I also get this lactic-acid feeling in my muscles and other patients suffer from weakness such as not being able to walk. But there are several reasons to think these are central rather than peripheral: due to things happening in the brain rather than in the muscle.
'oxygen tab gets closed down' is something that researchers would be able to measure for example by abnormal lactacte buildup, tissue damage or metabolic shifts which haven't been found in ME/CFS. Similarly the small and preliminary studies that we have that can simulate the muscle electrically suggest it did not reach its limit and that this limit comes from the brain.
Lastly, if there were a pathology in the muscle it would be pretty complicated to connect this to other symptoms such as photophobia or hyperacusis, gut sensitivity, orthostatic intolerance. So it would need to be connected to neurology anyway and probably to other body systems in order to make sense. And this all while remaining hidden for our current technology and tests, even at its greatest severity when it almost paralyses patients by the severity of symptoms!
If the lactic-acid feeling and weakness, however, could be caused by the brain almost everything would fall into place. it would also help to explain why PEM could be caused by any time of exertion, not just physical activity.