Several differences have been found between people with and without ME/CFS, including people with other diseases or who are sedentary. On the first test, people with ME/CFS exhibit lower performance and
heart rate, and on the second test, performance is even lower, while for controls, it is the same or slightly better. The largest decrease is in anaerobic threshold, which signifies a shift from
aerobic to
anaerobic metabolism at a lower level of exertion, and is not influenced by effort. Peak power output, heart rate, and VO2max also decrease, and in ME/CFS, but effort and familiarity with the test may affect VO2max and power. A 2-day CPET is hypothesized to measure PEM and its effect on physical functioning. However, its utility has not been completely confirmed, as many studies of it have been small. The cause of decreased performance is not understood. Proposals include impaired oxygen transport, impaired aerobic metabolism, impaired microvascular blood flow, loss of passive microvascular filtration and mitochondrial dysfunction. The test provokes symptoms by design, and recovery may be prolonged. In some cases, it may worsen the illness long-term, as such it should only be applied to patients when absolutely necessary. == Criticisms ==