Respiratory muscle function in health

Abstract

Normally both quiet breathing and augmented ventilation during physical exercise are sustained by the inspiratory muscles around the chest wall. Among these, the diaphragm is most important. The inspiratory muscles of the upper airways must also contract to permit inspiration to occur. The expiratory muscles of the chest wall and abdomen do not normally participate in breathing per se, but are critically important for coughing. The strength of respiratory muscle contraction, as estimated from maximum static inspiratory and expiratory pressures, depends on neural drive, the length at which the muscles contract, and the velocity with which they shorten during contraction. The intrinsic strength of the respiratory muscles also depends on whether they are hypertrophied, or atrophied consequent to weight loss. As a result of the structural and biochemical properties of their muscle fibers and their blood supply, normal respiratory muscles have a high level of endurance. However, even well-trained normal subjects may develop inspiratory muscle fatigue if they are subjected to severe inspiratory resistive loads, hypoxia, high levels of physical exercise at normal atmospheric pressure, or work under hyperbaric conditions. The respiratory muscles of normal subjects respond to training. Strength training regimens enhance respiratory muscle but not ventilatory endurance, and vice versa.(ABSTRACT TRUNCATED AT 250 WORDS)