The research ethics board suspended the study to investigate a complaint from a community member who is not enrolled in the study. This community member is worried about exercise in this population.There have been no complaints from participants.

Individuals with ME/CFS experience profound exercise intolerance and post-exertional malaise. This remote (app-based) pilot study explores whether light, fully self-paced, swimming may be a tolerable form of movement for people with ME/CFS and related conditions, due to the distinct physiological effects of water immersion. The horizontal posture and hydrostatic pressure of water supports venous return and reduces orthostatic stress, while cool water exposure may influence autonomic and inflammatory responses. We are recruiting adults with mild-to-moderate ME/CFS and related conditions for this study examining short-term symptom and autonomic responses to gentle swimming. Participants will choose their own intensity and duration and may stop at any time. A light cycling session is available as an optional comparator for those who feel comfortable doing so. [Note: this is not an exercise training or rehabilitation study, and participation is only intended for individuals who can tolerate some gentle activity and can be in public spaces without triggering post-exertional malaise. You should be comfortable with swimming, but flotation or other assistive devices are welcome]

Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a debilitating multisystem disorder characterized by extreme fatigue, sleep dysfunction, pain, and post-exertional malaise and/or post-exertional fatigue, and is accompanied by orthostatic intolerance, and neuroendocrine and immune manifestations. The defining feature of ME/CFS is the delayed onset (~12-48 h) exacerbation of symptoms following physical, cognitive, orthostatic, and even emotional exertion, termed post-exertional malaise (PEM). As such, despite the known benefits of exercise for improving function in most disease states including other multisystem inflammatory disorders like sepsis, exercising with ME/CFS is generally contraindicated to avoid a worsening of the condition.

It has been demonstrated that neurovascular dysregulation underlies much of the exercise intolerance in ME/CFS. When neurovascular function is normalized with pharmacologic cholinergic stimulation in ME/CFS patients performing upright cycling exercise, cardiac output, right atrial pressures, and maximal oxygen consumption are improved, possibly preventing post-exertional malaise. Water immersion can also be used to normalize cardiac output and right atrial pressures, as the prone/supine postures and hydrostatic pressure of the water act to passively encourage venous return. Furthermore, the human dive reflex (when the face is submerged in cool water) increases parasympathetic activation to the heart, slowing down heart rate. There is also evidence of anti-inflammatory effects of swimming in patients with cardiovascular, neurological, and rheumatological diseases, which could also improve inflammation in ME/CFS patients. The purpose of this study is to determine whether light-to-moderate swimming may be a more tolerable form of movement for individuals with ME/CFS.

Inclusion Criteria For ME/CFS participants:

• Be between the ages of 18-80
• Meet the criteria of (ME/CFS). The diagnostic criteria for ME/CFS according to the primary Canadian Consensus Criteria are: 1. Fatigue (significant physical and mental fatigue that is new onset, unexplained, persistent or recurrent, and substantially reduces activity level), 2. Post-exertional malaise and/or post-exertional fatigue (general feeling of discomfort, weakness and/or fatigue, and potentially worsening associated symptoms, following physical or mental exertion; slow recovery which is usually longer than 24 hours), 3. Sleep dysfunction (unrefreshing sleep or disturbances in sleep quantity or rhythm), and 4. Pain (significant degree of muscle and/or joint pain, and/or significant headaches of new type, pattern or severity). This may include individuals with (but not limited to) Post-Viral Fatigue Syndrome, Long-Covid 19, Overtraining Syndrome (OTS), and/or co-occurring Postural Orthostatic Tachycardia Syndrome (POTS), Fibromyalgia, and/or Mast Cell Activation Syndrome (MCAS).
• Experience some degree of post exertional malaise and have some degree of exercise intolerance upon physical exertion.
• Must have been previously physically active and be able to swim
• Must be considered moderate-to-mild CFS (not severe or very severe) with a FUNCAP27 score between 3.0-5.8.
• Must be able to do some light physical activity and go to public places without causing post-exertional malaise.

Exclusion Criteria for all participants:

• Individuals with cardiovascular, pulmonary, metabolic, renal, endocrine, autoimmune, neurological, inflammatory condition, infectious disease, or mental illness that makes exercise participation a risk will be excluded from the study.
• Individuals who are pregnant will be excluded from the study.
• Individuals with musculoskeletal injuries that prevent exercise participation will be excluded from the study

Inclusion Criteria for healthy controls:

• Be between the ages of 18-80
• Must be physically active and be able to swim.