Understanding Why Different Crashes Happen — And Why Pacing Must Be Individualised
If you live with an energy limiting condition such as Chronic Fatigue Syndrome, Long Covid, Post-Viral Fatigue or an autoimmune condition you have probably experienced post-exertional malaise.
Post-exertional malaise, often referred to as PEM, is a worsening of symptoms after increased physical, mental or emotional exertion. This can be experienced anywhere from immediately after exertion to several days later.
The onset of PEM may vary from person to person. It can happen immediately after an exercise or challenge, it can gradually come on a few hours later or the next day. It can last a few hours or a few days depending on the person.
Whatever your experience, you have probably experienced some version of this:
- You move your body
- You try to build capacity
- You feel okay in the moment (or maybe you don’t)
- And then… something crashes
When you’re in it, a crash can feel just like any other crash and you also just assume that whatever you are experiencing is what others experience too. But…
Not all post-exertional malaise (PEM) is the same.
And if we treat all crashes as identical, we can end up applying a good strategy to the wrong situation. This can be ineffective at best and stall progress over time.
In clinical practice, there are often three distinct physiological patterns behind what people label as “PEM.”
Understanding the difference can reduce fear and offer important insight into what type of support the body needs.
Because the goal is not avoiding movement.
It’s matching intensity to physiological capacity.
PEM Pattern One: The Autonomic Crash
(The “I hit a wall immediately” pattern)
For some people, symptoms occur during the activity itself or immediately afterwards.
You may notice sudden dizziness, shakiness, weakness or a strong need to lie down. Some describe it as “hitting a wall.” Others describe it as their body abruptly shutting off.
This pattern is often related to autonomic nervous system regulation.
The autonomic nervous system plays a key role in controlling blood pressure, heart rate and blood flow distribution. In some energy-limiting conditions, this regulation can become less stable. Blood may not return efficiently to the brain when upright, heart rate may rise disproportionately to effort, or blood pressure may fluctuate more than expected.
The result is not necessarily a lack of energy production, but a temporary instability in circulation and stress signalling.
This is why these crashes can feel sudden and dramatic.
It is also why they can often improve relatively quickly once horizontal, when the cardiovascular system is supported.
This pattern is sometimes seen alongside POTS-type physiology, though not everyone will meet formal diagnostic criteria.
Importantly, this kind of crash is regulatory in nature. It does not necessarily mean structural damage or long-term harm has occurred. It reflects a system that is currently working hard to maintain stability.
PEM Pattern Two: The “Wired but Exhausted” Crash
Pattern Two looks quite different.
You complete a workout — often strength-based, high intensity or interval-based, but not always as this can be relative — and initially feel energised. Later, however, you may feel restless, overstimulated or unusually hungry. Sleep may be disrupted. The next day you might feel mildly unwell or depleted.
This experience is often linked to crossing your anaerobic (glycolytic) threshold.
When exercise intensity rises above a certain point, the body relies more heavily on glycolysis (sugar burning) for energy production. Lactate levels increase, stress hormones such as adrenaline and cortisol rise, and sympathetic nervous system activation increases.
In a well-regulated system, this stress response resolves efficiently.
However, in a more unstable system the stress response may not downregulate as smoothly. The body remains in a slightly heightened state for longer than intended.
The result can be that “wired but tired” feeling.
This pattern does not mean that you are unfit or not strong enough to exercise. It suggests that current metabolic capacity and recovery signalling may not yet match the intensity of the activity performed. In other words, even though you feel okay in the moment, it is too much right now.
Capacity can change. But it needs to be built slowly in alignment with physiology, not in opposition to it.
PEM Pattern Three: The Delayed 24–48 Hour Crash
The third pattern is the one most traditionally associated with Chronic Fatigue and post-viral syndromes and forms part of the diagnostic criteria for Chronic Fatigue Syndrome or ME/CFS.
Symptoms may not appear immediately. In fact, you may feel relatively stable on the day of activity. Then, 24 to 48 hours later, you experience a noticeable worsening.
This may include heavy limbs, cognitive fog, sore throat, flu-like symptoms or a broader sense of systemic shutdown.
This delayed pattern is thought to involve immune and inflammatory signalling.
Physical, cognitive or emotional stress can influence cytokine activity and microglial activation in susceptible individuals. These pathways take time to build, which explains the delay between exertion and symptom escalation.
For individuals in this category, repeated threshold breaches can prolong instability. This is why pacing approaches often require a different level of care and precision compared to general deconditioning.
Again, this is not about fragility. It is about recognising the signalling pathways currently most sensitive within the system.
Why This Distinction Matters
When someone says, “I get PEM,” it is important to explore what that actually looks like.
Is it immediate and circulatory?
Is it metabolic and stress-driven?
Or is it delayed and inflammatory?
The timing and symptom pattern offer important clues.
If we apply an approach designed for immune-mediated delayed PEM to someone whose crashes are primarily autonomic, it may not cause harm, but it may not encourage progress.
If we apply high-intensity metabolic training to someone with inflammatory sensitivity, symptoms may worsen.
Individualisation is not a luxury in complex fatigue conditions. It is essential.
Supporting Each Pattern: Matching Strategy to Physiology
Once we have a clearer sense of which pattern is most prominent, the approach to pacing and recovery can become much more targeted.
Below is a general overview of supportive strategies often considered for each type. These are educational in nature and should always be individualised in discussion with your healthcare provider.
| Pattern | What’s Driving It (Often) | Support Focus | Practical Examples |
| Immediate Autonomic Crash | Circulatory instability, sympathetic spikes, difficulty regulating upright load | Nervous system stabilisation and blood flow support | Horizontal rest breaks, electrolytes, compression (if appropriate), heart rate monitoring, gradual upright tolerance work |
| Metabolic / Glycolytic Threshold Crash | Exceeding anaerobic threshold, lactate accumulation, prolonged stress hormone activation | Reducing intensity density and improving recovery signalling | Longer rest intervals, avoiding failure training, fuelling before/after activity, building aerobic base before adding intensity |
| Delayed Immune / Inflammatory Crash | Cytokine amplification, immune sensitivity, post-viral signalling | Energy envelope pacing and threshold protection, work Functionally to reduce inflammatory load (food, toxins, infections) | Tracking delayed symptoms, spacing exertion days, avoiding repeated “boom-bust” cycles, careful titration of cognitive and physical load |
A Closer Look at Each Approach
1. Supporting the Immediate Autonomic Pattern
When crashes are happening during or immediately after activity, the body often benefits from improved autonomic stability before intensity is increased.
This may include:
- Prioritising recumbent or semi-recumbent movement initially
- Ensuring adequate hydration and electrolyte intake
- Using heart rate monitoring to identify individual thresholds
- Building upright tolerance gradually rather than through willpower
The aim is to increase regulatory capacity, not to force adaptation through repeated collapse.
2. Supporting the Metabolic Threshold Pattern
When the issue is crossing the glycolytic threshold too frequently, the work often involves recalibrating intensity.
This may include:
- Reducing workout density (fewer exercises back-to-back)
- Extending rest intervals between sets
- Avoiding pushing to muscular failure
- Supporting fuelling around activity
- Building a stronger aerobic base before reintroducing high-intensity intervals
For many people, it is not exercise itself that causes issues — it is the mismatch between current metabolic capacity and training load.
When the stress response can downregulate more efficiently, tolerance often improves.
3. Supporting the Delayed Immune Pattern
When crashes are delayed and flu-like, pacing must be more conservative initially.
Support may include:
- Tracking symptoms 24–72 hours after activity
- Identifying patterns of delayed worsening
- Spacing demanding days
- Avoiding repeated threshold breaches
- Titrating both cognitive and physical exertion
- Address potential contributors to inflammatory load where appropriate (e.g. nutrition, environmental exposures, infections
This is not about permanent restriction. It is about stabilising inflammatory signalling so that gradual expansion becomes possible.
In some cases, additional work around immune modulation, gut health, sleep and nervous system regulation may also be part of a comprehensive plan. This is what we emphasize in the Nurturing Resilience Membership.
A Gentle Reminder
It is possible to experience elements of more than one pattern. The more sensitive your nervous system the greater the likelihood that two or all three patterns may be blended.
For example, someone may have autonomic instability, metabolic instability and delayed immune sensitivity simultaneously. In these cases, sequencing becomes important.
This is where generic advice such as “gradually expand” can feel confusing or unhelpful.
The body is rarely that binary.
The Goal Is Not Avoidance
It is understandable to become cautious around movement when crashes feel unpredictable.
However, the aim is not to avoid activity entirely.
The aim is to stabilise the body and understand your current physiological capacity and work within it.
Many people find that once the underlying pattern is identified and respected, stability increases. From that place of stability, capacity can often be built gradually and sustainably.
Energy-limiting conditions are complex. They involve interactions between nervous system regulation, metabolic health, immune signalling and psychological stress.
But complexity does not mean impossibility.
It simply means we must listen carefully to what the timing and quality of symptoms are telling us.
Where To Go From Here
If you recognise yourself in one of these patterns, you are not failing.
You are observing physiology.
And that observation is powerful.
When crashes are understood rather than feared, they become information. Information allows us to adjust, to sequence appropriately and to work with the body rather than constantly feeling at odds with it.
For some people, small adjustments to intensity and recovery make a meaningful difference.
For others, especially in more complex or long-standing cases, deeper work around nervous system regulation, metabolic health and immune signalling may be needed.
There is no one-size-fits-all pacing rule.
But there is a process of learning how your body responds — and building from there.
Wherever you are on your journey, progress rarely comes from pushing harder.
It tends to come from understanding more clearly.
And that clarity is something we can build.
