People with prolonged Covid who feel tired, unwell and painful for long periods after exercise should avoid intense exercise, researchers said.
Long is Covid thought to affect tens of thousands of people in the UK alone and a problem many experience is a worsening of symptoms for weeks after a single exercise.
Now experts say they have evidence that biological changes are to blame, such as severe muscle damage, mitochondrial problems and the presence of microclots in the body.
“It really confirms that there is something wrong in the body with the disease,” says Dr. Rob Wüst, an author of the study at VU University Amsterdam.
Published in the journal Nature Communications, the study involved 25 patients with prolonged Covid who reported experiencing malaise after exercising, and 21 people who had Covid but had fully recovered. None of the participants were hospitalized with Covid, while all were fit and healthy before contracting the virus and were of working age.
Each participant spent approximately 10-15 minutes on an exercise bike, while blood samples and skeletal muscle biopsies were taken from all participants a week before and the day after the task.
The results show that although there was considerable variation between patients, people with long Covid had on average a lower exercise capacity than healthy participants.
When the researchers analyzed the biopsies taken before exercise, they found that those with long Covid had a higher percentage of white fibers in their muscles than healthy participants. These fibers have fewer power-producing structures, known as mitochondria, within their cells, and fewer capillaries.
The team also found signs that the mitochondria in people with prolonged Covid did not work as well as in healthy participants.
Wüst said the findings partly explain why people with long Covid had a lower capacity for exercise.
The team also found that people with long Covid had more clumps of a protein called amyloid in their skeletal muscles, although there was no evidence that these “microclots” were blocking blood vessels as some researchers had previously suggested.
Comparison of the biopsies taken before and after cycling revealed that the function of the mitochondria actually worsened after exercise in those with long Covid, while these participants also had much more tissue damage after exercise and signs of the body who tried to recover.
“This could explain, for example, the muscle pain these patients experience after exercise,” said Wüst.
Wüst said the findings emphasize that people with prolonged Covid should not undertake intense exercise.
“It damages your muscles, it worsens your metabolism, and it can explain why you feel muscle soreness and fatigue for weeks after exercise,” he said.
Prof Betty Raman, from the University of Oxford, who was not involved in the study, said her own research suggests that mitochondrial dysfunction may play a role in the fatigue experienced by some long-term Covid sufferers. The latest study adds weight to the idea that treatments that improve mitochondrial health may be beneficial, she said.
But, Raman noted, that may not be the full story: “While it is plausible that fatigue is associated with these metabolic abnormalities, other contributing factors, such as persistent inflammation, may also play a role.”