As human beings, we like looking for hard and fast rules. If I do this, then I'll get that outcome. But when it comes to running related injuries, that's just not how it works, and we need to learn to be okay with that. For years, the go-to advice for avoiding injury has been the 10% rule: don't increase your weekly mileage by more than 10%. Doctors lean on it constantly. More recently, the industry shifted toward something called the acute:chronic workload ratio, or ACWR, comparing your training this week to the past few weeks. It's built into a lot of the watches on your wrist right now. A new study just put both of those tools to the test, and the results are surprising.
ARTICLE BELOW:
The Research
Researchers tracked over 5,200 runners for up to 18 months using GPS watches, one of the largest studies ever done on this topic. They compared three ways of measuring a "spike" in training: a single run's distance versus your longest run in the past 30 days, the standard ACWR, and a simple week to week comparison. Then they tracked who got injured.
Worth knowing upfront: this is an observational study, and injuries were self-reported through a weekly questionnaire the runners filled out themselves. Self-report always comes with some margin for error. But with a sample size this large, over 5,200 runners, it still carries real weight.
What They Found
The research suggests these are the key takeaways
🟢 This is a strong, well-designed study, but it's still observational. It shows a pattern, not proof that watching this number alone prevents injury.
A single run 10 to 30% longer than your recent longest run showed a 64% higher injury rate
A run 30 to 100% longer showed a 52% higher rate
More than doubling your longest recent run showed a 128% higher rate, the risk climbs the bigger the jump gets
ACWR, the tool a lot of us have trusted, actually ran backwards in this data, higher spikes were tied to lower injury rates
Week to week mileage change showed no meaningful relationship with injury at all
What This Means For You
The 10% rule and ACWR are the tools doctors and the industry have leaned on for years. It's notable that neither one held up here the way a single run standing out from your recent pattern did.
Here's the tension this creates. If you're building toward a marathon or an ultra, you eventually need those long run days, the ones that push volume and carry more risk by nature. You can't stack big jumps back to back either, so your build needs to stretch out longer, not get rushed.
And here's the part that matters most to me. Even runners who stayed under that 10% single-session threshold in this study still carried a bit of an increased risk. Follow the rules perfectly, and your risk doesn't hit zero. It's not whether you get hurt or you don't. It's how you manage it.
🏆 How To Apply It
Keep any single run within 10% of your longest run from the last 30 days when possible, especially heading into race-distance jumps
Give your longest efforts real recovery before stacking another big one, back to back increases add up even if each one looks "safe" on paper
Don't lean on your watch's ACWR score as your only safety signal, this study found it didn't predict injury the way we've assumed
Pair your mileage decisions with the basics that actually keep you durable: strength work, sleep, eating enough, and staying hydrated
When something starts to feel off, address it early rather than pushing through and hoping it resolves
This new model feeds right into something I already believe: it's not what you do on one day. It's how many days you do one thing. Chasing one perfect long run matters less than showing up consistently, making smart calls, and having a plan for when things don't go perfectly. Because at some point, they won't..
Hit reply and tell me, have you been using the 10% rule or ACWR from your watch?
-Keep running happy, healthy, and strong
Spencer
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