We’ve all heard the “10% rule” — the idea that increasing your weekly mileage by no more than 10% reduces injury risk. But recent research suggests that the size of an individual run may be more important than your total weekly mileage.
A 2025 British Journal of Sports Medicine study followed over 5,000 runners and analysed almost 600,000 running sessions. The researchers found that runners had a higher injury rate when they suddenly increased the distance of an individual run compared with their longest run over the previous 30 days.
For example, if your longest recent run was 10 km, suddenly running 15 km represents a 50% increase in your longest single-session load — even if your overall weekly mileage hasn’t changed significantly.
Interestingly, the study didn’t find the same relationship between injury and week-to-week changes in total mileage.
It's not just about your weekly mileage
Consider two runners who are both running around 35 km per week.
Runner A:
8 km 9 km 8 km 10 km
Runner B:
5 km 6 km 7 km 17 km
They have a similar weekly mileage, but Runner B has introduced a much larger single-session load.
The research suggests that this sudden increase in the distance of one run may be particularly relevant when considering injury risk.
This is especially important when runners introduce a longer run as part of marathon or half-marathon training.
What does this mean for runners?
Rather than focusing solely on your weekly mileage, consider:
How long is my longest run? How does today’s run compare with my recent longest run? Have I suddenly introduced a much longer session? Have I allowed enough time for my body to adapt? This doesn’t mean that 10% is a magic cut-off or that weekly mileage doesn’t matter. Instead, it highlights the importance of gradually progressing individual running sessions, particularly your long run.
So, when increasing your training, don’t just ask “How many kilometres am I running this week?” — also ask:
“How different is this run from what my body has recently been used to?”
Reference: Frandsen L, et al. How much running is too much? Identifying high-risk running sessions in a 5200-person cohort study. British Journal of Sports Medicine. 2025;59:1203–1210
Find Podium Physiotherapy on RLS
Podium Physiotherapy is an RLS Partner. From gym injuries and running-related pain to shoulder, hip, knee and back problems, its highly specialist physiotherapists help active people recover from injury and perform at their best. With over 20 years of combined clinical experience, the team provides expert assessment, hands-on treatment and tailored rehabilitation programmes, plus a dedicated Running Clinic using slow-motion video analysis. Run Love Sprint members can find Podium Physiotherapy in RLS Rewards, along with an exclusive member offer.
