Why extended rest days can increase injury risk for runners (and what to do about it)

The counterintuitive relationship between training breaks and tissue vulnerability.

Noah emailed recently with a screenshot of his training data. He’d done what every sensible runner does: after a long run pushed his weekly mileage into the orange zone, he took time off to recover. His weekly mileage chart showed his acute training load shot up before reducing slowly (Figure 1A). But when he stopped going for long runs the following week his injury risk score had done something unexpected: it had climbed into the orange (Figure 1B).

Noah was surprised his risk went up as his mileage was reducing, so he emailed to ask how myTrainingForecast estimates his risk of running injury.

It’s one of those counterintuitive patterns in training data that seems to defy logic: You do the responsible thing and rest, and your injury risk goes up. What’s going on? The answer reveals something fundamental about how our bodies adapt to training, and why consistency matters more than most runners realise…

The obvious risk, and the hidden one

Every runner knows about doing too much too soon. It’s why the 10% rule is popular, even if it has no scientific basis (Buist et al., 2008). But the general idea of “progressive overload” where we progressively increase training loads (distance, speed, hills) is well accepted and makes physiological sense. But suddenly spike your training load too high, and you’re asking tissues that aren’t prepared to handle the stress. Injury risk goes up. Simple.

But there’s a second path to injury risk that’s far less intuitive: rapid reductions in training load. When your training drops significantly, whether it’s planned recovery, illness, or just life getting in the way, you enter the “detraining” zone. And here’s the thing: research shows that this detraining zone (blue zone in Figure 1, above) predicts future injury just as surely as the red zone does (Blanch & Gabbett, 2016).

Why? The answer lies in understanding what “fitness” actually means.

Two systems / Two timelines

Here’s what happens when you take a few days easy after a hard week of running. That big training session, let’s say a 20-mile long run, drops out of your 7-day window. Your acute training load (last 7-day mileage) falls. But your chronic training load (last 4-week average mileage) stays relatively high because that big week is still in there, just further back in time.

Your ACR drops. You might even dip into the blue zone. On paper, you look undertrained. And in a sense, you are.

But here’s the critical part that Noah’s email helped me realise we haven’t explained clearly enough: our cardiovascular and musculoskeletal systems don’t adapt on the same timeline!

Your aerobic fitness (heart, lungs, blood) responds quickly. A few easy days and you still feel pretty strong. You could probably go out and run at a decent pace without your breathing getting out of control. From a cardiovascular perspective, you’re still fit.

Your connective tissues are a different story.

Tendons, ligaments, the microscopic structures in your muscles, the bone remodelling that happens in response to impact: all of these adapt slowly to training stress. We’re talking weeks and months, not days. Some studies show it requires up to two months before measurable changes in collagen structure occur.

But here’s the kicker: they also detrain faster than you’d think. A study on rat patellar tendons found that after just four weeks of detraining, tendon structure and mechanical properties had already deteriorated significantly (Frizziero et al., 2011). The researchers noted that “sudden discontinuation of physical activity has a negative effect on tendons” and that “tendon injuries are more frequent in athletes that change type, intensity and duration of training“.

Tendon injuries are more frequent in athletes that change type, intensity and duration of training.

Frizziero et al., 2011

This isn’t just about rats. Studies on human Achilles tendons found similar patterns: Research shows that improving tendon stiffness takes at least 8-12 weeks of consistent training, but within a few weeks of reduced loading, these tissues start losing their conditioning (Kubo et a., 2012). Not completely, but enough to matter.

Figure 2 below shows the scale of the problem: Achilles tendon required 2 months of consistent training to significantly get stronger (+50%), but only 1 month of rest to return to pre-adaptation levels!

So when you bounce back to your normal training (because you feel fine, because you’re “only” returning to what you were doing two weeks ago), you’re asking tissues that have started to detrain to handle loads they’re no longer fully prepared for…

This is the mismatch. Your brain says “I’m ready.” Your heart and lungs say “Let’s go.” But your Achilles tendon, your plantar fascia, your tibial stress response? They’re telling a different story.

What the data actually predicts

I should be clear about what we’re measuring here. The injury risk score in myTrainingForecast isn’t telling you that you’re injured right now, or even that you will definitely get injured. It’s estimating the probability of an overuse injury developing over the next week or so, based on patterns in your training load.

When you see that risk climb after an extended rest, it’s the algorithm flagging a common pattern: runners in this situation (undertrained relative to their recent fitness) often try to return too quickly to their previous training levels. And when they do, many get injured.

It’s not the rest itself that’s risky. It’s what typically happens next.

Think about the most common scenario: you get a cold, take five days completely off, feel better, and go straight back to your 40-mile weeks because that’s “normal” for you. Your lungs might handle it, but your Achilles don’t. Or you get busy with work, run sporadically for two weeks, then try to make up for lost time with a big weekend. Or you nail a marathon, take a sensible long break, then resume training as if nothing happened.

In each case, the cardiovascular fitness is largely intact. The tissue conditioning is not.

The practical problem

The tricky part is that rest is essential, it’s when adaption happens, and sometimes reduced training is exactly what we need. Race tapers work (Houmard et al., 1994). Taking time off when you’re ill or injured is the right call. The issue isn’t the rest but rather managing the return to running.

This is where most runners often go wrong. We plan the down time carefully but treat the ramp-up as if it’s automatic. “I’ll just get back to normal.” But what’s normal? The training load your body was handling two weeks ago isn’t necessarily what your tissues can handle today.

This is exactly what the planner in myTrainingForecast is designed for. Every day it recalculates what “green zone” means for you right now, given both your longer-term fitness and your recent (de)training. It’s showing you the progressive path back to consistency.

Back to Noah’s question

So what should Noah do? He took some extended rest after a hard week (the right move). His injury risk climbed, not because he rested, but because the algorithm knows what usually comes next: Runners in his situation typically try to resume their normal training too quickly.

The answer isn’t to avoid rest. It’s to respect the recovery on both ends: ease into the down time, and ease back out of it.

If Noah uses the planner to guide his return (gradually rebuilding his training load based on where his tissues actually are, not where his cardiovascular fitness is), that orange risk score will come back down. Not immediately, but steadily. And crucially, without him having to learn this lesson the hard way, with an actual injury.

That’s the real lesson here. Not that rest is dangerous, but that consistency is protective. And sometimes, protecting that consistency means resisting the very natural urge to make up for lost time.

If you want to dig deeper into the science behind ACR and training load, there’s more detail on our help page. And if you’re curious about how personalised injury risk monitoring works in practice, you can sign up for free and try it yourself.

References