This study evaluates the correlation between the 6-min walking test (6MWT) with gases measurement, and the peak cardiopulmonary exercise testing (CPET) using incremental cycling with gases and workload measurement, in order to determine if the 6MWT detects impairment in exercise tolerance and if it avoids the post-exertional malaise that the peak CPET causes on decreasing levels of physical activity, in participants affected by chronic fatigue syndrome/ myalgic encephalomyelitis (CFS/ME). Physical activity level at baseline (usual activity, the parcipant will not be given any directions) will be recorded during 7 days, 24 hours/day. Afterwards, the 6MWT will be performed. After this test, the physical activity level will be collected again during 7 days, 24 hours/day. Peak CPET will be carried out 14 days after 6MWT to make sure that the basal levels are recovered, and finally, physical activity level will be collected again during 7 days, 24 hours/day.