The kind of running injury risk science we’re always waiting for

• Jun 12, 2026 • by Paul Ingraham
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A weekly nugget or two of pain science news and ideas for patients and pros, usually 400–1000 words. The blog is the “director’s commentary” on the core content of PainScience.com: a library of major articles and books about common painful problems and popular treatments. See the blog archives or updates for the whole site.

Bright, blurry, warm-toned photo of legs running on pavement.

The injured always want to know why me? Science rarely has a satisfying answer, but today it can tell you at least this much: Achilles tendinitis probably causes gait glitches, and not the other way around, according to a big new Czech study with a better-than-average design. Jandacka et al gave us some precious “prospective” evidence: that is, they followed 900 thoroughly measured adults (runners and non-runners) for a year to see who got Achilles tendinitis, and who didn’t, and — thanks to the prospective part — maybe a bit of the tricky why of it.

None of the subjects had ever had Achilles tendinitis before. But after a year, there were 23 new cases in their study subjects, 20 in runners, 3 in the non-runners. That’s not many cases — the study’s main weakness. But it’s big compared to other trials like this, it’s an unusually good design, and it mostly just reaffirmed the obvious…

Running volume is the main risk, and exactly how you run is not so important. Many traditional biomechanical scapegoats were exonerated by this study — most notably footfall patterns — because they “do not significantly influence the risk of Achilles tendinopathy, challenging common recommendations to alter running footfall pattern.” Classic physio advice like “avoid forefoot strike,” among other usual suspects.

The risk drivers according to Jandacka et al.

Just how risky is a lot of running for your heel tendons? 67% riskier with every additional 12 kilometres per week. The injured group ran 3.5× more per week than the uninjured group. (Based on fitness-tracker data, not self-report.) To run more is to get hurt more! Duh? Too obvious? You’d think so. But never underestimate the degree to which people like to make up more complicated stories about why we get hurt, so it’s always worth emphasizing the “obvious.”

This next bit is a bit more interesting…

The injured runners were already in trouble when they entered the study! 🤯 They just didn’t know it yet. Although asymptomatic and with no history of Achilles tendinitis, their tendons were visibly flawed on MRI (the VIMATS scoring system). Healthy tendons easily score 100%. Tendons with symptomatic Achilles tendinitis score more like 50 out of 100. The runners who seemed fine but ended up injured? 95. Just a little compromised: a bit of thickening, a bit rough. The earliest clear signs of degeneration.

So, who gets injured? The people who had a quiet head start on being injured! Maybe they had a head start because they were also the runniest runners, but we’ll need more research to determine that. Clearly we need to be studying who develops silent tendon degeneration, before they even know anything’s happening.

“Tendinitis” versus “tendonitis”: Both spellings are acceptable these days, but the first is the more legitimate, while the second is just an old misspelling that has become acceptable only through popular use, which is a thing that happens in English. The word is based on the Latin “tendo” which has a genitive singular form of tendinis, and a combining form that is therefore tendin. (Source: Stedmans Electronic Medical Dictionary.)

“Tendinitis” vs “tendinopathy”: Both are acceptable labels for ticked off tendons. Tendinopathy (and tendinosis) are often used to avoid the implication of inflammation that is baked into the term tendinitis, because the condition involves no signs of gross, acute inflammation. However, recent research has shown that inflammation is actually there, it’s just not obvious. So tendinitis remains a fair label, and much more familiar to patients to boot.

Interestingly, age did not move the needle — in defiance of every aging athlete’s cynicism about getting old! The odds were against them at any age if they ran a lot. Or had that head start.

Jandacka et al did flag two running form issues that were linked to more Achilles tendinitis: weaker ankle inversion during stance, and not turning the foot out as far. Although those signals were fairly strong in this study, I wouldn’t put too much stock in them: they’re new ideas in the field, might well never be reproduced by another study. They also probably aren’t causes that can be tinkered with. We always want our risks to be “modifiable,” avoidable. Of course, reality doesn’t always cooperate. But an unmodifiable risk is news you cannot use, so even if it’s true it won’t change how you run.

But I suspect it’s not true. I bet that this signal would disappear in a do-over study.

The prospective data dream

Jandacka et al. have given us the kind of evidence about risk factors for injury/illness we’re always waiting for, and can never really get enough of: prospective data from following people over time, which can actually shed some light on causal relationships by showing us what came first. This is far superior to the “just another correlation” conclusions that we get from far cheaper, easier observational studies that only give us a snapshot of a group of people at one time.

“Prospective” is potent. Basically all previous data on this topic was retrospective or cross-sectional, or prospective-but-tiny, making this experiment fundamentally better despite the fairly small group of limping runners they had to analyze after a year.

It’s kind of embarrassing that we have so little science like this, but that’s the hand we’ve been dealt so far. The authors argue their exoneration of footfall pattern is more trustworthy because the retrospective studies likely detected injury-driven gait adaptations, the actual opposite of gait glitches that cause Achilles tendinitis.

We still need a ten-year study of ten thousand runners. Don’t hold your breath.

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I’ve updated my guide to Achilles tendinitis with a citation to Jandacka et al — most of this nuance is all just packed into one new footnote and the bibliography.