Assessment of Exercise Response in Chronic Fatigue Syndrome / Myalgic Encephalomyelitis.

Inclusion Criteria: * To be diagnosed by a medical specialist in internal medicine with experience in this disease. For this, the participant must fulfill with the diagnostic criteria of CFS/ME, following the recommended criteria for the diagnosis of 2015. * Prior signing of the informed consent. Exclusion Criteria: * Present any of the diagnoses considered excluding of the CFS/ME, according to the international criteria recommended for clinical diagnosis and the selection of subjects for research: * Primary psychiatric disorders * Somatoform disorders * or Substance abuse * Present any of the absolute or relative contraindications, to perform exercise tests, described in previous investigations: * Decompensated heart failure * Acute myocardial infarction (less than 3 days) * Syncope * Unstable angina * Cardiac arrhythmia poorly controlled * Endocarditis, myocarditis or acute pericarditis * Acute pulmonary edema * Moderate or severe cardiac valvular stenosis * Suspected dissection or dissecting aortic aneurysm * O2 saturation at rest less than 85% * Acute renal failure * Untreated thyrotoxicosis * Acute infection * Uncontrolled hypertension (greater than 200-120 mmHg) * Hypertrophic obstructive cardiomyopathy * High-grade atrioventricular block * Significant pulmonary arterial hypertension * Advanced or risky pregnancy * Significant diselectrolithmia * Severe symptomatic aortic stenosis * Severe anemia * Pulmonary embolism * Acute thrombophlebitis * Traumatologic, orthopedic or neurological pathology that does not allow to walk or cycling. * Psychic incapacity to understand the instructions of the tests. * Present comorbidity of Multiple Chemical Sensitivity Syndrome. * Have performed the peak incremental CPET in the last 3 years.