Glenn Wellmann DPT

Every runner eventually asks some version of the same question: how much more can I run today without paying for it next week? A large 2025 study finally put a number on it, and the number is smaller than most people want to hear.

The study, briefly

Researchers followed 5,205 recreational runners from more than 87 countries for 18 months as part of the Garmin-RUNSAFE Running Health study, capturing 588,071 individual running sessions from GPS watches and asking runners weekly whether they had picked up an injury. Average age was 45.8 years, 22 percent were women, median running experience was 9.5 years, and anyone already dealing with a musculoskeletal problem was excluded at the start (Frandsen et al., British Journal of Sports Medicine, 2025;59(17):1203 to 1210).

What makes it useful is the exposure they chose to measure. Instead of tracking weekly mileage, they compared each single run against that runner’s longest run in the previous 30 days, then sorted every session into four buckets: a regression or an increase up to 10 percent, a small spike of more than 10 up to 30 percent, a moderate spike of more than 30 up to 100 percent, and a large spike of more than 100 percent. Runners moved between those buckets run to run, and the analysis tracked who got hurt.

What they found

Compared with staying inside 10 percent of your longest recent run, the rate of overuse injury climbed as soon as runners went past it.

• A small spike of more than 10 up to 30 percent carried a 64 percent higher rate of overuse injury (hazard rate ratio 1.64, 95 percent confidence interval 1.31 to 2.05). That is the finding that surprises people: a jump most runners would call modest already moved the needle.
• A moderate spike of more than 30 up to 100 percent came in at 52 percent higher (1.52, 1.16 to 2.00), essentially in the same range as the small spike rather than worse.
• Doubling your longest recent run raised the rate by 128 percent (2.28, 1.50 to 3.48). One run more than twice your recent longest, and the injury rate more than doubled.
• Overall, 1,820 of the 5,205 runners (35 percent) reported a running related injury, and 72 percent of those were overuse rather than traumatic. By 200 running sessions the cumulative incidence of an overuse injury was 30.5 percent.
• The popular weekly metrics missed all of this. Acute to chronic workload ratio spikes above 10 percent were associated with a lower injury rate, not a higher one, and the week to week ratio showed no significant relationship with injury at all.

That last point is worth sitting with. The load metric most running apps and coaches quote, comparing this week’s volume to the last few weeks, did not flag the risky sessions. The single run comparison did.

The catch

This is an observational cohort study, not a trial, so it cannot prove that capping your progression at 10 percent prevents injuries. The authors say so directly: a randomized trial is still needed. A few other honest limits:

• Injuries, and whether they were overuse or traumatic, were self reported through weekly questionnaires, so some misclassification is likely.
• Load was measured as GPS distance only. No intensity, no pace, no elevation, no surface. A hilly 6 miles and a flat 6 miles counted the same.
• Exposure was analyzed in broad bins, which is why the moderate spike and the small spike look so similar. Real risk almost certainly does not step up in tidy blocks.
• Staying under 10 percent is not a guarantee. A 1 to 10 percent increase still carried a 19 percent higher rate, though that result was not statistically significant (95 percent confidence interval negative 10 percent to 57 percent, p equals 0.22).
• The cohort skewed male, averaged 46 years old, and was experienced. Newer runners may respond differently.

What this means for you

The practical shift here is what you compare today’s run against. Most runners audit their week. This study says the risky decision is a single session, judged against the longest run you have actually done in the last month.

Here is how I turn that into a rule in the clinic:

1. Know your number. Open your watch app and find your longest single run in the past 30 days. That distance, plus about 10 percent, is your ceiling for any one run this week. If your longest recent run is 6 miles, today’s ceiling is roughly 6.6.
2. Grow the long run, then hold it. Add roughly 10 percent to the long run, then repeat that distance for two or three weeks before pushing again. Stacking several 10 percent jumps inside a single week is still a large change in load even though each step looks small.
3. Plan your big days instead of stumbling into them. A race, a group run that turned into 12 miles, or a spontaneous trail day is where the doubling happens. If a big jump is on the calendar, put an easy or off day on either side of it.
4. Do not let a metric override how you feel. Distance was the only load this study measured. Pace, hills, and hard surfaces all add stress that never showed up in the data, so a hard 5 mile hill workout is not the same as an easy 5 miles even if your watch says it is.
5. Rebuild your ceiling after time off. Two weeks away from running resets what your body considers normal. When you come back, your 30 day longest run is smaller, so your ceiling is lower.
6. Add strength work. This study did not test strength training, but a meta-analysis of 25 trials in 26,610 participants found strength training reduced sports injuries substantially (relative risk 0.315, 95 percent confidence interval 0.207 to 0.480) while stretching showed no protective effect (Lauersen et al., British Journal of Sports Medicine, 2014;48(11):871 to 877). Two sessions a week is a reasonable target.

And the honest reframe: 35 percent of these runners got hurt over 18 months. Load management lowers the odds, it does not zero them out. If something starts talking to you, the useful response is to hold your long run flat for a couple of weeks rather than to quit running entirely, because the tendon or bone in question still needs load to adapt.

Where to go from here

If you are already dealing with something that flares every time you build mileage, guessing at your own progression gets expensive in lost weeks. A home exercise program gives you a written build plan plus the strength work underneath it, matched to your mileage and your history. If you are coming back from an injury and want objective numbers before you trust your leg with a real training block, that is what return to sport testing is for. And if knee pain is the thing derailing your builds, start with my six week plan for kneecap pain in runners, which pairs a loading progression with the same mileage rules described here.

References

• Frandsen JSB, Hulme A, Parner ET, Moller M, Lindman I, Abrahamson J, Simonsen NS, Jacobsen JS, Ramskov D, Skejo S, Malisoux L, Bertelsen ML, Nielsen RO. How much running is too much? Identifying high-risk running sessions in a 5200-person cohort study. British Journal of Sports Medicine. 2025;59(17):1203-1210. doi:10.1136/bjsports-2024-109380. https://pubmed.ncbi.nlm.nih.gov/40623829/ (full text: https://bjsm.bmj.com/content/59/17/1203)
• Lauersen JB, Bertelsen DM, Andersen LB. The effectiveness of exercise interventions to prevent sports injuries: a systematic review and meta-analysis of randomised controlled trials. British Journal of Sports Medicine. 2014;48(11):871-877. doi:10.1136/bjsports-2013-092538. https://pubmed.ncbi.nlm.nih.gov/24100287/