Chronic obstructive pulmonary disease (COPD) and asthma are characterized by airflow obstruction and significant increase of respiratory muscle workload, with concrete risk of ventilatory pump failure. Respiratory muscles, the main component of this pump, undergo structural and functional changes during the course of these diseases. Aim of the present paper is to analyze modifications of respiratory muscles in COPD and asthma. An analysis of the most important controlled clinical studies released during the past years was carried out. The patients suffered from chronic obstructive pulmonary disease and asthma. In COPD, respiratory muscles have to cope with an increased load, an intrinsic weakness and a mechanical disadvantage, especially in the diaphragmatic length-force relationship; in patients with acute asthma, the main features are a massive hyperinflation and a persistent inspiratory muscle activity during expiration. Modifications of respiratory muscles deserve great consideration not only for the complete comprehension of the underlying physiopathologic aspects of these diseases, but also for the optimal clinical management: a reduced pulmonary hyperinflation in COPD place the respiratory muscles in a better position of the force-length curve while great care must be payed to the metabolic and nutritional aspects. During asthmatic crisis respiratory muscles are subjected to a sort of intense training but anyway persistence of bronchospasm in most severe attacks can lead to exhaustion of the ventilatory pump and need of mechanical ventilatory support.