What's the ten percent rule? It's the commonly stated rule that you shouldn't increase mileage by more than ten percent a week.
Imagine you just took up running. You run 3 miles a week. Are you seriously going to limit yourself to 3.3 miles the second week? You could, but it'd be a waste of time. You could easily go to 4 or 5 miles without getting injured. Similarly, if you're an experienced marathoner, you could easily bump your mileage from say 35 to 43 miles in one week. That's over twenty percent. The extra 8 miles will not get you injured if you have the experience. Increasing 10% a week will surely get you injured too. 50, 55, 60.5, 66.5, 73.2, 80.5, 88, 97.4... Doubling your mileage in 8 weeks is a recipe for disaster.
As far as we know, the ten percent rule has never been tested in a peer reviewed study. It is just a gimmick. It sounds neat and may somewhat limit runners that are overambitious, but does it really work. Probably not.
Experts and coaches are afraid to take the blame if someone gets injured by increasing their mileage too quick. Increasing your mileage too quickly WILL get you injured. But the magic number is not 10%. It varies based on current mileage and intensity. It varies based on your propensity to get injured. Some runners will get injured beyond any mileage per week.
So is the 10% rule stated out of laziness? We don't think so. Fear is a better explanation. People are afraid to go against the grain. We believe the ten percent is stated simply because it has been around so long and is stated to deflect responsobility. Someone stating only ten percent increase takes no risk. If a runner gets injured from increasing more than ten percent, they can state the rule. If they don't, they can also say they were just stating the common rule.
The ten percent rule is completely and utterly useless. It is too conservative at some mileage ranges and too aggressive at others. File this one in circular storage. It will not prevent injury. It gives runners a false sense of confidence against injury. It has no place in a running program whatsoever.
Cut-back mileage weeks probably work better. Be careful increasing mileage, but don't limit yourself unnecessarily. Going from 39 miles to 46 miles is probably just fine if you've been at 40 miles for a long time. Going from 40 to 44 to 48 to 53 is probably less safe. Speedwork will limit advances in mileage and this is definitely not accounted for with the 10 percent rule. Still other runners will never tolerate higher mileage. If you always get injured beyond a certain point, you probably shouldn't ever go above that mileage.
A better "rule" often stated is don't do speedwork unless you've been over 20 miles per week for a long time. Still, this one needs context, but it is more useful than the worthless gimmicky ten percent rule. Harsh? Not really. If a rule is pointless, it has no place in running.