Substantial reduction or impairment in the ability to engage in pre-illness levels of occupational, educational, social, or personal activities

A substantial reduction or impairment in the ability to continue pre-illness activities is required for most definitions of myalgic encephalomyelitis and chronic fatigue syndrome currently in use.

Inability to "push through"
A person who is able to "push through" (ignore symptoms or carry on but with substantial suffering) while carrying on their usual activities will not meet the diagnostic criteria, but may have diagnosis such as idiopathic chronic fatigue, chronic fatigue (symptom), chronic pain, or chronic fatigue instead. A person with ME or CFS who attempts to continue their pre-illness activities will be physically unable to do so, patients may collapse, experience periodic paralysis, lose too much of their cognitive ability to be able to work, or have delayed symptoms which are significantly more severe. For example, attempting to work full-time but then being forced to spend all weekend bedbound, and being unable to cook meals, clean the home, or engage in social activities or parenting. The fatigue|cognitive fatigue and potentially severe sensory symptoms mean that even those with electric wheelchairs for mobility may have severe difficulties with accessibility in many areas of life.

ME/CFS and co-existing illnesses
A person who experiences a substantial enough reduction in ability to carry out activities that is caused by symptoms of a different illness would also not meet the ME/CFS criteria. For example, a person with a severe mitochondria or neurological disease who experiences that greatly reducces their activity level but has ME/CFS symptoms that do not substantially impair would not meet the ME/CFS criteria. They may well some ME/CFS symptoms added as additional diagnoses.

Millions Missing
The inability to take part in many normal aspects of daily life can result in ME/CFS patients feeling like they are missing and invisible, especially those who become mostly housebound or bedbound. The MillionsMissing patient campaign highlights the involuntary reduction in everyday activities caused by ME/CFS. Patients who cannot attend can ask friends who display a pair of their shoes to represent this.

CDC ME/CFS (since 2017)
A CDC core symptom is: '''Greatly lowered ability to do activities that were usual before the illness. This drop in activity level occurs along with fatigue and must last six months or longer.'''

Clinician's CDC ME/CFS version
Also known as the IOM Systemic Exertion Intolerance Disease criteria (2015). A substantial reduction or impairment in the ability to engage in pre-illness levels of occupational, educational, social, or personal activities that is a result of ME/CFS symptoms, which is a clinicial version of the equivalent CDC criteria for ME/CFS.

NHS ME/CFS criteria (2021)
The NICE ME/CFS diagnostic criteria 2021 is not fully in use yet. It requires that: the person's ability to engage in occupational, educational, social or personal activities is significantly reduced from pre‑illness levels.

Fukuda CFS criteria (1994)
The fatigue must result in: substantial reduction in previous levels of occupational, educational, social, or personal activities. The Fukuda criteria has been retired.

International Consensus Criteria for ME (2011)
This is not typically in clinical use but is preferred by a number of ME patient groups. The operational note in the post-exertional neuroimmune exhaustion (PENE) group of criteria in the ICC states that: ''For a diagnosis of ME, symptom severity must result in a significant reduction of a patient's premorbid activity level. Mild (an approximate 50% reduction in pre-illness activity level), moderate (mostly housebound), severe (mostly bedridden) or very severe (totally bedridden and need help with basic functions). There may be marked fluctuation of symptom severity and hierarchy from day to day or hour to hour. Consider activity, context and interactive effects. Recovery time: e.g. Regardless of a patient’s recovery time from reading for ½ hour, it will take much longer to recover from grocery shopping for ½ hour and even longer if repeated the next day – if able. Those who rest before an activity or have adjusted their activity level to their limited energy may have shorter recovery periods than those who do not pace their activities adequately. Impact: e.g. An outstanding athlete could have a 50% reduction in his/her pre-illness activity level and is still more active than a sedentary person.'' 

Canadian definition (2003)
Mostly used for research, the CCC definition for ME/CFS, which is regarded as the most narrow definition and typically defines a more severely ill group, states that: "The fatigue should be severe enough to substantially reduce the patient's activity level, usually by approximately 50%. When considering the severity of the fatigue, it is important to compare the patient’s activity level to their premorbid activity level. For example, a former world class athlete could have a substantially reduced activity level and still exceed the norms for sedentary persons. Some patients may be able to do some work, but in order to do that they have had to eliminate or severely reduce other aspects of their life activities. Such interactive effects should be considered in the assessment of whether activity reduction is substantial."

Prevalence
100% - all diagnostic criteria with a criterion of a substantial reduction in activity level have this is a compulsory, core symptom.

Symptom recognition
A substantial reduction in pre-illness activity level is recognized symptom of many diagnostic criteria for ME or CFS but is not required by the retired Chronic fatigue syndrome definitions:
 * Oxford criteria

Possible causes

 * Mitochondria dysfunction is known to limit the energy available to patients with ME/CFS, and this cannot be resolved by diet, medication or stimulats
 * Post-exertional malaise is the hallmark symptom
 * Exertion intolerance is a required symptom, this is not simply exercise intolerance but a reduced tolerance to mental / cognitive tasks, and often to sensory stimulus
 * Rest does not significantly relieve symptoms, this is a required symptom, but it can limit them from becoming worse
 * Immune system dysfunction causes immune symptoms, these vary but can be flu-like symptoms, much greater susceptibility to viruses are common
 * Orthostatic intolerance which causes a range of symptoms is very common, and causes symptoms when a person moves from sitting to standing or stands still, some patients need wheelchairs because of this
 * There are management techniques and strategies to limit and manage symptoms but there is no treatment that addresses ME/CFS directly, and there is no cure