[en] The strength of respiratory muscles is crucial for quality of life, especially for patients with lung or neurological disorders. Several methods can be used to assess them: i.e direct or indirect, voluntary or involuntary. The conventional spirometry method assesses indirectly the respiratory function by measuring maximum inspiratory and expiratory pressure, which reflects the coordination of all respiratory muscles. Techniques of magnetic and electrical stimulation can detect a specific muscle dysfunction, independently of the patient's motivation, however they are invasive methods as measurements are made in the esophagus and stomach. The voluntary techniques are generally non-invasive and require patient's cooperation. Measures of respiratory muscle strength, coupled to a conventional spirometry, can predict the development of ventilatory support in patients at risk, and may be useful during weaning from mechanical ventilation in intensive care units.