Issue #17: Running Injury Risk from Training Volume

✍️ Author’s Note

Welcome back to The Threshold Lab, where each week we dissect a training topic, dig into the research and translate it into practical strategies. I’m Stephen Pelkofer, an aspiring HYROX Elite 15 athlete and science/data nerd. This week’s edition is focused on running injuries and a specific 5200 person cohort study that is making the rounds on the internet.

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👨‍💻 Introduction

Topic: Running Injury Risk from Training Volume

Total weekly mileage is such a common metric and topic of discussion among runners. Increasing this number too quickly is often blamed or assumed to increase injury risk. But what if the real story comes down to how a single run stacks against your recent training? A massive cohort study of over 5,200 recreational runners sought to answer this exact question by examining how training load relates to injury risk. The findings challenge some long-held beliefs and offer practical guidance for structuring your training.

Reference study: How much running is too much? Identifying high-risk running sessions in a 5200-person cohort study.

⚡ Fast Finds for the busy reader:

• Don’t increase your long run too soon: injury risk skyrockets if you dramatically increase your longest run.
  

• Week-to-week training load is less predictive of injury: Gradual progression over time for mileage or training load is a safe way to build.

🔬 Deep Dive

Study Details

Researchers followed 5,205 recreational runners for an 18-month period, tracking their training via Garmin watches and pairing those logs with self-reported injury data. This was one of the largest studies on running injuries to date, making it especially valuable compared to smaller lab-based studies. Instead of relying on recall or estimates, the GPS data gave researchers a clear picture of training loads at both the weekly and single-session level. Injuries were defined as any musculoskeletal pain or issue that caused a runner to reduce or stop training.

The key metrics the research team wanted to test were:

1. Weekly Training Load: Changes in weekly mileage, including the acute-to-chronic workload ratio (ACWR) → how one week compares to the average of the past four weeks.
   

2. Single-Session Distance Load: How much longer a runner’s longest run in a given week was compared to their longest run in the previous 30 days.

Single Session vs Weekly Training Load

Conventional wisdom, and many coaching handbooks, warn against increasing your weekly mileage too quickly, often citing the “10% rule.” This study revealed a more nuanced picture.

• Weekly Training Load (ACWR): Surprisingly, changes in weekly mileage and even high ACWR values did not strongly predict injury risk. In other words, fluctuating mileage week-to-week wasn’t as dangerous as once thought.
  

• Single-Session Distance: The standout finding was that a sudden increase in the length of a single run was the biggest predictor of injury. If a runner’s longest run in the past 30 days was, say, 10 miles, and they suddenly went out and did 13, the risk of injury spiked significantly.

The data showed a clear correlation (not causation!):

• A single run that was 10-30% longer than the prior month’s max increased injury risk by about 64%.

• Runs >30% longer than the previous max raised the risk by over 100%, more than doubling it.

Put simply: one overly ambitious long run can derail your season more than modest swings in weekly mileage.

What They Found

The study’s headline conclusion is that single-session spikes matter more than weekly mileage shifts. Many runners think injuries are the product of slow, cumulative overload, but this evidence points to “trigger events.” A body can tolerate steady load and even gradual increases, but it often breaks down when hit with a sudden, unaccustomed challenge.

Injuries in the study were most commonly associated with these outsized long runs, not with slightly higher total weekly mileage. This suggests that training plans should prioritize gradual progression of the long run, even more than obsessing over total mileage counts.

Limitations

Like any study, there are caveats. This was good data to see, but I think there are so many limitations to it.

• Self-reported injuries: injuries relied on runner self-reporting, which can be inconsistent and noisy.

• Population studied: The runners were mostly recreational. Results may not apply the same way to elites, who have higher conditioning and different adaptation profiles.

• Definition of injury: The study used a broad definition (any musculoskeletal pain that altered training). This captures more injuries but also lumps together minor niggles with more serious issues.

• Causation vs correlation: While the sample size here is pretty large (5205 runners), the study can’t perfectly separate whether long runs caused injuries or simply coincided with them. This wasn’t a randomized controlled trial that had a treatment and control group, so we can’t infer causal relationships.

• Lack of focus on single session intensity: More distance doesn’t always mean higher training load – I would have liked to seen some data focusing on the intensity of the sessions as well.

• Confounding variables: age, sex, training history, injury history – I didn’t see cross-tabs of any of these cuts of data.

Time for some lab notes ⬇️

🧪 Lab Notes

In every issue of this newsletter, the “Lab Notes” are going to be the protocols that you can apply to your training and routine right away. The goal of this section is to translate the science into actionable steps for the reader, whether you’re a recreational runner/HYROX competitor, or someone pushing the limits of their peak potential.

• Respect the Long Run: If you don’t have a long run training history, don’t increase your longest run by more than ~10% at a time week over week. Build long runs conservatively over several weeks. If you’re a HYROX athlete, I would argue that having a long run more than every 10-14 days probably isn’t needed.
  

• Weekly Mileage: While this study showed no correlation between weekly totals and injury risk, you should probably still avoid massive leaps in training load week over week.
  

• Week over week isn’t just Monday-Sunday: This is something I focus on myself and with all of my 1:1 athletes. Training should be viewed as more of a continuous time series of stress, recovery, and adaptation. Your body doesn’t just magically reset every morning. It’s nice to see pretty strava graphs that show gradual progression in weekly miles, but remember that you may have hit an even higher mileage peak within a different 7-day window (say Wednesday-Tuesday) that you won’t see on your strava graph.

• Intensity also causes stress: This review focused mainly just on distance of the longest run. From personal experience, a very intense interval day often causes way more fatigue and acute stress than a longer, easier, run. You should probably be careful with how much harder you make your hard workouts each week as well.

That’s it for this edition of the Threshold Lab. Happy training!

References

Schuster Brandt Frandsen J, Hulme A, Parner ET, et al - How much running is too much? Identifying high-risk running sessions in a 5200-person cohort studyBritish Journal of Sports Medicine 2025;59:1203-1210.