A study of over 5,200 runners reveals that most running injuries don’t develop gradually—they strike during a single session. According to researchers, a large portion of runners are misled by the guidelines from their sports watches.
Researchers from Aarhus University shed new light on how running injuries actually occur with their study — and this picture differs significantly from what was long assumed.
In what is now the largest research in this field, they discovered that so-called overuse injuries don’t necessarily build up gradually. On the contrary: many injuries occur suddenly, sometimes during just a single workout.
“Our study represents a fundamental shift in understanding how overuse injuries develop in runners. For a long time, we thought injuries built up slowly, but it turns out many complaints can be traced back to a single bad training session,” says Rasmus Ø. Nielsen, associate professor in the Department of Public Health at Aarhus University and lead researcher of the study.
The 5 Key Takeaways
- Many running injuries occur acutely – They happen during a single workout, not after months of overuse.
- A sudden increase in distance raises the risk – Especially if the distance exceeds what was run in the past 30 days.
- The algorithm in sports watches is often wrong – Many commonly used methods are based on flawed science.
- The widely used 10% rule isn’t as safe as thought – Even small percentage increases can raise the risk.
- The study calls for better guidance based on current data – And advocates for real-time alerts during running.
The researchers followed 5,205 runners from 87 countries over an 18-month period. They found that injury risk increases significantly when someone runs a longer distance in one go than they have in the past 30 days. The bigger the jump, the greater the risk.
Misleading Guidance for Millions of Runners
According to Rasmus Ø. Nielsen, the research results cast a critical light on how the technology industry has applied the term “scientifically proven.” Millions of runners worldwide use sports watches that provide training advice — both to improve performance and prevent injuries.
But according to Nielsen, the underlying logic of that advice is often flimsy.
“In practice, this means millions of people are guided incorrectly every day. They think they’re training in a scientifically sound way to prevent injuries, while the algorithm they’re using can’t actually predict injury risk accurately,” he says.
Glossary
- Algorithm – A series of calculations used to make decisions or predictions
- Chronic load – The average training load over multiple weeks
- Paradigm shift – A fundamental change in how we view or understand something
- Team-based validation – A method tested within team sports but not necessarily applicable to individual athletes
No Real Evidence for the Advice
The current algorithm, known as the Acute:Chronic Workload Ratio (ACWR), was introduced in 2016. Since then, it has been incorporated into the software of multiple sports watch manufacturers. Professionals such as physical therapists also use it to estimate injury risk.
The ACWR compares the recent training load from the past week with the average of the three weeks before. Based on that ratio, it’s recommended not to increase the load by more than 20% per week.
Associate Professor Rasmus Ø. Nielsen is affiliated with the Department of Public Health at Aarhus University. As coordinator of the international RUNSAFE project, he has been working on injury prevention in runners for years. His research focuses on the influence of training load, technique, and algorithms on injury risk. With his clinical background as a physical therapist and his scientific experience in epidemiology, he bridges the gap between practice and prevention.
According to Rasmus Ø. Nielsen, however, the algorithm was developed based on a small study with only 28 participants, focused on team sports. Due to the limited scope of that study and data manipulation, there is virtually no solid evidence for using this method with runners.
Real-Time Guidance Based on New Algorithm
The research team has spent the past eight years developing a new algorithm to better protect runners from injuries. This approach is intended as an alternative to existing, insufficiently supported methods.
Rasmus Ø. Nielsen emphasizes that he and his colleagues have no commercial interests in rolling out this new system. The algorithm will be made freely available to runners, healthcare providers, companies, and organizations — with the goal of making training safer.
He hopes that sports technology companies will integrate these insights into their existing systems. “I envision sports watches eventually providing real-time feedback during running. For example, a color-coded warning system: green for low risk, yellow for increasing risk, and red when the chance of injury is really high,” Nielsen says.
Study Facts
The Garmin-Runsafe Running Health Study followed 5,205 runners from 87 countries over 18 months. Together, they logged 588,071 running sessions. During the study, 35% of participants sustained a running-related injury.
The results show clear links between sudden distance increases and injury risk, compared to the longest distance run in the past 30 days:
- Increase of 10–30%: 64% higher injury risk
- Increase of 30–100%: 52% higher injury risk
- Increase of more than 100%: 128% higher injury risk
Verified Sources
- “How much running is too much? Identifying high-risk running sessions in a 5200-person cohort study” – Jesper Schuster Brandt Frandsen et al., July 7, 2025, British Journal of Sports Medicine – https://doi.org/10.1136/bjsports-2024-109380 The study was funded by the Aarhus University Research Foundation and the Danish Rheumatism Association. Garmin and other external parties had no influence on the design, execution, analysis, or publication of the study.
- Thanks to SciTechDaily.com
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Frequently Asked Questions
What types of knee injuries are there?
There are various types of knee injuries, including runner’s knee (iliotibial band syndrome), jumper’s knee, meniscus tears, and overuse of tendons or ligaments.
How long does it take to recover from a knee injury?
Recovery time varies greatly depending on the severity of the injury. Mild overuse can heal within days to weeks, while more serious injuries like a torn meniscus can take months.
What should you do if you have a knee injury?
Rest, ice, possibly apply a compression bandage, and elevate the joint are first steps. For persistent complaints, it’s advisable to consult a physical therapist or sports medicine doctor.
What are the symptoms of a knee injury?
Pain during movement, swelling, stiffness, instability, or a popping sensation can indicate a knee injury.
What does a torn meniscus feel like?
This can feel like sharp pain on the inside or outside of the knee, sometimes accompanied by locking or a ‘clicking’ sensation during movement.
How do you recognize an overused knee?
An overused knee often feels dull or achy, especially after exertion. The pain typically increases when climbing stairs, squatting, or standing for long periods.
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