08/02/2026
"Long COVID can affect the body’s ability to produce and sustain energy. Common symptoms include profound fatigue, muscle weakness, breathlessness, dizziness, brain fog, disrupted sleep and fluctuations in heart rate.
Many people also experience post-exertional malaise, or PEM. A short walk, household task, conversation or period of concentration may lead to a delayed increase in exhaustion, pain, brain fog, flu-like symptoms or autonomic disturbance. The activity may appear small, while its physiological cost is very high.
What is happening inside the muscles?
Research suggests that oxygen can reach the lungs and circulate through the bloodstream effectively, while the working muscles may have difficulty taking it up and using it.
Muscles use oxygen and nutrients to produce a steady supply of energy. This is called aerobic energy, and it supports sustained activity.
When the muscles cannot use oxygen efficiently, they turn to anaerobic energy - quick energy made from glucose. This supports shorter periods of activity and leads to faster fatigue.
Researchers have also found changes in mitochondrial function. Mitochondria are the parts of our cells that produce energy using oxygen and nutrients. When they work less efficiently, the muscles depend upon anaerobic energy sooner.
Specialised exercise testing has found reduced oxygen extraction in some people with Long COVID, even when their heart and lungs are delivering oxygen adequately. Muscle biopsies have also found:
Reduced capacity to produce energy using oxygen
A greater proportion of fast-fatiguing type IIx fibres
More hybrid type IIa/IIx fibres
Changes within the muscles following strenuous activity
These findings help explain why ordinary activity can require so much effort and why recovery may take so long. Long COVID varies between individuals, and circulation, immune activity, autonomic regulation, oxygen exchange and mitochondrial function may all contribute.
Slow and fast muscle fibres
Type I, or slow-twitch, fibres are designed for gentle, sustained activity. They use oxygen efficiently, contain many mitochondria and resist fatigue. They support posture, standing, walking and rhythmical movement.
Type II fibres provide greater force and speed. Type IIa fibres combine strength with some endurance, while type IIx fibres produce rapid power and fatigue quickly.
The nervous system recruits slow fibres first during gentle, low-force movement. As more force or speed is required, it adds progressively faster and more powerful fibres.
Muscle biopsies suggest that some people with Long COVID have a greater proportion of fast-fatiguing fibres. ...."
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