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Pain, fatigue & chronic conditions

ME/CFS is defined by the crash after effort, not by tiredness

Why post-exertional malaise sits at the center of ME/CFS, how it overturned advice on graded exercise, what the PACE trial fight is about, and what newer research found.

By the Health Research USA editorial team

Published Last updated October 2026

Written from the sources listed below; links checked October 2026. It touches on medicines, doses or health conditions and has not yet been reviewed by a licensed clinician, so use it to prepare for a conversation with yours. How we research and review

Sources22, listed below Reading time14 minutes
On this page
  1. The Symptom at the Center
  2. How Many Americans Have It
  3. How the Exercise Advice Was Reversed
  4. What a Close-Up NIH Study Found
  5. After COVID-19, and the Search for Causes
  6. Questions to Ask About Activity Advice
  7. The Takeaway

It was a good Saturday. You walked to the market, cooked for friends and stayed up talking. Sunday felt fine. On Monday you could barely lift your head, reading one email took three tries, and the slump lasted until Friday. Then a well-meaning clinician suggested adding a little more exercise each week.

That delayed collapse has a name, post-exertional malaise (PEM), and in myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) it is the feature that separates the illness from ordinary exhaustion. In 2021 and 2022, according to the National Center for Health Statistics (NCHS), 1.3% of US adults said a health professional had told them they had ME/CFS and that they still had it.

Putting PEM at the center of the definition did more than sharpen diagnosis. It helped overturn years of treatment advice built on steadily increasing exercise, and researchers on each side of that reversal still disagree, in print, about what the evidence shows.

The symptom at the center

In 2013 the Department of Health and Human Services asked the Institute of Medicine (IOM), part of the National Academies, to define the illness. Its 2015 report called ME/CFS a serious, long-lasting illness that affects the whole body and can drastically curtail daily life, and it named three required symptoms.

The first is a substantial drop in the ability to carry on work, school, social or personal life at pre-illness levels, lasting more than six months and accompanied by fatigue that rest does not much relieve. How that six-month line works is laid out in our explainer on how long fatigue lasts. The second is PEM. The third is unrefreshing sleep. A person also needs either cognitive impairment or orthostatic intolerance, meaning symptoms that worsen when upright. If symptoms are not present at least half the time, at moderate intensity or worse, the report says the diagnosis should be questioned.

The IOM defined PEM as a flare of some or all of a person’s symptoms after physical or mental effort, or after the strain of being upright, that reduces how well they function. The Centers for Disease Control and Prevention (CDC) adds emotional effort to the triggers and notes that, for some patients, sensory overload from light and sound can bring it on. The key detail is how little it takes: the exertion is something that would not have caused trouble before the illness, and CDC says the relapse can last days, weeks or longer.

Orthostatic intolerance, CDC says, is measured by heart rate and blood pressure changes during standing, by bedside readings lying and standing, or by tilt-table testing. The thinking problems that often come along are examined in our piece on brain fog.

So the common belief that ME/CFS is just being very tired misses the definition. Fatigue is required, but so are a characteristic crash, unrefreshing sleep and either cognitive or upright-posture problems. The IOM also recommended a new name, systemic exertion intolerance disease, and described the belief that the illness is psychological in origin, or imagined, as a misconception.

How many Americans have it

The NCHS figure comes from the National Health Interview Survey, which questioned 57,133 adults in 2021 and 2022. Women reported ME/CFS more often than men, 1.7% against 0.9%. The rate climbed with age, from 0.7% among adults under 40 to 2.0% among people in their 50s and 2.1% in their 60s, then fell to 1.4% at 70 and older. It was 2.0% among adults below the poverty line against 1.1% among those at twice the poverty line or more, and 1.9% outside metropolitan areas against 1.0% in large central cities.

By race and ethnicity, the survey found 1.5% of White adults, 1.2% of Black adults, 0.8% of Hispanic adults and 0.7% of Asian adults. Those are diagnoses people reported, and getting a diagnosis depends on reaching a clinician who makes one. The authors note that undiagnosed adults are not counted and that the true figure may be higher.

CDC’s fact page, reviewed in May 2024, gives a wider range: 836,000 to 3.3 million people in the US, with about 9 in 10 undiagnosed. It says the illness is most common between ages 40 and 60, more common in adolescents than in young children, and that Black and Latino people may be more affected. That hedged statement and the survey’s pattern measure different things, so neither cancels the other. The 2015 IOM report estimated 836,000 to 2.5 million Americans, 84% to 91% of them undiagnosed, at a cost to society of $17 billion to $24 billion a year. CDC adds that some people are homebound or even bedbound.

How the exercise advice was reversed

The trial at the center

The PACE trial, which The Lancet published in 2011, was led by P. D. White of Queen Mary University of London and enrolled 641 patients at six UK clinics, using an older diagnostic definition known as the Oxford criteria. Everyone got specialist medical care; three groups also got adaptive pacing therapy, cognitive behavioral therapy (CBT) or graded exercise therapy (GET), in which activity is increased step by step. Participants rated their own outcomes and knew which treatment they had.

At 52 weeks, fatigue scores were 3.4 points lower with CBT and 3.2 points lower with GET than with specialist care alone. Physical function scores were 7.1 and 9.4 points higher. Adaptive pacing made no difference. Serious adverse reactions were recorded in 1% to 2% of every group, and subgroups meeting two other definitions of the illness showed similar results. The authors concluded that CBT and GET could be added safely to moderately improve outcomes. Funders included the UK Medical Research Council and the Department for Work and Pensions.

A 2015 follow-up led by M. Sharpe of the University of Oxford reached 481 participants, three-quarters of the total, a median of 31 months after enrollment. CBT and GET gains had held, but the pacing and usual-care groups had improved too, and the groups no longer differed much. The authors cautioned that 44% had received more treatment after the trial ended.

In 2018, a team led by C. E. Wilshire of Victoria University of Wellington in New Zealand reanalyzed the trial using data released through a Freedom of Information request. Co-authors included a researcher affiliated with the Irish ME/CFS Association and the journalist David Tuller of the University of California, Berkeley. Following the trial’s original protocol as closely as they could, they found CBT and GET did not significantly beat the control group after correcting for multiple comparisons, recovery rates were low in every group, and secondary gains were almost all self-reported. In their words, the modest self-reported effects “do not exceed what could be reasonably accounted for by participant reporting biases.”

Agencies change course

In July 2017, Tuller reported in Undark magazine that CDC had removed information on graded exercise and the CBT approach from the main section of its ME/CFS website. Tuller disclosed that he had worked with physicians and patient groups to challenge both therapies. A separate CDC features page still mentioned graded exercise at the time, and CDC told him it had rewritten the pages to avoid confusion and still believed exercise could help some patients.

CDC’s current pages, reviewed in 2024, name neither therapy. They say there is no cure or approved treatment, and that standard exercise recommendations for healthy people can harm people with ME/CFS; the clinician page says they can substantially harm. CDC also counts deconditioning as a harm, so it favors keeping up whatever activity a patient tolerates. The clinician page also states that its content is not a federal guideline or treatment recommendation, so the most detailed federal advice arrives with that caveat attached.

Cochrane, the international evidence-review network, went through its own turn. Its exercise review, amended in 2019, pooled eight trials with 1,518 people, all diagnosed by CDC’s 1994 definition or the Oxford criteria, with searches only to May 2014. Compared with passive care, exercise probably reduced fatigue at the end of treatment, by about 3.4 points on a 33-point scale. The long-term effect and the risk of serious harm were very uncertain.

Editor in Chief Karla Soares-Weiser’s September 2019 statement said the amendment gave more weight to the limits of applying that evidence, given the definitions of ME/CFS the trials had used, and that the review still rested on a question and methods from 2002. Cochrane promised a full update with patient-advocacy partners. An editorial note dated December 2024, amended in early 2025, says a pilot project to involve stakeholders in that update has been discontinued. The 2019 review was never withdrawn.

England moved furthest. In a November 2020 announcement of its draft, the National Institute for Health and Care Excellence (NICE) said no program based on fixed incremental increases in physical activity, GET included, should be offered; that CBT is not a cure but may help as supportive care; and that ME/CFS can be confirmed after three months of symptoms instead of four. The final guideline, published in October 2021, kept those positions and made PEM a core symptom, according to a 2022 summary by researchers from the CureME group at the London School of Hygiene and Tropical Medicine and the Myalgic Encephalomyelitis Association.

The pushback

The trial’s investigators did not accept the reversal. In 2023, White and 50 co-authors, among them PACE investigators T. Chalder of King’s College London and Sharpe, plus US researchers including D. J. Clauw of the University of Michigan and K. Kroenke of Indiana University, wrote in the Journal of Neurology, Neurosurgery and Psychiatry: “This U-turn in recommendations from the previous 2007 guideline is controversial.”

They argued that the NICE committee created a new definition that downgraded confidence in trial results, set aside trial data on harm in favor of lower-quality surveys, read GET as fixed increments when trials had run it as collaborative and adjusted to symptoms, and endorsed energy management without supporting research. The result, they warned, could be patients denied helpful treatment.

Where does that leave a reader? On the library’s evidence scale, GET for ME/CFS earns the verdict of positive evidence with a contested interpretation: modest, mostly self-reported gains in trials that used older definitions, harms measured with very low certainty, and a reanalysis that disputes even the gains. Neither proven to work nor proven harmful fits the record. How graded and adapted activity is used today, and what pacing means, are taken up in the library’s articles on those subjects; CBT as a treatment is weighed in our coverage of psychological strategies.

What a close-up NIH study found

In February 2024, a National Institutes of Health team based at the National Institute of Neurological Disorders and Stroke (NINDS) published an intensive study in Nature Communications. It sought people whose ME/CFS followed an infection, and a panel had to agree unanimously on each case. Only 10% of those reviewed were accepted, and even among accepted cases another cause emerged over time in 20%. The final comparison was 17 patients against 21 healthy volunteers. Within four years, four of the 17 had recovered fully on their own.

The patients reported more depressive and anxiety symptoms but did not meet criteria for a psychiatric disorder, and their psychiatric histories did not differ. Their maximum grip strength matched the volunteers’, yet they could not hold a moderate grip as long, and that was accompanied by lower activity in a region on the right side of the brain. They also had lower fitness on exercise testing, lower levels of chemicals related to catecholamines, a family of nervous-system messengers, in spinal fluid, and immune differences, some of which differed by sex. NINDS summarizes the study as suggesting that infections may trigger immune problems that lead to chemical changes in the brain.

One phrase set off a fight. On a button-pressing task, patients chose the harder option less often, and the authors concluded that effort preference, which they defined as how much effort a person subjectively wants to exert, rather than fatigue, was the defining motor behavior of the illness. They also reported that patients described sustained effort bringing on PEM, and that conscious or unconscious pacing might explain the results.

A 2025 analysis in Frontiers in Psychology, by three independent scholars and researchers at University College London and Reed College, reanalyzed the public data. Patients completed only 65% of the hard tasks they chose, against 96% for volunteers, and seven of the 15 patients tested had lower completion rates than any volunteer. The critics argue the task measured ability, not willingness. The NIH authors themselves called the study exploratory, said it showed correlation rather than cause, and noted its small size could detect only large effects. It did not show that people with ME/CFS simply don’t want to try.

After COVID-19, and the search for causes

NINDS says ME/CFS most often starts after a viral or bacterial infection, and COVID-19 offered a large test. The NIH-funded RECOVER-Adult study, reported in 2025 by a team whose lead author is at the Bateman Horne Center in Salt Lake City, applied the IOM criteria to self-reported symptoms. Of 11,785 infected participants, 4.5% met them, against 0.6% of 1,439 who were never infected. PEM was the most common ME/CFS symptom among the infected, reported by 24%, and 88.7% of the post-COVID cases also met the study’s long COVID definition.

That 4.5% is a research-cohort figure, not anyone’s personal risk. More than 7,000 of the infected participants enrolled more than 30 days after infection, which may have drawn in more people with lingering symptoms. Long COVID itself is the subject of our article on lasting symptoms after infection.

The search for causes continues. NINDS says NIH published a research roadmap in 2024 naming eight priority areas, from immunity and energy processing to genetics and blood flow. A 2026 analysis by researchers at RTI International notes there are still no established biomarkers and no FDA-approved drugs for ME/CFS, and that gene findings have been inconsistent across studies. CDC adds that stimulant medicines may help concentration for some people but can feed a push-and-crash cycle, a trade-off for the prescriber to weigh. On outlook, NINDS says children and teens are usually more likely than adults to recover partly or fully. How benefits programs treat ME/CFS is a question for our piece on chronic illness and disability.

Questions to ask about activity advice

Because so much hinges on PEM, these questions help test any plan:

  • Did we talk about crashes specifically? Ask whether the clinician has asked about worsening after mental, emotional or sensory effort, not just physical activity.
  • What else has been checked? In the NIH study, another cause surfaced later in one accepted case in five.
  • Have my heart rate and blood pressure been checked lying and standing? That is how CDC says orthostatic intolerance is assessed.
  • If a program is proposed, what sets the pace? Fixed increases on a schedule are what NICE rejected; a plan that adjusts to symptoms is a different thing.
  • Who adjusts the plan, and what happens after a crash?
  • If a medicine is suggested, how will we tell whether it helps or fuels a push-and-crash cycle?

When to get help sooner

A diagnosis covers the symptoms already assessed. New or different symptoms deserve their own look, since causes can surface later. Fainting or nearly fainting when you stand needs prompt evaluation. Some people become housebound or bedbound, CDC notes, and a care plan that accounts for that is worth raising with the care team directly. If the illness has brought thoughts of suicide, call or text 988 to reach the 988 Suicide & Crisis Lifeline, free, confidential and open around the clock. If you are in immediate danger, call 911.

The Takeaway

ME/CFS is defined by what happens after effort, not by how tired someone feels. That feature reshaped the diagnosis in 2015, and CDC and NICE now frame their activity advice around avoiding the crash, while the investigators behind the original exercise trials argue the reversal went too far.

The most useful step is to describe the crash precisely: what triggered it, when it hit and how long it lasted. Then ask how any activity advice accounts for it.

Sources

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