Here is an article which explicitly states some things I had deduced, but had not found in print. Dangerous Curves : A Perspective on Exercise, Lactate, and the Anaerobic Threshold Jonathan Myers; Euan Ashley Chest. 1997;111(3):787-795. doi:10.1378/chest.111.3.787 My bolding. Those changes in lactate levels are particularly significant for ME/CFS patients because we appear to have defects in the mechanisms that clear lactate. Even when it is cleared from peripheral blood it can persist in muscles or cerebrospinal fluid, where it has been found by magnetic resonance spectroscopy (MRS). Other papers suggest that lactate levels are the least reliable of the indicators discussed in this paper. This could be an indirect indication that some researchers had encountered patients with problems clearing lactate.