The accessory muscles are the subclavian sternocleidomastoid (pulls the clavicle and sternum upward to increase the chest cavity superiorly), the scalene muscles (three scalene muscles help raise the uppermost two ribs) and the pectoralis minor .
Inspiratory muscles: The main ones are diaphragm and muscles Intercostal external . Their function is to produce increased ribcage. The contraction of the diaphragm promotes the reduction of the bottom of the ribcage, which expands vertically. The external intercostal muscles elevate the cervical and anterior rib cage, changing the angle of the ribs and elongating the anteroposterior thickness of the rib cage. Inspiration is an active phenomenon ribcage expansion, mainly due to contraction of the inspiratory muscles, which constitute a true respiratory pump.
Expiratory muscles: Principals Are abdominals and internal intercostal . Their function is to produce the decrease in rib cage. The abdominal pull down the chest reducing the thickness and force the upward displacement of the abdominal contents, which also pushes up the diaphragm decreasing the size of the thoracic cavity. The internal intercostal exert traction ribs down, it decreases the size of the chest. Normally the expiration is passive and occurs mainly by relaxing the diaphragm.