Thesis Explaining Fatigue and Post-Exertional Malaise in Long COVID and ME/CFS: From Skeletal Muscle To Exercise Immunology, 2026, Charlton

[RichieG123]

Noticed that @RichieG123 is listed as an author on this preprint. It looks really interesting and potentially quite significant for the field. I wondered if you had any comments to make on the paper, discussion here, or your team's future research directions based on these findings?

It is very interesting, but my involvement in this particular study was relatively minimal. I was involved in the data collection of the original cohort, but since getting those biopsies, I haven't done much with the subsequent analysis of satellitete cells etc. presented here. Braeden is a friend so we have discussed these data a lot, but not a great deal more than that, unfortunately. I will probably be involved in the write up phase of the paper. That said, it is already on my to-do list: Braeden will defend his PhD shortly in early November, and I will be a paranymph at the defence. So, I need to get reading! His thesis is very thick compared to the average PhD!

 

[V.R.T.]

paranymph

I have learned a new word today, thank you!

 

[Jonathan Edwards]

Apparently it does not refer to a skydiver in a tutu but something much more prosaic.

 

[SNT Gatchaman]

I keepcoming back to the question of why, if muscle damage is occurring enough to contribute to symptoms, why are simple things not abnorkal like serum creatine kinase?

Yep it's probably the key question on the muscle side of things.

We have had elevated results, eg Eccentrically Induced Skeletal Muscle Damage in Patients With Chronic Fatigue Syndrome CFS, With Reference to Overtrained Athletes (1995) —

Recruitment of 25 subjects was undertaken. From these 25 subjects three experimental groups were formed of; Chronic Fatigue Syndrome (CFS)+ eccentric damage n = 8, Control Damage (CD) + eccentric damage n = 10, and Control (ND)+ no-damage n = 7

Serum creatine kinase efflux peaked in the CFS & CD groups 4 days post damage with a peak mean of 8388 ± SEM 1224 (UI/L) in the CFS group and a peak mean of 2583 ± SEM 753 (UI/L) in the CD group. As expected the ND group serum CK levels remained constant.

Annotated fig 4.2 —

The greater concentration of serum CK in the CFS group following damage when compared to the control groups is of particular interest, indicating a greater degree of eccentrically induced damage. Whilst it is accepted that serum CK activity is "a useful tool for the detection of muscle damage, since it is generally considered to be highly sensitive and relatively specific to muscle" (Dioszeghy & Mechler, 1988, p. 175), doubt remains about the concentration of serum CK being used as a determinant of the amount of damage due to the large inter-subject & gender differences reported in serum CK efflux (Ebbling & Clarkson, 1989 ). Evans & Gannon, ( 1991, p. 104) state: "It is likely that postexercise rise in circulating CK activity is a manifestation of skeletal muscle damage but not a direct indicator of it".

It's uncertain whether the groups do peak at the same time, or whether there might be a delayed peak in ME/CFS (potentially 2 days later).

They measured at day 1, 2, 4, 6, 8, 12 ... so possibly either HC or ME could have peaked at day, 3, 4 or 5

Hypothesis 1 (after damage, muscle force characteristics of subjects with CFS will be different from the Control Damage (CD) group) was fully proved with isometric muscle force characteristics along with dynamic force differences being significantly different between the groups after the damage. Hypothesis 2 ( after damage, the contractile properties of skeletal muscle as determined by the level of low frequency fatigue (LFF), will differ in subjects with CFS when compared to the CD group) was satisfactorily proven by LFF in CFS patients having significantly greater depth, and lasting longer. Hypothesis 3 (the time course of serum CK efflux, and the serum CK concentration in subjects with CFS will be different from the CD group) was partially proved, in so far as the serum concentration of CK was significantly higher in CFS patients following the damage bout, however the time course of the increased serum CK activity was no different from the control group.

If you're measuring CK at rest and only to 24 hours post exercise, you'd miss any peak around day 4. Eg Patients with chronic fatigue syndrome performed worse than controls in a controlled repeated exercise study despite a normal oxidative phosphorylation capacity (2010) —

And it is likely that also their muscle mitochondria are normal, since 24 h after strenuous exercise CK did not leak to the blood, as is the case in patients with defective oxidative phosphorylation.

If there is increased collagen 4 around the muscle cells as indicated in this thread's work then you could have:

1) impaired diffusion of oxygen from circulation to muscle (mitochondria)
2) impaired movement of large molecule waste and damage products back to circulation via lymphatics (regardless of thickened basement membrane around the capillaries themselves)

#1 could cause a major metabolic problem for muscle cells but #2 could act to stop us identifying the results and nature of this problem.

Is it possible that resting levels of CK are lower in ME because an impediment in it reaching the lymphatics and then serum in order to be measured? (beyond activity reduction/PEM avoidance).

Spoiler: Skippable detour on CK assay types

A question might also be: how is CK being measured? Is it by its enzymatic activity or by immunoassay? If it's by enzymatic activity then CK catalytic activity has a half-life and for low-range but still abnormal levels it might be dropping before we get a chance to see it (even though the CK itself is still elevated). This was thought to be a problem for myocardial infarction extent assessment, which has been superseded by troponin for decades.

Effects of lymphatic transport of enzyme on plasma creatine kinase time-activity curves after myocardial infarction in dogs. (1978)

It's possible that some of the studies looking at resting CK levels using enzymatic assay report spuriously low CK.

Enzymatic Measurement

Patients with chronic fatigue syndrome performed worse than controls in a controlled repeated exercise study despite a normal oxidative phosphorylation capacity (2010) —

Plasma creatine kinase (CK) is usually considered a marker of non-specific muscle damage. In the plasma the activity of CK was measured, as a surrogate measure of a lowered oxidative phosphorylation in skeletal muscle.

Muscle abnormalities worsen after post-exertional malaise in long COVID (2024) —

Creatine Kinase c702 Roche Diagnostics NAC-activated

Neither did we observe an increase in muscle breakdown products, such as creatinine and creatine kinase, in the plasma of both groups

Unspecified Assay Type

Evidence of Clinical Pathology Abnormalities in People with Myalgic Encephalomyelitis/Chronic Fatigue Syndrome ME/CFS from an Analytic Cross-Sectional Study (2019) —

CK isoenzymes, mainly produced in skeletal muscle (CK-MM) and heart muscle (CK-MB), and aldolase were analysed in a sub-sample of 50 cases (25 severe and 25 non-severe cases) and 25 controls. According to the proximity of the collection site, blood samples were analysed by the haematology, clinical pathology, and immunology laboratories at the Norfolk and Norwich University Hospital or the Royal Free Hospital in London; standard NHS laboratory protocols were used for all tests [8].

The reduced CK values were mainly due to low values of CK-MM isoenzyme, for which the mean was 44 U/L (interquartile range (IQR) = 24–86) in severe cases, compared to 73 (IQR = 37–94) in the mild/moderately affected and 90 (IQR = 41–238) in healthy controls (p = 0.03).

Also using the UK Biobank Assessing diagnostic value of microRNAs from peripheral blood mononuclear cells and extracellular vesicles in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (2020) —

Among the 30 clinical parameters evaluated at the UK Biobank in the blood samples studied, only creatine phosphokinase (CPK) levels showed statistically significant differences between groups (p < 0.05), with increased decreased levels by approximately 1.5-fold in ME/CFS patients.

Blood creatine phosphokinase (CK) levels appeared significantly reduced in our ME/CFS studied cohort (t-student, p < 0.05), in agreement with the recent report by Nacul et al. including evaluation of this variable in 56 severe ME/CFS patients.

Evaluating Routine Blood Tests According to Clinical Symptoms and Diagnostic Criteria in Individuals with Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (2021) —

The samples were delivered to the central laboratory at Aker Hospital within 30 min from collection and analyzed consecutively. Some serum samples were transported refrigerated to other hospitals for further processing. Standard OUS laboratory protocols were used for all collection, tests, and transport.

Our main findings include a lower creatin kinase (±CK) value in routine blood samples among patients than among controls, and lower in those with severe function impairment ME/CFS compared to moderate and mild function impairment.

Immunoassay

Hand grip strength and fatigability: correlation with clinical parameters and diagnostic suitability in ME/CFS (2021) —

Creatine kinase (CK, immunological UV-test) and lactate dehydrogenase (LDH, photometric test) were determined from blood samples drawn after the second HGS assessment at the Charité diagnostics laboratory (Labor Berlin GmbH, Berlin, Germany).

Further we found that higher CK levels are associated with poorer Recovery Ratio and higher Fatigue Ratio in women. CK plays a central role in the energy supply of the muscle and muscular strain can increase the serum concentration of CK, although there are inter-individual differences. A previous study found that CK concentrations are lower in patients with severe ME/CFS. This is not in contrast to our study as CK was analyzed after exertion in our study.

Fluge and Mella used aptamer microarray proteomics in Charting the circulating proteome in ME/CFS using cross-system profiling to uncover mechanistic insights (2026)

This doesn't use enzymatic activity for the assay (SomaScan) and still found reduced muscle breakdown products (but higher myoglobin).

The reduced release of intracellular skeletal muscle proteins appears to represent a distinct ME/CFS phenotype that is supported by our data. […] Although decreased leakage of muscle proteins could theoretically result from low physical activity or reduced muscle mass, this explanation is not well supported, as the analyses were adjusted for age, BMI, and sex, which partly account for such effects. While the underlying mechanisms remain unclear, it seems likely that muscle tissue homeostasis and turnover are affected, potentially linked to previously reported muscle abnormalities and contributing to hallmark ME/CFS symptoms such as fatigue and PEM.

And although not CK Temporal dynamics of the plasma proteomic landscape reveals maladaptation in ME/CFS following exertion (2025) —

muscle-related proteins MYBPC1, KLHL41 and MYL3 showed significantly decreased levels in patients with ME/CFS at D2PRE [morning of 2nd CPET].

 

[Jonathan Edwards]

We have had elevated results, eg Eccentrically Induced Skeletal Muscle Damage in Patients With Chronic Fatigue Syndrome CFS, With Reference to Overtrained Athletes (1995) —

Yes but that is an artificial situation. We know that unaccustomed eccentric contraction in untrained muscle gives damage and a rise in CK. We would expect people with ME/CFS to be unaccustomed to forearm eccentric contraction

 

[Creekside]

People with ME/CFS have learned to do everything gently so that even the steps they take are likely a modest strain on the muscle.

That would probably be worthy of study: using cameras, body sensors, etc, to measure changes in muscle usage in people with physically-limiting diseases. That might correct some biases in findings in other studies. Reduced amount of activity is a major factor, but the subtle changes due to trained avoidance might be significant.

 

[rvallee]

Maybe it's just that damage isn't the right description, in that it's not the same type as DOMS, and certainly not on the same timescale, but there is definitely something odd with how my muscles react to small strain, tire out and start hurting very quickly. This seems way too fast for there to be physical damage to the muscle fibres, even though it feels a bit similar. Definitely not the same, it's easy to distinguish from DOMS, and not just because it's not delayed, the sensation is just different.

But there's definitely something there, something that isn't fixed, that doesn't happen all the time. No idea how common this is, though, I might be the odd one out, but it doesn't seem so.

 

[Creekside]

even though it feels a bit similar.

For any system that includes signal processing and transmission, it's possible for abnormalities in the signalling to mimic hardware problems. I, for example, experienced muscle aches, but I don't think those sensations were due to physical muscle problems.