Another study found that despite the increased skeletal muscle CSA and 6MWD, 3-6 months of high-intensity cycling training did not significantly affect serum irisin levels in patients with COPD at different stages (the majority were in Global Initiative for Chronic Obstructive Lung Disease mild-to-moderate [I-II] stages; total of 20 patients) [17]. Here, FNDC5 is linked to chronic obstructive pulmonary disease.