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Before COVID, you could work, walk, exercise or manage everyday tasks without giving much thought to recovery. Now, those same activities leave you exhausted, your heart rate rises more easily, and returning to your previous routine is taking longer than expected.
You may have been told that illness and rest have left you deconditioned. The explanation sounds reasonable: you have been less active, so gradually increasing activity should help you regain your fitness. But what if those attempts repeatedly make you feel worse?
A discussion published in the British Journal of Sports Medicine examines this question: how much can deconditioning explain Long COVID, and what does that explanation leave out?
What does deconditioning actually mean?
Deconditioning describes physiological changes that develop after reduced activity or prolonged rest. These can include muscle weakness, changes in circulation and a reduced ability to tolerate exertion. Cardiac deconditioning refers specifically to the heart’s adaptations to inactivity.
These changes can contribute to difficulties for someone who has been unwell for an extended period. However, their relationship with the illness matters.
In Long COVID, illness may restrict activity first, with deconditioning developing afterwards. Its presence does not automatically explain why someone initially became unable to tolerate their previous level of activity.
Why is deconditioning an incomplete explanation?
The authors argue that not all physiological findings in Long COVID fit an explanation based on inactivity alone. Studies involving selected groups of patients have identified impaired cardiac filling during exertion and reduced oxygen extraction by peripheral tissues. Inflammatory or structural cardiac changes have also been reported in some patients.
These findings are not present in everyone with Long COVID. However, they illustrate why reduced exercise tolerance should not automatically be attributed to a lack of training. For example, insufficient filling of the heart before it contracts is different from weakness of the heart muscle. Different findings require different explanations and an assessment of what they mean for the individual.
What if trying to regain stamina makes me worse?
Delayed, prolonged symptom worsening after physical or cognitive activity may indicate post-exertional malaise, or PEM.
When planning an activity, it is important to understand both what someone can do and what happens afterwards: which symptoms become worse, when deterioration begins and how long recovery takes.
The World Health Organization recognises this pattern and recommends energy conservation approaches, including pacing, in guided rehabilitation. For someone experiencing PEM, an activity programme that requires predetermined increases in exertion despite worsening symptoms is inappropriate.
Assessment for PEM should therefore come before advice to rebuild stamina. Activity recommendations then need to reflect the individual’s response, including any deterioration over the following days.
Can mitochondrial function tests provide an answer?
Mitochondrial and endothelial dysfunction, autoimmunity and disrupted circulatory regulation are being investigated as potential contributors to Long COVID. No single mechanism has been established that explains every symptom in every patient.
Mitochondrial abnormalities have also been studied in muscle biopsies from people with Long COVID. However, a research measurement in a particular tissue does not automatically establish the value of a commercial test marketed as a “mitochondrial function assessment”.
Before considering such testing, important questions include: what does it measure, how reliable is the result, and can it meaningfully inform a treatment decision? Identifying a possible disease mechanism and developing a clinically useful test are separate steps.
Why does care need to be individualised?
People with Long COVID do not all have the same combination of difficulties. PEM may dominate for one person, while another experiences a racing heart and difficulty remaining upright. Others have problems with breathing, sleep or concentration. Symptoms can overlap and change over time.
Assessment therefore involves considering alternative diagnoses, identifying symptom patterns and involving relevant specialties where appropriate. Recommendations need to be coordinated: improving one symptom may offer limited benefit if the same intervention worsens another symptom.
Non-pharmacological measures also need a clear purpose. For example, compression garments may help selected patients with orthostatic symptoms, but they are not a treatment for every manifestation of Long COVID. Measures should be chosen according to the clinical indication and the person’s tolerance.
How do we assess whether treatment is helping?
Trying a medicine because someone else found it helpful differs from a clinically supervised treatment trial.
Clinical follow-up starts with a defined treatment goal and a record of baseline symptoms and function. Benefits, adverse effects and everyday functioning are then reviewed. Other changes that could influence the apparent response also need to be considered. Where feasible, treatment changes are introduced in a way that allows their effects to be assessed.
At Immuno Centar, where possible, we complement symptom monitoring with data from patients’ own wearable devices. Heart rate and activity readings are interpreted alongside the person’s account of symptoms and recovery. A wearable can help reveal patterns, but it cannot independently identify the cause of symptoms or guarantee that an activity will be tolerated.
This approach supports ongoing reassessment and adjustment of the care plan to the individual.
When should you seek clinical assessment?
If you are struggling to return to your previous activities after COVID, particularly when attempts to increase exertion trigger delayed deterioration, that pattern deserves clinical assessment.
At Immuno Centar, our approach focuses on understanding the individual symptom pattern and tailoring assessment, monitoring and treatment accordingly. We aim to distinguish emerging research findings from tests and interventions that can meaningfully inform clinical care.
To enquire about an assessment, please contact Immuno Centar.
This article is prompted by the discussion by Charlton and colleagues published in the British Journal of Sports Medicine in 2026, with additional sources and clinical commentary from Immuno Centar.
Reference: , et al. Post-exertional malaise and the myth of cardiac deconditioning: rethinking the pathophysiology of long covid.