In muscle abnormalities, the muscle fibres become more easily fatigued after exercise than in people without post-COVID fatigue.
Fatigue is one of the most common symptoms of long Covid.
While most people who catch Covid do not become severely ill and get better relatively quickly, some have long-term problems after recovering from the infection - even if they were not very ill in the first place.
"These abnormalities in the results on objective tests show that fatigue in long Covid is a measurable disease and these tests may, in time, help us understand how changes in the nervous system contribute to fatigue," said Demetris Soteropoulos, Senior Lecturer at Newcastle University, who led the research.
The researchers carried out a battery of behavioural and neurophysiological tests on people suffering from post-Covid fatigue and compared them to people without fatigue. A group of 37 volunteers with post-Covid fatigue underwent a range of well-established non-invasive tests. Their results were compared to those of 52 control subjects, matched for age and sex, who underwent the same tests. The tests which provided 33 sets of data included a startle reaction time test, electrocardiogram and transcranial magnetic stimulation.
"We know that many people have faced criticism or even disbelief when they report long Covid, so by being able to provide an independent measure, we can help medical teams provide continued support," said research associate Anne Baker, who is a co-author of the paper.