When Movement Feels Like a Threat: Could PEM Be a “Fear Response”?

Everyone says: “Exercise is good for you!” “Just do more, you’ll feel better!”

But if you live with Post‑Exertional Malaise (PEM), you know that even a short walk can trigger days of symptoms  — exhaustion, brain fog, visual disturbances, pain — just to name a few.

You’re left thinking: Why is exercise making me worse?

What if your body isn’t reacting to effort—it's reacting to threat?

Photo by Emmeline T. on Unsplash

What if your body isn’t reacting to effort—it's reacting to threat?

I suffered from severe, relentless PEM for many years. It was brutal. And even now, I still get symptoms when I’ve pushed beyond my limits.

I went from being physically active, working a hands-on job, loving sport and the outdoors — to being completely bedbound.

Post-Exertional Malaise (PEM) meant that any kind of exertion — physical or mental — made my symptoms worse. It had me beat. I mean, what do you do when even taking a shower triggers a crash?

At first, I was hopeful that rest and time would fix it. But as the weeks, months, and years went by, it became clear this wasn’t going to resolve on its own. In fact, it got worse. I lost everything — my job, my independence, my identity.

Lying in a darkened room, cut off from the world, avoiding every stimulus — light, sound, movement. My system was so reactive, constantly buzzing inside like I was wired to an electric socket.

I tried everything I could: every supplement, every hack. Desperate to feel even slightly better… but nothing helped. In fact, most of it made me worse.

And to top it off, I couldn’t sleep. I’d lie there for hours, and when I finally drifted off, I’d be jolted awake — gripped by a hellish nightmare, heart racing, my body reacting as if I were in danger.

There was no escaping it. Day or night, the punishment was relentless.

Eventually, I stopped chasing quick fixes — and started studying. The body, the brain, the nervous system. Bit by bit, I began to join the dots and make sense of what might actually be happening.

What helped me wasn't time or rest or another round of pills. It was understanding.

I learned how my nervous system might have become so reactive that any kind of stress — physical, emotional, or mental — could trigger PEM.

That understanding gave me something I hadn’t had in years: a foothold, and a way forward.

So what’s really going on under the hood?

The science is finally starting to explain it.

Buckle up. Let’s take a look.

Science Says: Exercise and Stress Activate the Same Systems

Hang on a minute — let’s read that again.

Exercise and Stress Activate the Same Systems.

Understanding this was a huge turning point for me. It gave me a language for what might be happening in my body — and why exertion of any kind, even mental effort, could cause such a crash.

• A review of voluntary exercise — including wheel running — shows it activates systems involved in the stress response, particularly the hypothalamic-pituitary-adrenal (HPA) axis, though adaptations over time can modify this effect. PMC

• One study found that voluntary wheel running can initially cause hyperactivation of the HPA axis, likely due to increased adrenal sensitivity to ACTH; these changes were attenuated with longer training. PubMed

  
  

• Another experiment demonstrated that six weeks of voluntary wheel running accelerated habituation of corticosterone responses to repeated loud-noise stress, indicating an adaptive, stress-buffering effect. PMC

  
  

• Exercise also triggers acute activation of the sympatho-adrenal (sympathoadrenal) system, producing rises in catecholamines and cortisol — the same hormonal surge seen during psychological stress. PMC

  
  

• Meanwhile, stress-neurobiology research shows HPA activity is driven by neural circuits that release CRH, ACTH, and cortisol in threat contexts, underscoring the shared physiological circuitry between stress and exertion. The Journal of Physiology

In short: exercise and psychological stress run through the same physiological wiring.

Here’s where PEM changes the story: In a healthy nervous system, the stress response from exercise usually triggers adaptations that build resilience over time.

But with PEM, the nervous system has become so hypersensitive that even normal stress responses to physical or mental exertion are amplified and prolonged. Instead of adapting, the body treats the exertion like an ongoing threat — much like chronic psychological stress. The same systems get activated, but they stay locked in overdrive, leading to severe crashes and symptom flares.

Here’s something else I found interesting from a different condition that applies —

Challenges for Exercising with PTSD:

During exercise, the heart beats faster, blood pressure increases, there may be shortness of breath, etc. These are all symptoms associated with anxiety. A client who has PTSD may be deterred from exercising because these reactions are similar to those that occur during an episode. source

✅ Clear takeaway:

• Exercise → ↑ heart rate, ↑ blood pressure, ↑ ACTH, ↑ cortisol (HPA axis)

• Psychological stress → the exact same → ↑ SAM + ↑ HPA response

So, what does this mean for PEM?

For me (and many others experiencing PEM), our nervous systems may react to physical or mental exertion as if it’s a threat — similar to how someone with PTSD experiences anxiety symptoms during exercise. This activates the body’s stress systems (like the HPA axis and sympathetic nervous system) and pushes them into overdrive, leading to the severe crashes and symptom flares typical of PEM.

In other words, PEM isn’t just about “using up energy.” It’s about the nervous system being stuck in a heightened state of alert — misreading normal activity as danger and responding with an intense stress reaction that takes a long time to calm down.

What about the delayed reaction? People often ask how this theory explains the delay before symptoms hit. In my opinion, it makes perfect sense. If the nervous system gets stuck in sympathetic fight-or-flight mode, the crash can come later because the body can’t transition back into rest-and-digest.

Say you’ve overexerted yourself. In the moment, you might not feel too bad. But that evening, you’re restless and can’t sleep because your system is still locked in that stress state. Even if you do manage to sleep, your body may remain in fight-or-flight all night. You wake up exhausted, with a whole range of symptoms, because you haven’t truly rested — your body has been in survival mode the entire time. For many of us, that survival state never really switches off — and the symptoms just keep rolling on, year after year. This also aligns with the brain prediction error theory — where the brain keeps predicting danger or overreacting to a stimulus long after the original trigger has gone. In PEM, it’s as if the brain and nervous system are still bracing for impact, even when the activity is over.

I explore this in more detail in my post on Brain Prediction Errors.

Understanding PEM this way helps explain why, for me, a graded exercise program didn’t work and actually made my symptoms worse. The exercise wasn’t helping my body heal — pushing through activity unintentionally reinforced the body’s stress response, deepening the nervous system dysfunction and causing symptoms to worsen.

Another thing that got me thinking...

Post-Exertional Malaise (PEM) still lacks a definitive biomarker or any routine test or scan that can objectively confirm it. Nothing. Absolutely nothing.

That blew my mind.

Here we have a symptom that is completely disabling, punishing, and life-altering — and yet there’s no biological evidence you can point to. No abnormal blood results. No scan showing structural damage. No inflammation markers. Every medical avenue I researched led to a dead end. And all the online noise about mitochondrial dysfunction, blood pooling, or weird metabolic markers? Interesting theories — but none of it really adds up in any concrete, clinically useful way. Not yet.

❌ What We Still Don’t Have:

• No FDA-approved blood test or imaging scan that can definitively confirm PEM.

• No consistent abnormal lab values across all patients with PEM.

• No structural damage that shows up on MRI, CT, or other scans.

And ironically, that’s exactly what made the nervous system threat response theory make even more sense to me. The fact that researchers — even after all this time — still haven’t found anything “broken” or consistently wrong in the body suggests that the system itself is dysregulated, not damaged. It's not about destruction — it’s about dysfunction.

The body is reacting as if it's under threat. And if the nervous system is stuck in that loop, constantly misinterpreting safe stimuli as danger, then of course PEM happens. The crash isn’t from what we did — it’s from how our system perceived what we did.

What Happens Inside

You walk to the kitchen—nothing intense. But:

• Heart rate rises

• Breathing deepens

• You feel a little “on edge”

To your brain, that looks a lot like fear or stress. It doesn’t know you're only cooking dinner—it senses a “challenge,” then fires off:

• Cortisol surge

• Sympathetic arousal

• Energy diverted

• Shutdown mode

Then comes PEM: hours or days of delayed symptoms and slowed recovery. Bottom line?

Your nervous system can’t always tell the difference between going for a jog, cooking a meal, mental effort, or stress and worry. It’s responding to everything as if it’s dangerous.

That’s why PEM feels so brutal, unpredictable, and unfair. And why understanding it — not fearing it — is the first step to recovery.

Making sense of it...

Now for me, this made sense. I began to understand how all of this might work — and slowly started making the changes I needed to improve how my nervous system was reacting at that automatic level.

No one was going to help me get better. There was no medication, no cure. I’d seen enough doctors and specialists to realise that.

That was a big turning point — the moment I understood that if I wanted to heal or improve, I had to make it happen myself. This wasn’t about blaming myself. It was about recognising that I had a serious problem with my nervous system — as serious, in my opinion, as a heart condition or any other major illness.

The nervous system is a physical system that controls everything from movement and vision to heart rate, digestion, and hormone balance. Mine wasn’t working as it should, and if I wanted any quality of life, I had to make changes to help it function as well as possible.

I know many of you don’t subscribe to this idea — and that’s fair, because this is just a theory I’m discussing here. But the more I hear from people, especially recently with Long COVID, the more I see a pattern: many people, including myself, are treating their symptoms as nervous system dysfunction, and they’re getting better by committing to nervous system work.

They can’t all be wrong.

At the same time, there are plenty of people who say they have tried this approach — and it didn’t work. That also has to be respected.

What’s stopping the nervous system from recovering? There are many possible reasons, and the truth is, we don’t fully understand them yet.

Some believe there’s a different process driving the symptoms — something not directly related to nervous system dysfunction — and we absolutely need more research and support for everyone, no matter their path.

In the meantime, people are improving and recovering using these nervous system-based techniques.

And that matters too.

References

1. Central Mechanisms of HPA Axis Regulation by Voluntary Wheel Running This review discusses how voluntary wheel running activates systems involved in the stress response, particularly the hypothalamic-pituitary-adrenal (HPA) axis, with adaptations over time. Source: PMC3010733 

2. Voluntary Wheel Running Initially Increases Adrenal Sensitivity to ACTH This study found that voluntary wheel running can initially cause hyperactivation of the HPA axis due to increased adrenal sensitivity to ACTH; these changes were attenuated with longer training. Source: PubMed 19008482 

3. Chronic Voluntary Wheel Running Facilitates Corticosterone Response Habituation An experiment demonstrated that six weeks of voluntary wheel running accelerated habituation of corticosterone responses to repeated loud-noise stress, indicating an adaptive, stress-buffering effect. Source: PMC2600621 

4. Sympatho-Adrenergic Activation by Endurance Exercise This review discusses how exercise triggers acute activation of the sympatho-adrenal system, producing rises in catecholamines and cortisol—the same hormonal surge seen during psychological stress. Source: PMC5884654 

5. Endocrine Responses of the Stress System to Different Types of Exercise This review examines the adaptive endocrine responses of the HPA axis, catecholamines, cytokines, and growth hormone to different types of exercise, highlighting the shared physiological pathways between stress and exertion. Source: PMC10023776