Ilaria Rosella Pagliaro A massive study from Denmark reveals that a single overambitious run — not weeks of training — can cause injury, and current smartwatch algorithms aren't equipped to prevent it.
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Running injuries don’t build up — they can happen overnight
We’ve heard it a thousand times: running injuries creep up slowly, the body wears down little by little, and sends subtle warning signs before something gives. And in the age of smartwatches, many of us have trusted those devices to steer us safely through our training. But a new study from Aarhus University in Denmark turns all of this on its head. According to the researchers, it doesn’t take weeks of overtraining to get hurt — one single, poorly planned run can be enough.
And yes, even if you’re the kind of runner who follows your smartwatch’s advice to the letter, you might still find yourself sidelined. The culprit? The algorithms behind those devices, which, it turns out, aren’t actually designed to predict injuries at all.
Too much, too soon: the real reason runners get hurt
The Danish study — the largest ever conducted on this topic — followed over 5,200 runners from 87 countries, tracking their workouts for 18 months. The data is stark: out of 588,071 recorded runs, 35% of the participants suffered at least one running-related injury.
But what’s truly surprising is when these injuries happen. Forget the idea of slow wear and tear. Most injuries occurred suddenly, often after a single run where the athlete pushed too far beyond their usual limits.
That old rule of thumb — “never increase your mileage by more than 10%” — takes a hit here. The numbers paint a different story:
- Increasing your longest run by 10–30% compared to the previous month? Injury risk rises by 64%.
- Increase by 30–100%? The risk jumps 52%.
- And if you go all out and double your distance (over 100%) — injury risk skyrockets by 128%.
Even more modest jumps under 10% aren’t necessarily safe. In essence, every time you raise the bar too quickly, you’re entering dangerous territory — whether you feel it or not.
The algorithms in your watch are misleading you
If you think your smartwatch is there to protect you from injury, think again. Most of these devices rely on a now-widespread calculation known as the Acute:Chronic Workload Ratio (ACWR). It compares your training volume from the past week to your previous three weeks, suggesting you stay within a 20% increase to avoid injury.
The problem? According to Professor Rasmus Ø. Nielsen, lead author of the study, these models weren’t made for running. “They’re based on outdated research, with tiny sample sizes and weak methodologies,” he explains.
And the consequences go far beyond casual joggers. “Every day, millions of people get bad advice from their smartwatches. They believe they’re following a scientific method, but the algorithm can’t actually predict injury risk,” says Nielsen.
Even worse, these same systems are now used by clinics, physiotherapists, and coaches. Despite their popularity, Nielsen argues that there’s no solid scientific foundation supporting them.
A smarter, safer system is on the way — and it’s free
Fortunately, the researchers didn’t just stop at pointing fingers. Over the past eight years, they’ve been developing a new algorithm built specifically to help runners avoid injuries — one that’s more accurate, personalized, and free to use.
This new tool will be available to companies, healthcare providers, coaches, and individual runners, and can be integrated into existing sports watches. The idea is to offer real-time risk alerts, much like a traffic light system:
- Green means you’re safe.
- Yellow is a caution zone.
- Red means you’re pushing too far — and it’s time to slow down or stop.
It’s like having an intelligent running buddy, one that knows when you’re about to cross the line before your tendons or joints do.
Nielsen and his team envision this tool as a new standard in sports tech, replacing the flawed systems currently in use. And most importantly, it won’t cost a thing.
