In this post we discuss why training spikes, not weekly volume, can cause injuries in runners and gym goers alike.
At a glance
The “don’t increase weekly running volume by more than 10%” rule doesn’t hold up — a 2025 study of 5,205 runners found no meaningful link between weekly increases and injury risk.
What actually predicted injury: a single session significantly longer than anything done in the previous 30 days — not the weekly average.
The same principle applies to resistance training: a sudden jump in weight can feel completely fine at the time and still cause problems.
Reason one — tendons lag behind muscles: you can feel strong enough to handle more weight before your connective tissue has adapted.
Reason two — overloaded joints can inhibit nearby muscles: the nervous system protectively reduces activation around a stressed joint, and neighbouring muscles may increase tension to compensate.
Reason three — overload isn’t just local: research shows reduced neural drive to the overloaded muscle and increased demand on others, often surfacing a day or two later rather than immediately.
Bottom line: feeling fine during a session isn’t a reliable signal. Plan load increases progressively rather than spiking them in the moment, so your tendons, joints, and nervous system have a chance to adapt.
Introduction
If you’ve spent any time around runners, you’ve probably heard the 10% rule. Don’t increase your weekly mileage by more than 10% compared to last week. It’s practically gospel in running circles.
There’s just one problem: the largest study ever conducted on this exact question found it doesn’t hold up.
What the research actually found
A 2025 study published in the British Journal of Sports Medicine followed 5,205 runners for 18 months, tracking over half a million individual running sessions. It’s about as large and rigorous as this kind of research gets.
The researchers tested the classic weekly percentage rule directly. A weekly increase of more than 10% showed no meaningful association with injury risk. Neither did the more sophisticated acute:chronic workload ratio that many coaching apps now use.
What did predict injury? A single run that was significantly longer than anything the runner had done in the previous 30 days — regardless of what their weekly total looked like.
In other words: your weekly average can look perfectly sensible on paper while hiding the exact thing that gets you injured.
Why this matters beyond running
I see the same pattern in the gym, and it applies just as much to resistance training as it does to running.
Someone who trains consistently, sensibly, progressively — and then one day adds a bunch of weight to the bar because it feels good, because they’re motivated, because last week’s number suddenly feels too easy.
And it does feel fine. That’s exactly the problem. There are three separate reasons why “fine” isn’t a reliable signal, and it’s worth understanding all of them.
Reason one: your tendons haven’t caught up yet
Muscles adapt to load relatively quickly. Tendons and connective tissue adapt far more slowly — often over months, not days. That mismatch means you can genuinely feel strong enough to handle a big jump in weight while your tendons haven’t caught up yet.
This is also why the damage often doesn’t show up in the moment. Tendon and connective tissue issues frequently develop over the 24 to 72 hours after a session, not during it. By the time it hurts, the session that caused it is long gone — which makes it very hard to connect cause and effect, and very easy to repeat the mistake.
Reason two: overloaded joints influence muscle function
There’s a second, less well-known mechanism worth understanding, and it comes from a well-established area of research: arthrogenic muscle inhibition, or AMI.
AMI describes a reflex your nervous system uses to protect a joint. When a joint experiences stress, irritation, or altered signalling from its surrounding receptors, the nervous system can reflexively reduce activation of the muscles crossing that joint.
When one muscle becomes inhibited like this, a neighbouring muscle often increases in tension to compensate. Research on the knee, for example, has shown that when the quadriceps become inhibited, tension in the hamstrings frequently increases to keep your knee stable.
Most of the direct research on this phenomenon comes from injury and post-surgical settings — after an ACL reconstruction or a knee replacement, for instance, where the trigger is clear.
But the underlying logic holds regardless of the trigger. A joint under more mechanical stress than it’s currently prepared for doesn’t need a diagnosed injury to send altered signals — and it’s a reasonable inference that a similar protective, compensatory pattern can show up after a heavy session, even one that felt completely fine at the time.
Reason three: overloaded muscle isn’t just a local problem
After unaccustomed or heavy eccentric exercise researchers have repeatedly found a measurable drop in voluntary activation: the ability of the nervous system to drive a muscle contraction.
One study found that this drop in voluntary activation tracked closely with delayed-onset muscle soreness — both became measurable roughly a day after the triggering exercise, in muscle that showed no signs of structural injury beyond the normal stress of a hard session.
In a further study, researchers found not just reduced drive in the overloaded muscle but increased drive in a neighbouring muscle as a direct result. Researchers induced DOMS in the glute max muscle of their subjects, then measured muscle activity during sprinting while that soreness was present. While glute max was sore, activity in the hamstrings — and in a quadriceps muscle — increased significantly, alongside a shift in hip and knee mechanics.
The researchers interpreted this as the hamstrings compensating for reduced output in the glute max and explicitly flagged it as a plausible contributor to increased hamstring strain risk.
Putting the three reasons together
A session that overloads a muscle or joint beyond its current capacity doesn’t necessarily announce itself with pain at the time. It can instead show up a day or two later — as tendon or connective tissue discomfort, as tightness in other muscles, or as reduced performance in the muscle you trained. Three different mechanisms, all pointing the same way: what you feel during a session isn’t necessarily a good guide to what that session actually cost you.
What this means in practice
The equivalent of the running study’s finding, applied to resistance training, would be something like this: instead of tracking your average weekly training load, avoid big increases in load in any one exercise.
A few practical translations:
- A “good day” where everything moves easily is not a green light to jump weight significantly — it’s a sign your programming is on track, not an invitation to break from it.
- Increases in load are far safer when planned in advance as part of a progressive structure, not decided mid-session because you’re feeling strong.
- New tightness or discomfort appearing a day or two after a heavier session is worth paying attention to, even if nothing hurt at the time — it may be a downstream signal rather than an unrelated coincidence.
- If you want to increase the weight you’re working with, do it in a controlled, planned way — rather than an in-the-moment spike — that gives your tendons, your joints, and your nervous system a chance to adapt.
Summary
The old advice — don’t increase your weekly volume by more than 10% — turns out to be measuring the wrong thing, at least for runners. The real risk sits in single sessions that spike well beyond your recent history, not gradual weekly averages.
The same logic applies in the gym, for three compounding reasons. Your tendons adapt slower than your muscles, so feeling capable of lifting more tells you nothing about whether your connective tissue has caught up. Your nervous system may protect an overloaded joint by inhibiting the muscles around it, creating tightness in neighbouring muscles. And separately, overload itself can quietly reduce your nervous system’s drive to muscles and increase the demand from others, potentially increasing your risk of injury.
Feeling fine in the moment isn’t the test. Progressive, planned increases in load protect you. Sudden ones — even when they feel completely fine — are where problems tend to start.