Completing a solid and prolonged period of running training, whilst at the same time staying clear of injury, is the key to great health, fitness and performance. There are many different components that go into both a great running program and injury prevention, however a recent large study (Frandsen et al 2025) of 5,205 runners, followed over 18 months, has given us a powerful new insight into how running injuries happen — and the one factor you can monitor to help reduce injury risk!
Rather than injuries developing gradually over weeks as many training programs assume, the research found that sudden increases in the distance of a single run are the strongest predictor of overuse injury in recreational runners.
The Key Finding: Watch That One Run
In the Frandsen study the distance of each athlete’s run was compared to their longest run in the previous 30 days. When a runner’s single session was more than 10% longer than their recent longest run, their risk of sustaining an overuse injury went up significantly.
Here’s how the risk changed:
- 🟡 Small spike (10–30% longer): ~64% higher injury risk
- 🟠 Moderate spike (30–100% longer): ~52% higher injury risk
- 🔴 Large spike (>100% longer): ~128% higher injury risk
(Compared to runners who kept a run within 10% of their recent longest distance.)
What this study challenges is the common approach of monitoring weekly mileage”, because the results show that most injuries are linked to one big spike in a single session rather than gradual week-to-week changes.
Common Ways Runners Monitor Training Load (and The Limitations)
Many runners already monitor their training load, but not all methods detect the biggest injury risk identified in recent research — a sudden increase in the distance of a single run. The table below compares common approaches and highlights why single-session spikes matter.
METHOD | WHAT RUNNERS TRACK | WHY IT’S POPULAR | KEY LIMITATION FOR INJURY PREVENTION |
**Weekly mileage (km per week) | Total distance run across the week | Simple, familiar, often follows the “10% weekly rule” | Weekly totals can hide a large spike in one run, which is now known to be a major injury trigger |
**Acute : Chronic Workload Ratio (ACWR) | Last 7 days compared to the previous 3–6 weeks | Used in elite sport; built into many apps | Averages load over time, so a single overly long run may not be flagged |
**Session RPE (rate of perceived exertion): (how hard it felt) | Perceived effort × duration | Easy, captures fatigue and stress | Long runs can feel easy but still overload tissues if they’re much longer than recent runs |
**GPS / watch load scores | Distance, pace, elevation, algorithms | Automated, looks sophisticated | Composite scores can give a false sense of safety and miss distance spikes |
**Longest run in past 30 days (+10% rule) | Compares each run to the longest run completed recently | Simple, specific, tissue-focused | Requires runners to consciously track their longest recent run |
Why the “Longest Run + 10% Rule” Is Different
Physio’s often talk about “tissue capacity” and when we do, we are referring to the capacity, or ability, of the tissue (be it muscle, tendon, ligament, bone, cartilage etc) to tolerate and absorb the loads you are putting through it. If load outweighs capacity, injury risk is high.
Unlike many traditional methods, the approach highlighted by Frandsen et al. focuses on tissue capacity during a single session level.
✔ It asks: “Is this run longer than anything my body has handled recently?”
✔ It uses the previous 30 days, not abstract averages
✔ It targets the most common trigger for overuse injury — a single distance spike!
What you want to avoid is the temptation to go much further than you have in the past month!
A Sensible Progression Strategy
So how should you progress if you’re training for a goal like a half-marathon or marathon?
✔ Track your longest run over 30 days.
✔ Limit single-session increases to ~10% or less.
✔ Build up gradually over weeks — not in one big push.
✔ Listen to your body and allow recovery between hard or long sessions.
This approach lets your body adapt to the extra distance in a manageable way, helping to reduce strain on your tissues. You’ll be amazed if you give yourself enough training time how those 10% increments will add up!
Using % as a Reality Check
This is a fantastic and simple rule to follow, and all of these new studies and findings certainly keep the SSPC physio’s on their toes in our quest to stay at the forefront of health and medical research. But it’s never quite as simple as it seems!
If you are just starting out as a runner, and you are managing 2km, it may not be practical to increase by 200m at a time. But I still strongly encourage you to “think in percentages” at the very least. If you have been running 2km and are thinking of going to 3km, that’s a 50% increase so puts your injury risk in a high category. Maybe bring it back to 2.5km for a few runs.
Likewise, if you are thinking in minutes, and your 2km run takes 12 minutes, it’s very easy to decide to increase to 20 minutes – but that’s not just an 8 minute increase, it’s a 66% increase on your previous longest run. So, thinking in % terms can give a bit of a reality check and be a good guide when deciding what your next increase will be.
In Summary
This was a great study in that it assessed over 5000 runners at recreational level and really emphasised that it is important to give yourself a realistic time frame to be able to slowly build up towards your longer runs – the longer the time frame, the less likelihood that you have to “spike your loads”! Running injuries are complex, and definitely not caused by a single factor only, but this study gives us an actionable rule you can use right away:
👉 A single run that exceeds your recent longest run by more than ~10% significantly increases injury risk.
Keep this in mind next time you plan your runs — especially if you’re tempted by a big “hero session.” Progress slowly and consistently, and you’ll be much less likely to end up in our clinic with an overuse injury!
Anthony Lance
SSPC Physiotherapist
References available on request
