[Muscle fatigue and dyspnea].

Calm breathing requires little effort on the part of the inspiratory muscles to overcome the elastic and non-elastic resistances of the chest-lung system. Spontaneous expiration is passive and does not engage the respiratory muscles. In the event of restriction or obstruction, however, there are changes in these two types of resistance and their distribution. Extra work is required and the blood flow through the muscles has a decisive influence on their performance. Histologically detectable differences in the number of fast or slow, glycolytic or oxidative fibres in the respiratory muscles correspond to functional differences. Some muscles are required to supply the force or energy of contraction, others are concerned with endurance and others with intermediate situations. The diaphragm is mainly composed of slow-oxidative fibres. It is thus characterised by its strength, low tension, high repetitive capacity and incredible endurance. This brief review describes the main concepts of the physiology of the respiratory muscles. Reference is also made to the mechanisms of muscle fatigue and dyspnea, two independent or associated clinical situations that point to the risk of progression to respiratory insufficiency.