Fluge and Mella used an expensive bit of kit called the Seahorse analyser, which measures glycolysis through the lactate production and mitochondrial activity through changes in oxygen levels.
They tested normal healthy muscle cells that had been grown in the lab. But they added to those cells serum taken from either ME/CFS patients or healthy controls. Serum is the fluid left over after blood has clotted and it contains small molecules and other soluble substances.
They have data for 12 people with ME/CFS and 12 healthy controls, a relatively small sample.What they found was, surprisingly, that the muscle cells produced more lactate and burned more oxygen when they were incubated with ME/CFS serum than when incubated in serum from healthy controls. And the effect was particularly strong when the cells were made to work hard.