 It has become increasingly recognized that skeletal muscle abnormalities are common and profound in patients with chronic inflammatory status. Based on the unique character of obstructive sleep apnea OSA, repeated intermittent apnea and hypopnea caused by the upper airway muscles collapse during inspiration, it is logical to speculate some adaptations in skeletal muscle of patients with OSA. The purpose of the first part is to investigate the inspiratory muscle function in patients with different severity of OSA. Participants firstly underwent polysomnography assessments will be recruited. The measurements includes Epworth sleepiness scale, strength and endurance of inspiratory muscles, and fatigue test during either maximal voluntary efforts or in response to cervical magnetic stimulation CMS with simultaneously sEMG recordings. In addition, the exercise capacity with simultaneous sEMG recordings for diaphragm and vastus lateralis during the exercise testing will also be tested. The second part is designed to examine the metabolic characteristics of lower extremity in patients with OSA. Blood samples will be drawn and proton magnetic resonance spectroscopy is used to detect intramyocellular and extramyocellular lipid contents IMCL and EMCL. The third part will detect the effect of exercise training on the measured parameters described as above in patients with OSA. Participants will be randomized into exercise group, inspiratory muscle training group IMT, and CPAP group. Exercise protocol includes 30-minute aerobic exercise at the intensity of 70-85 VO2max, and 20-minute general resistance exercise. IMT group will receive inspiratory muscle strengthening by device. 