Comparable RCTs of clinical therapeutic effects using respiratory physiology-oriented IMT device in different operation principle in COPD are so far lacking. Therefore the investigators perform an adequately powered RCT on the effects of two IMT device application as an pulmonary physical therapy in patients with COPD with inspiratory muscle weakness.

Chronic obstructive pulmonary disease (COPD) is defined as sustained expiratory flow limitation small airway disease which is a major cause of morbidity and mortality worldwide. Dyspnoea is the most vital symptom lead to exercise-limiting of COPD which is the major reason of physical activities reduction and consequently contribute to dysfunction of skeletal muscle including diaphragm. Diaphragm sarcopenia is well documented as an important extrapulmonary manifestation of COPD. It has been suggested that reductions in exercise capacity are conversely related to the muscle weakness present in these patients and consequently impact negatively on quality of life. It also contributes to hypoxaemia and hypercapnia in progressive stage.

Mutiple pulmonary rehabilitation programs are regular physical care for stable patients with COPD to improve extrapulmonary disease manifestations. Inspiratory muscle training (IMT) serve as one of pulmonary rehabilitation program that aim at diaphragmatic improvement has been used constantly and is extensively studied in recent years in stable patients with COPD.

From meta-analyses of randomised controlled trials (RCTs) in patients with COPD, it can been concluded that IMT as a stand-alone therapy was proved to increase inspiratory muscle strength and endurance, decrease dyspnea, improve exercise capacity and the quality of life. It was also concluded that COPD patients with inspiratory muscle weakness which defined as PI,max< 60 cmH2O were more likely to significantly improve inspiratory muscle strength and functional exercise capacity when IMT was applied.

IMT is defined as persistent breathing training using the inspiratory training device. Inspiratory muscle trainers were classified into inspiratory resistive trainers® and inspiratory threshold trainers® according to operation principle. The former contain build-in spring-loaded valve which provides a continuous, pre-determined inspiratory load during the entire inspiratory phase; The latter do not provide a constant inspiratory load to ensure the intensity of inspiratory training. There are great differences in operation principle between the two inspiratory muscle trainers above. Inspiratory resistance that generated by inspiratory resistive trainers depend on inspiratory flow rate, however threshold load is independent of inspiratory flow. Intensity of loads in the two IMT devices can be repectively adjust.

However, therapic effect of IMT remains undefined due to disunity of IMT devices and loads in previous studies. Various devices and loads arouses different training intensity which is the key point of securing training effect. The optimized IMT program with proper devices and loads should be figure out. The investigators' previous study elaborated the detailed respiratory physiological response to IMT in COPD. Comparable RCTs of clinical therapeutic effects using respiratory physiology-oriented IMT device in different operation principle in COPD are so far lacking. Therefore the investigators perform an adequately powered RCT on the effects of two IMT device application as an pulmonary physical therapy in patients with COPD with inspiratory muscle weakness.

Inclusion Criteria:

1. Severe and very severe COPD (postbronchodilator FEV1/FVC under 70% and FEV1 under 50% of the predicted value (GOLD C and D group);
2. Inspiratory muscle weakness（Maximal Inspiratory Pressure under 60cmH2O）;
3. Bronchial dilation test (BDT) negative;
4. No history of pulmonary rehabilitation.

Exclusion Criteria:

1. severe orthopaedic problems having a major impact on daily activities;
2. previous inclusion in rehabilitation programme (<1 year);
3. diagnosed psychiatric or cognitive disorders
4. progressive neurological or neuromuscular disorders.