Also found in: Medical.
the skeletal muscles that increase and decrease the volume of the thorax during respiration. The main work of the respiratory muscles is done during inspiration, since expiration occurs passively, as a result of the pull of the thoracic walls and the elasticity of the lungs and intercostal muscles. (The internal intercostal muscles and the abdominal muscles participate in labored exhalation.) The diaphragm is the principal muscle used in inspiration, in which the external intercostal muscles also participate. An increased rate of inspiration is promoted by certain muscles of the neck and the superficial muscles of the chest. Also involved in labored respiration are certain muscles of the back and the shoulder girdle.