Neuromuscular diseases are characterized by the progressive loss of muscle strength, sometimes also affecting the respiratory muscles. When respiratory muscles are involved, the main consequences are ventilatory difficulties, decreased vital capacity, and reduced chest wall expansion. Common symptoms are dyspnoea, tachypnoea, use of respiratory accessory muscles, paradoxical respiration, difficulty in coughing, and infections. The reduction of respiratory muscle functioning is believed to reduce functional capacity, limiting activities of daily living and worsening quality of life. 
The hypothesis of respiratory muscle training relies on the belief that respiratory muscles are skeletal muscles that may respond to training in the same way as any other muscle. Some training techniques are based on the improvement of strength and endurance and can be classified as inspiratory and expiratory muscle training. Strength training involves high-pressure and low-flow loading of specific inspiratory or expiratory muscles, whereas endurance training involves low-pressure and high-flow loads of both inspiratory and expiratory muscles. 
Nevertheless, the effects of respiratory muscle training in people with neuromuscular diseases are uncertain. This Cochrane Review aims to synthesize the available evidence on the effectiveness and safety of respiratory muscle training in this population.
The Review identified 11 studies involving 250 patients with amyotrophic lateral sclerosis, Duchenne muscular dystrophy, Becker muscular dystrophy, limb-girdle muscular dystrophy, and myasthenia gravis. 
The Review is limited by the heterogenicity of the studies as well as the measurements. Respiratory muscle training may produce some improvements in lung function in patients with amyotrophic lateral sclerosis and Duchenne. Regarding physical function and quality of life, respiratory muscle training may have no clear effect, but this aspect was studied only in patients with amyotrophic lateral sclerosis. The studies did not report whether respiratory muscle training had a preventive effect, such as decreasing the number of unscheduled hospitalizations for infections or worsening of respiratory failure. In conclusion, respiratory muscle training may improve lung capacity and respiratory muscle strength in some neuromuscular diseases, but studies did not show any significant effect on physical functioning or quality of life

Comment by Sara Laxe