In Pursuit of Understanding the Crash
What Post-Exertional Physiology Tells Us About ME/CFS
Myalgic encephalomyelitis is characterized by post-exertional malaise (PEM).1,2 PEM is variously defined, but it involves an abnormal multi-system and multi-symptom response to often-trivial levels of exertion. It frequently results in severe disability. Everyone’s specific pattern of signs and symptoms is different, but they all have in common a worsening after exertion. Solving PEM would solve ME/CFS.
The failure of medicine and science to understand ME/CFS has been a shortcoming of theory, not technology. For decades, clinicians have assumed that exertion reveals health. The idea is relatively simple, and it holds up most of the time. If you stress the system and observe the resulting performance of physiological systems under load, then you can infer capacity, and counterfactually, pathology. However, ME/CFS is an important exception to this rule.
Functional capacity in people with ME/CFS may be significantly compromised. But there is often a delay between the exertion and the worsening.3 Clinicians and scientists long have assumed this delay must signal a problem with motivation, beliefs, and fear. However, what ME/CFS research has surfaced slowly over time is something that medicine is still in its infancy in understanding: what happens after exertion, the biology of recovery. And the data in ME/CFS are now too coherent to dismiss.
Failure of Biological Recovery is Unusual and Unique to ME/CFS
The most consistent finding in ME/CFS research is also the most instructive. Patients cannot reproduce energy output on repeat exertion, whether physical, cognitive, emotional, or environmental. I’m an exercise physiologist, so I understand this literature the best and the literature is probably best developed related to physical exertion. Across multiple centers all over the world, two-day cardiopulmonary exercise testing (CPET) demonstrates that people with ME/CFS show objective declines in volume of oxygen consumed and workload at submaximal exertion on the second day, despite showing maximal effort on both days.4-6 ME/CFS is a crisis of reproducibility in science, just not the way we normally talk about it.
This failure of biological recovery does not occur in healthy controls, deconditioned people, or in other chronic diseases, including heart failure, chronic obstructive pulmonary disease, and multiple sclerosis.7 It is not fatigue. Rather, it is failed metabolic recovery. Attempts to dismiss this finding have relied on underpowered null studies or inappropriate analyses that erase stratified effects. (More on how ME/CFS researchers have used and abused CPET studies in future articles.) Larger and more carefully analyzed studies continue to confirm impaired reproducibility as a defining signature of PEM.4
If medicine and science treated exertional responses the same way it treats glucose testing, the pathophysiology of ME/CFS would have become obvious decades ago. And we would avoid the constant misapplications of the biopsychosocial approach to ‘treat’ the pathophysiology of ME/CFS that still divert precious attention, time, and resources today.
PEM is a Failure of Bioenergetic Recovery
Exercise physiology tells us the process of exertion is not completed at task termination. Recovery means further exertion. It requires mitochondrial substrate switching, redox normalization, autonomic rebalancing, immune system activation, and tissue repair signaling. However, this sequence breaks in ME/CFS. Multi-omics studies show that after an initial bout of exertion, ME/CFS patients exhibit continued impairment TCA cycle flux and fatty acid β-oxidation, reduced ATP generation and altered AMP/ADP ratios, persistent redox imbalance, complement activation and innate immune amplification, and lipidomic signatures consistent with inflammatory repair failure.8,9 All of these are worse after effort and correlated with symptom severity.
Simply put, the bodies of people with ME/CFS can not efficiently do the work of recovery. And because these abnormalities are found in people with ME/CFS but not matched deconditioned controls, we can confidently say that PEM is not deconditioning.
The Immune System Signals Persistent Danger in PEM
Normal exercise produces a transient inflammatory response followed by rapid resolution. However, people living with ME/CFS show the opposite pattern. A diverse body of studies demonstrate Exaggerated complement (C4a) activation post-exercise, abnormal toll‑like receptor and IL-10 gene expression, and heightened oxidative stress with delayed antioxidant responses.8,10 These immune changes align with PEM.
Recent large multi-omics studies have gone further, documenting post-exercise innate immune hyperreactivity alongside metabolic collapse. These findings suggest that exertion re-engages a persistent danger response within the immune system rather than resolving it.8 Some people are still tempted to think PEM is just exercise intolerance. These data suggest PEM is also exertion-triggered immune system dysregulation.
Autonomic Control Collapses in PEM
The autonomic nervous system is the conductor of the orchestra of physiological recovery. However, the conductor is drunk and the orchestra has become poorly coordinated in ME/CFS. Large multisite studies show common autonomic symptom burden in ME/CFS, including orthostatic intolerance, impaired heart rate variability, and abnormal blood pressure regulation. Severity of these signs and symptoms are correlated with illness burden.11 Invasive CPET studies demonstrate that exertion decouples neurovascular control, which impairs venous return and cardiac preload.12 These data indicate exertion destabilizes control systems that normally stabilize the body during recovery.
Exertion and Time Are the Key Variables Science and Medicine Have Ignored
Failing to consider previous exertion destroys the validity of cross-sectional outcomes measurements in ME/CFS.13 Taking blood samples, obtaining questionnaires, and collecting other outcomes measures must consider the patients signs, symptoms, and previous exertion. Important physiological differences are apparent in people living with ME/CFS on “good days” versus “bad days.”14 This understanding needs to be incorporated into research and clinical practice.
One assumption in ME/CFS research has been that recovery physiology happens on “clinic time.” However, PEM commonly unfolds over 24–72 hours, with recovery lasting days to weeks to even months. Recovery-focused studies show ME/CFS patients report return-to-baseline times averaging nearly two weeks after a standardized physical exertion compared with around 48 hours in healthy, deconditioned controls.3,15,16 This means researchers and clinicians need to plan for extended observation times when PEM is involved.13 When variability is a key characteristic of the disease, more measurements are needed to separate signal from noise.
Post-Exertional Physiology is Not Optional Science in ME/CFS
The emergence of post‑exertional physiology forces medicine to confront an inconvenient truth. We have engineered entire research and treatment approaches without verifying that post-stress recovery worked. People with ME/CFS have been harmed because we have ignored the physiology of recovery, leading to misapplications of exercise, cognitive-behavioral therapies, and a piecemeal approach to testing repurposed pharmacological therapies.
Post-exertional physiology now sits at the uncomfortable intersection of metabolism, immunology, and cardiovascular function. Science and medicine must break down the conceptual silos between them, so exertion will not continue to be weaponized.
Key References
1. Carruthers BM, Jain AK, DeMeirleir KL, et al. Myalgic Encephalomyelitis/Chronic Fatigue Syndrome: Clinical Working Case Definition, Diagnostic and Treatment Protocols. Journal of Chronic Fatigue Syndrome. 2003;1(1):7-115. doi:10.1300/J092v11n01_02
2. Carruthers BM, van de Sande MI, De Meirleir KL, et al. Myalgic encephalomyelitis: International Consensus Criteria. J Intern Med. Oct 2011;270(4):327-38. doi:10.1111/j.1365-2796.2011.02428.x
3. Davenport TE, Stevens SR, Baroni K, Van Ness M, Snell CR. Diagnostic accuracy of symptoms characterising chronic fatigue syndrome. Disabil Rehabil. 2011;33(19-20):1768-75. doi:10.3109/09638288.2010.546936
4. Keller B, Receno CN, Franconi CJ, et al. Cardiopulmonary and metabolic responses during a 2-day CPET in myalgic encephalomyelitis/chronic fatigue syndrome: translating reduced oxygen consumption to impairment status to treatment considerations. J Transl Med. Jul 5 2024;22(1):627. doi:10.1186/s12967-024-05410-5
5. Lim EJ, Kang EB, Jang ES, Son CG. The Prospects of the Two-Day Cardiopulmonary Exercise Test (CPET) in ME/CFS Patients: A Meta-Analysis. J Clin Med. Dec 14 2020;9(12)doi:10.3390/jcm9124040
6. Franklin JD, Graham M. Repeated maximal exercise tests of peak oxygen consumption in people with myalgic encephalomyelitis/chronic fatigue syndrome: a systematic review and meta-analysis. Fatigue: Biomedicine, Health & Behavior. 2022;10(3):119-135.
7. Larson B, Davenport TE, Stevens SR, Stevens J, Van Ness JM, Snell CR. Reproducibility of Measurements Obtained During Cardiopulmonary Exercise Testing in Individuals with Fatiguing Health Conditions: A Case Series. Cardiopulmonary Physical Therapy Journal. 2019;30(4):145-152. doi:10.1097/CPT.0000000000000100
8. Che X, Ranjan A, Guo C, et al. Heightened innate immunity may trigger chronic inflammation, fatigue and post-exertional malaise in ME/CFS. NPJ Metab Health Dis. Sep 3 2025;3(1):34. doi:10.1038/s44324-025-00079-w
9. Heng B, Gunasegaran B, Krishnamurthy S, et al. Mapping the complexity of ME/CFS: Evidence for abnormal energy metabolism, altered immune profile, and vascular dysfunction. Cell Rep Med. Dec 16 2025;6(12):102514. doi:10.1016/j.xcrm.2025.102514
10. Nijs J, Nees A, Paul L, et al. Altered immune response to exercise in patients with chronic fatigue syndrome/myalgic encephalomyelitis: a systematic literature review. Exerc Immunol Rev. 2014;20:94-116.
11. Issa A, Lin JS, Chen Y, et al. Autonomic Dysfunction in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS): Findings from the Multi-Site Clinical Assessment of ME/CFS (MCAM) Study in the USA. J Clin Med. Sep 5 2025;14(17)doi:10.3390/jcm14176269
12. Joseph P, Arevalo C, Oliveira RKF, et al. Insights From Invasive Cardiopulmonary Exercise Testing of Patients With Myalgic Encephalomyelitis/Chronic Fatigue Syndrome. Chest. Aug 2021;160(2):642-651. doi:10.1016/j.chest.2021.01.082
13. Soares L, Davis H, Spier E, et al. Recommended long COVID outcome measures and their implications for clinical trial design, with a focus on post-exertional malaise. EBioMedicine. Dec 19 2025;123:106083. doi:10.1016/j.ebiom.2025.106083
14. Aitken A, Sawyer A, Iwasaki A, et al. Digital physiological biomarkers predict within-person symptom changes in complex chronic illness. NPJ Digit Med. Mar 24 2026;9(1)doi:10.1038/s41746-026-02543-3
15. Moore GE, Keller BA, Stevens J, et al. Recovery from Exercise in Persons with Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS). Medicina (Kaunas). Mar 15 2023;59(3)doi:10.3390/medicina59030571
16. Mateo LJ, Chu L, Stevens S, et al. Post-exertional symptoms distinguish Myalgic Encephalomyelitis/Chronic Fatigue Syndrome subjects from healthy controls. Work. 2020;66(2):265-275. doi:10.3233/WOR-203168