"My concern about making post-exertional malaise an absolute criteria for diagnosis is if you don't have a consistent, validated way of measuring it that clinicians can use easily, it's big barrier," Unger explained at the meeting. A few minutes later, when committee member Donna Pearson asked again about PEM being included in the criteria, Unger replied, "I think everybody agrees that it's very characteristic and a very important symptom, and clinicians will tell you that they can recognize it, they can elicit this information from patients, but to make that quantifiable and to make it easily implementable is another question...."
"Sometimes represented as METs or metabolic equivalents, oxygen consumption may be assessed directly using cardiopulmonary exercise testing with measured gas exchange (Milani et al., 2006), or estimated from heart rate or other indicators of effort such as time and/or distance travelled. Assessment of effort is critical when exercise is used as a physiological stressor to elicit symptoms in CFS patients or for assessments of functional capacity as part of clinical trials. Heart rate as a percentage of age-predicted maximum is the most recognized indicator of subject effort for both maximal and submaximal exercise protocols. However, the maximal heart rate response to exercise varies widely in the general population (Balady et al., 2010) and has been shown to be blunted in some subjects with CFS (e.g., VanNess et al., 2003) and also in fibromyalgia (Ribeiro et al., 2011).
"As an alternative to heart rate, the peak respiratory exchange ratio (RER) is acknowledged as the most valid and reliable gauge of subject effort (Balady et al., 2010). Because it can only be obtained from ventilatory expired gas analysis, RER may not be available in all exercise studies. Similarly, submaximal exercise protocols do not provide for the measurement of peak RER. In such instances selecting alternative measures that can accurately assess effort both within and across subjects is particularly important."
In addition, in Table 2 in the paper, the authors cite actigraphy and pedometers for activity assessments.