely increase over time.”
“But if I increase by 10% *every* week –”
“No, silly. Don’t do that.”
And then the conversation usually winds down with nothing established except that someone has advised caution and tried to pin a number on it without explaining the need for the number. Let’s say then 30-miles-a-week runner in the discussion about hoped to increase his mileage over the course of a year by some significant but undetermined amount. Since everyone agrees (I think) that increasing by 10% every week would quickly lead to unmanageability, assume that the runner plans to increase his weekly mileage by 10% per month. Got that?
So in January the guy averages 33 miles a week, then in February kicks it up to 36.3, etc. Sounds conservative enough, but observe what happens by late fall. The formula for determining how much a fixed percentile increase will lead to an increase from some baseline total (useful in calculations involving interest earned and mortgage payments) is
M = Mie(r)(t),
Where M = mileage, Mi is the initial value (in this case 30),e is a constant (2.718; this is the base of the so-called natural logarithm), r is the rate of increase expressed as a decimal (0.10), and t is elapsed time (in this case, 12 months). Plugging these in, we find
M = (30)(2.718)(1.2),
or 99.6 miles. While this is possible, it’s not something most runners would try even if their bodies allowed it.
It’s not the advice to be careful here that jump-starts my agitator so much as the inherent contradiction involved in putting a fixed number to such vague use. It would seem more sensible, and just as simple, to advise people to not increase their weekly average by more than 10 miles a month or something (this works for me since it’s roughly what I prescribe when writing training plans for people, depending on where they are to start with, of course) or better yet, to ultimately find out for themselves what they can handle, since things like running surface and intensity factor into this issue to a tremendous extent.