Well this is some good news. Via the article:

We saw various abnormalities in the muscle tissue of the patients. At the cellular level, we saw that the mitochondria of the muscle, also known as the energy factories of the cell, function less well and that they produce less energy," says Rob Wüst, Assistant Professor at Department of Human Movement Sciences at the VU University. "So, the cause of the fatigue is really biological. The brain needs energy to think. Muscles need energy to move. This discovery means we can now start to research an appropriate treatment for those with long-COVID

Source: Amsterdam UMC