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There are objective indicators of maximal effort built into the CPET. One of those is the Respiratory Quotient (or Respiratory Exchange Rate), which is the ratio of CO2 exhaled to O2 inhaled. This cannot be controlled or faked by the patient. If the RQ gets to 1.1 or higher, the patient has exerted maximal effort, and accusations of slacking off are completely absurd at that point...
There are disputes within the research community, unrelated to ME/CFS, about exactly what constitutes the threshold for this switch, which we would do well to avoid, where possible.