De Becker et al,1 using a bicycle ergometer, found that the maximal workload and oxygen uptake attained by their patients with chronic fatigue syndrome (CFS) were almost half those achieved by the control subjects. The observation that the resting heart rate was higher in patients with CFS, as in other studies, led De Becker et al1 to suggest that "alteration in cardiac function is a primary factor associated with the reduction in exercise capacity in CFS." As someone whose reported2 recovery from CFS was characterized by a dramatic normalization of physical functionality, thanks to the glucocorticoid2 and mineralocorticoid3 properties of licorice, an old remedy for Addison disease,2 I believe that adrenal insufficiency, rather than alteration in cardiac function, may primarily account for the reduction in exercise capacity in CFS.
Baschetti R. Chronic Fatigue Syndrome, Decreased Exercise Capacity, and Adrenal Insufficiency. Arch Intern Med. 2001;161(12):1558-1559. doi: