Most running injuries are overuse injuries. They don’t come from a single misstep. They come from asking a tissue to absorb more than it has been prepared for. The open question has always been which measure of “more” actually predicts the injury.

The largest study of runners yet published gives the clearest answer available. It followed 5,205 runners across 588,071 recorded sessions and asked which sessions came before an injury.[1] The signal wasn’t the weekly total. It was the individual run that went well past what that runner had recently done.

This fits what we know about how bodies adapt. Your cardiovascular system adapts in days. Your muscles adapt in weeks. Your tendons, ligaments, and bones adapt in months. A run far longer than your recent longest asks the slowest-adapting structures to do something they haven’t been prepared for, on that day, in that session.

The Jump That Predicts Injury

The reference point in that study was the runner’s longest run in the previous 30 days. Sessions that exceeded it by more than about 10% were followed by more overuse injuries, and the biggest jumps carried the highest rate of all. Compared with sessions that stayed inside that limit, sessions 10-30% over were injured at roughly 1.6 times the rate, sessions 30-100% over at roughly 1.5 times, and sessions that more than doubled the recent longest at roughly 2.3 times.[1] The authors’ own advice to runners was to avoid exactly that jump.

Notice what that number is. It’s not a weekly total, and it’s not a ratio of one training period against another. It’s one measurement of one run, against that runner’s own recent history.

How Spikes Happen

Single-run spikes. Your longest run in the past month was 6 miles. This week’s long run is 12. Your weekly total may look reasonable and the plan may look sensible on paper, but that one session is exactly the jump the research points at.

Comeback spikes. You took two weeks off for life, illness, or travel, then came back and picked up where you left off. The problem isn’t the calendar. It’s that your longest recent run has shrunk, so the run you used to do comfortably is now a large jump.

Motivation spikes. You feel good, you sign up for a race six weeks out, and you double your training overnight. Your body wasn’t consulted.

Catch-up spikes. You missed three runs this week and try to make them up on the weekend. Saturday and Sunday absorb the whole backlog.

What About the Acute:Chronic Ratio?

If you’ve read about training load before, you’ve probably met the acute:chronic workload ratio: your recent training divided by your longer-term baseline, with a 0.8 to 1.3 band widely described as the safe zone. This article used to teach that band as settled science. It isn’t, and we’d rather say so plainly than quietly delete it.

Three things changed our mind.

The band was never validated. The pooled evidence for the 0.8 to 1.3 sweet spot amounts to a weak trend with a confidence interval so wide it tells you close to nothing. This isn’t us reading the reviews against the grain. Both of the reviews that pooled the band describe a trend in its favour, and in both cases their own numbers refuse to back them up. The 2025 review’s figure for the band carries a range wide enough to cover almost any answer, and its authors close by saying the ratio needs to be used with caution.[6] The earlier review reports two results for that same band depending on how training load was measured, and they disagree with each other. The one measured by distance, which is what a running app actually tracks, was associated with more injuries inside the so-called safe zone, not fewer.[5]

The ratio adds no information. The acute load is part of its own denominator, which manufactures correlation out of arithmetic. Swap in contrived baselines and you reproduce nearly the same odds ratios. The ratio beats neither acute load on its own nor a null model. The methodologists who took it apart concluded, in their words, “We suggest ACWR be dismissed as a framework and model.”[3][4]

In runners it points the wrong way. A study of 435 Dutch recreational runners found the highest predicted injury probability at the LOW end, below 0.70, not the high end.[2] The 5,205-runner study reported a negative dose-response for the ratio, running in that same backwards direction, and no relationship at all for the week-to-week version.[1] The ratio came out of team sports, and most of its evidence is still team-sport data. It doesn’t transfer to running.

So the ratio is gone as a guardrail. The question it was standing in for is a real one, and it now has a better answer.

How Pacewright Guards Against Spikes

Three systems do the actual protecting, in this order.

The single-session spike cap comes first. No run Pacewright prescribes will jump far beyond your longest run of the past month. This is the guard that maps onto the strongest runner-specific evidence available, so it’s the one that binds hardest. If today’s planned long run would clear the limit, the engine shortens it and shows you why.

Volume caps sit on top of it. Your weekly mileage can only grow so fast, and the cap tightens as your mileage climbs. A single run can be within its own limit while the week as a whole is still rising too quickly, so both get checked.

Your feedback overrides both. After each run you tell Pacewright how it went: your effort, how it felt, whether anything is sore. That catches what the other two can’t see, and it can pull the plan back regardless of what the numbers say.

Returning from time off runs the same machinery. The engine doesn’t resume where you stopped, because your longest recent run has shrunk in the meantime. It restarts below your old level and rebuilds from there.

DIAL: Are You Training the Right Amount?

Those guards answer “is this run safe?” They don’t answer the question runners actually ask, which is “am I doing enough?” That one gets its own number: DIAL, short for Dose In Adaptive Limits.

DIAL places your current training on a single axis between two points. Your floor is the least training that still moves you forward. Your ceiling is the most your body can safely absorb right now. Where you sit between them reads three ways:

  • Dial it up. You have room to grow. You’re under your floor, and the plan can build.
  • Dialed in. You’re in the productive band. Enough to keep adapting, sustainable, safe.
  • Dial it back. Ease off. You’re at or near your limits.

DIAL and RFI are a pair. RFI tells you how fit you are. DIAL tells you whether you’re loaded right.

One thing DIAL is not: the guardrail. It describes your dose and nudges the plan. The spike cap, the volume caps, and your feedback are what hold the line. A gauge telling you to ease off is information. A cap that won’t prescribe the run is protection.

What We Don’t Know

Replacing a false certainty isn’t the same as selling a new one, so here are the limits on everything above.

The single-session finding is observational. It’s the largest and newest runner cohort available, built on recorded sessions rather than training diaries, which is why we weight it heavily. It’s still one cohort, and it shows association rather than proven cause.

The inverse ratio findings may partly run backwards. A runner with a low ratio may have a low ratio because they’re already hurt or already backing off, which would make low ratios a symptom rather than a cause. That doesn’t rescue the 0.8 to 1.3 band. It’s a reason not to flip the claim over and start telling people that training less is dangerous.

Base as protection is under-tested. It’s biologically plausible that a runner with months of consistent training absorbs a hard session better than one without it. In runners it hasn’t been tested well enough for us to promise it. Pacewright builds your base gradually because that’s a sound principle, not because we can quote you a number for what it buys.

Total volume still matters on its own. High mileage is among the strongest injury factors in its own right. A well-built base doesn’t buy immunity from the amount you’re running.

The Short Version

Look at the longest run you’ve done in the last month. The run that gets you hurt is usually the one that goes well past it. Pacewright won’t prescribe that run, and if you go out and do it anyway, it will see it in your logged data and adjust what comes next.