A study of over 5,200 runners reveals that most running injuries don’t build up gradually; they strike during a single session. According to researchers, millions of runners are being misled by the guidance offered by their sports watches.
A groundbreaking study from Aarhus University is challenging long-standing beliefs about how running injuries happen.
In what is now the largest study of its kind, researchers discovered that overuse injuries in runners do not always build up slowly over time, as many have thought. Instead, these injuries often appear suddenly, sometimes during just one workout.
“Our study marks a paradigm shift in understanding the causes of running-related overuse injuries. We previously believed that injuries develop gradually over time, but it turns out that many injuries occur because runners make training errors in a single training session,” explains Associate Professor Rasmus Ø. Nielsen from the Department of Public Health at Aarhus University, who is the lead author of the study.
Tracking 5,205 runners from 87 countries over an 18-month period, the team found that the risk of injury climbs sharply when a runner increases their distance in a single session compared to their longest run in the previous 30 days. The more dramatic the increase, the greater the chance of injury.
Incorrect guidance for millions of runners
According to Rasmus Ø. Nielsen, the results cast critical light on how the tech industry has implemented so-called “evidence.” Millions of sports watches worldwide are equipped with software that guides runners about their training – both for training optimization and injury prevention.
However, the algorithm used for injury prevention is built on very thin scientific grounds, according to Rasmus Ø. Nielsen.
“This concretely means that millions of runners receive incorrect guidance from their sports watches every day. They think they are following a scientific method to avoid injuries, but in reality, they are using an algorithm that cannot predict injury risk at all,” he says.
Non-existing evidence behind guidance
The current algorithm, called “Acute:Chronic Workload Ratio” (ACWR), was introduced in 2016 and is now implemented in equipment from companies that produce sports watches, while organizations and clinicians, such as physiotherapists, also use the algorithm.
The ACWR algorithm calculates the ratio between acute load (last week’s training) and chronic load (average of the past 3 weeks). The algorithm recommends a maximum 20% increase in training load to minimize injury risk.
According to Rasmus Ø. Nielsen, the algorithm was originally developed for team sports and was based on a study with 28 participants. Due to the few participants in the study combined with data manipulation, the evidence base for using the algorithm to prevent running injuries is therefore “non-existent.”
Realtime guidance
The research team has therefore worked for the past eight years to develop a new algorithm that will be much better at preventing injuries for runners.
Rasmus Ø. Nielsen emphasizes that he and the other researchers behind the study have no commercial interests in launching a new algorithm as a potential replacement for a method he himself criticizes.
The algorithm will be made freely available to runners, companies, clinicians, and organizations who can use it actively to guide training and injury prevention.
Rasmus Ø. Nielsen hopes that the new insights will be implemented in existing technology.
“I imagine, for example, that sports watches with our algorithm will be able to guide runners in real-time during a run and give an alarm if they run a distance where injury risk is high. Like a traffic light that gives a green light if injury risk is low, a yellow light if injury risk increases, and a red light when injury risk becomes high,” explains Rasmus Ø. Nielsen.
Facts about the study
The Garmin-Runsafe Running Health Study followed 5,205 runners from 87 countries over 18 months.
Participants recorded 588,071 running sessions, and 35 percent of participants sustained a running-related injury during the study.
The study documents concrete risk increases with increased running distances compared to the longest run in the past 30 days:
- 10-30 percent increase: 64 percent increased injury risk
- 30-100 percent increase: 52 percent increased injury risk
- Over 100 percent increase: 128 percent increased injury risk
The study also shows that injury risk increases with progressions over 1 percent (in the interval between 1 and 10 percent), which questions whether the frequently used 10% rule for safe training progression is accurate.
Reference: “How much running is too much? Identifying high-risk running sessions in a 5200-person cohort study” by Jesper Schuster Brandt Frandsen, Adam Hulme, Erik Thorlund Parner, Merete Møller, Ida Lindman, Josefin Abrahamson, Nina Sjørup Simonsen, Julie Sandell Jacobsen, Daniel Ramskov, Sebastian Skejø, Laurent Malisoux, Michael Lejbach Bertelsen and Rasmus Oestergaard Nielsen, 7 July 2025, British Journal of Sports Medicine.
DOI: 10.1136/bjsports-2024-109380
The study received external funding from Aarhus University Research Foundation and the Danish Rheumatism Association. Garmin and external funders had no influence on study design, research questions, data collection, data processing and statistical analysis and/or interpretation of data as well as the writing and publication process.
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