My 2 cents...
When thinking of PEM or PER (Post exertional relapse) we first have to separate having used our brain or muscles (including heart and lungs), from the symptoms and disabilities this causes.
E.g in severe cases of ME CFS:
Physical - MUSCLE:
One may be stable, used to walking around the house in episodes, mostly confined to the sofa or bed.
On an energetic rare day, one may be able to walk/stand more than usual, even leave the home. This will cause a latent reaction. A reduction in function of physical or mental capacity, such as difficulty thinking, vertigo, pain, overwhelming exhaustion, weakness etc. This weakness is not expected, explained and a huge reaction to minor exertion. This is hallmark of ME, as is exercising the brain....
Mental - BRAIN USE:
One may be able to talk for limited bursts, use the PC for episodes, watch a short TV show.
Talking for extended periods and concentrating may cause an extreme reaction, such as not be able to physically move limbs, speak, and induce severe vertigo, pain, and very severe headache (on top of the permanent ME headache severe PWME report they never experienced before the condition started).
There are two main theories postulated by the medical community for such an extreme reaction in people with 'normal' muscle function at rest, and normal muscle pathology..
1) ME is a neurotic belief in ME. (CDC, British Psych Lobby approach). This idea has failed.
2) ME is a mitochondrial infection/reaction within parts of the CNS that causes massive energy dysfunction reducing in abnormal nerve firing/cell to cell communication with a combined immune disorder involved in low grade inflammation of the CNS that is 'irritated' by these energy cells being 'used' (using your brain or body).
We know, and science is slowly proving, 2) is the accurate view but will need a huge amount of scientific input to now prove, due to the mainstay of research using Fukuda CFS criteria (CDC), and not researching the severely affected to get the data!
In my view, the massive 'exhaustion' ME causes from using your brain, can have a more extreme affect
than physical effort if we are debating how much 'effort' the person uses Vs muscle power and cardio respiratory input. E.g a PWME with ME who concentrates for 1hr hard, may have a worse relapse, than the same PWME who walks around for 30 mins.
There has to be an explanation for this, the simple one being the energy defect is brain centered/CNS, and not muscle based. It could also be explained by oxygen utilization. Someone far clever than I will be able to tell you the oxygen and glucose demand on the BRAIN from reading for 1hr, Vs the oxygen and glucose demand on the BODY for gently walking around for 30 mins. If your brain cells cannot use glucose efficiently, then your brain cells won't work correctly and will 'wilt' quickly.
This utilization of oxygen from exercise in people with elevated oxidative stress AT REST (PWME CFS) will cause a cascade of further oxidative stress in the body (from gentle exertion) and DNA damage, that presumably is higher than 'thinking' with the brain. The question is though, is the brain in PWME already undergoing significant oxidative stress at rest? Is 'thinking' with exhausted mitochondrial cells in PWME's brain what causes the lactate to rise in the CNS and not physical exertion. (CFS research shows, high CSF lactate in sufferers). Could this explain why concentrating with 4 people in a room may annihilate a PWME more than walking up the staircase 4 times, which A PWME may also find very taxing, but not as relapsing?
To confuse matters further we must remember here that the 'delayed payback' of PEM and PER can be interchangeable with BRAIN vs MUSCLE.
*One time period you CAN talk for 1 hr without huge relapse.
*Another time period the staircase you could walk 4 times in one day, 1 attempt and you can't get off the floor.
And vice versa....
Either way, main stream medicine and the wider public (due to propaganda by the psych lobby) don't believe patients with severe ME are 'severe' other than from the process of de-conditioning. For me (biased view), it's obvious. The brain's cells used for cognition and some other areas in PWME simply don't work at rest efficiently, and become so depleted (when used) so badly the ME sufferer produces signs of neurological dysfunction (mimic 'real' neurological diseases) I've mentioned before, or worsening of the following if present at rest:
Worsening of pain.
Eyelid twitching.
Eye ball involuntary movement.
Eye drooping.
Stammering or Stuttering.
Slurred speech.
Gait changes (balance).
Vertigo and dizziness.
Blurred vision, double vision.
The above symptoms are rarely witnessed in a doctors office, because it takes TIME to relapse the patient from thinking/talking. A GP appointment may be only 5 to 15 mins, not 30-60 mins. Doctors thus rarely 'see' the patients neurologically decline in front of them. Conversely, partners and carers......do. Another phenomena of 'statistical data' (never produced) missing PER rather than PEM, is patients tend to 'save up' for doctors appointments and/or the appointment is at the 'start' of the patients day. Doctors appointments don't occur at 7pm onwards, when the patient may be massively worse. Doctor's don't see this.
These symptoms in severe PWME are in reaction to MENTAL exertion, and thus the brain cells ATP is depleted/cells are likely inflamed and targeted by a form of autoimmune process. No other condition does this to this level, to my knowledge in which if rested the patients can return to 'normal'. (Normally disabled). Yes MS, Parkinson's can and will relapse from exertion of mental effort, but these people have permanent damage, such as paralysis, or the Parkinson walk ('shuffling gate'). Hence, ME is disbelieved.
None of the above extreme PER or PEM can be predicted for or managed, hence 'Pacing' does not and cannot 'fix' the underlying cause by trading of energy from one task to another or simply cutting back on total energy expenditure.
Pacing to 'manage' ME and return people to consistent activity levels, is a myth. The immune system activation simply doesn't allow for consistency, neither does the metabolic ATP defect. Each day we feel very ill, ghastly, a little better, or like we're dying - irrespective of what we do. This is how ME works. We are made worse by any activity, but we fluctuate anyway, even when we don't over exert and do literally, nothing. We also relapse, via stress or infections (inflammation and hormone associated) or indeed, 'unknown' factors of the pathology no one understands...yet.
Pacing is a sensible common sense approach to energy management, not over doing things. All patients with ME CFS do this once they are diagnosed and (late in the day usually) forced to learn how to listen to their bodies when experiencing this terrible illness. However, in ME, pacing as an effective 'therapy' does not apply.
From listing to patients, and ignoring the CDC's disinfo about GE, we can see that:
*ME patients are in a permanent state of metabolic and thus bodily exhaustion AT REST.
*Activity makes us worse and causes an immune response (raised lymph glands, sore throat, sneezing in some).
*Brain activity can produce equal or worse response to physical activity - a surprising finding if ignoring neurology.
*Severe grade patients can have permanent relapses from moderate activity who do 'pace', who don't boom n' bust.
For PWME, the CDC's 'Post Exertional Malaise' (feeling a bit off), is an insult to the nature of LOSS OF FUNCTION PWME suffer from minimal exertion of brain or body. Post exertional relapse, is thus a better term. (PER), in my view as patients dontt feel worse, they become disabled from activity/worsen existing disabilities. That cannot ever be described medically, as a 'malaise'. The CDC's description of CFS, for ME or ME/CFS is thus insufficient and incorrect.
NB: Sufferers less ill who haven't developed a severe form of ME won't experience these reactions to the level severe grade patients will. This allows for inevitable disagreement in patients to occur.
A concern of mine, is there is nothing stopping mild or moderate patients pushing themselves into a severe form of ME, from repeat, or even 'one off' over exertion that results in a strong immune response, apparent immune suppression (if repeated) and then an infection, that one-day 'gets' you, and you literally become severe grade and screwed for life. Multiple PWME have told me this story, they were told to exercise, told to ignore symptoms told to mix CBT with GE and ended up severe grade for ever. Conversely, it appears less severely affected people were diagnosed earlier, and didn't damage the bodies for as long. Is this why time and time again we hear that ex exercise and adrenaline junkies end up severely affected? It would seem logical these unfortunate people are experiencing massive oxidative injury (whilst moderately ill), unaware, that they have ME CFS, and unaware that this can causes them to end up housebound.
It greatly worries me a percentage of PWCFS end up as PWME, needlessly, but pushing (within or over limits) only
to become permanently disabled believing the were safe. I am biased, because I was one of these people.
As anyone with severe ME CFS will tell you, no one is 'safe' with activity, as we have no idea what the pathogen reservoir is (if any), and what causes us to get worse, other than simply expending energy! The sensible thing to do is listen to your body, and achieve some form of activity level, that generally doesn't cause severe worsening of symptoms.
For PWME severely affected, they will agree, but point out they have over whelming symptoms AT REST and are in permanent state of metabolic energy crisis (defective ATP), meaning the concept of PEM to them is silly. Many would argue, that people with such bad at rest exhaustion are original ME, and not CFS Fukuda criteria.
With biomarkers, we can finally decide once and for all, but the biomarkers won't be of any use in a heterogeneous cohort of non severely affected. Catch 22. We need to compare severe Vs Moderate Vs mild to come to a conclusion about the scientific and medical reasons of PEM and PER, and which subset of ME CFS has 'it and why.