Inspiration

• Primary muscles: diaphragm, scalenes, parasternals

• Accessory muscles: sternocleidomastoids, upper trapezius, pectoralis major and minor, subclavius, and possibly the external intercostals

Expiration

• Primary muscles: none active during tidal (resting) expiration

• Accessory muscles: abdominals including the rectus abdominis, transversus abdominis, and internal and external obliques; pectoralis major; and possibly the internal intercostals

Parasternal intercostals.  The parasternals, a portion of the internal intercostals, are active during resting inspiration. They function to stabilize the rib cage and prevent inward movement of the superior aspect of the chest wall.

Accessory muscles of inspiration. The sternocleidomastoid (SCM), upper trapezius, pectoralis major and minor, and subclavius muscles are all active during deep or labored inspiration. These muscles become increasingly active with greater inspiratory effort, which occurs frequently during strenuous physical activity.

The accessory muscles of inspiration may become the primary muscles of inspiration and may become active during resting inspiration when the diaphragm is ineffective or weak as the result of pathology. For example, paralysis of the abdominals as the result of a spinal cord injury reduces the support to the viscera (when the patient is in an upright position) which, in turn, allows the diaphragm to assume a flattened rather than a normal dome-shaped position. Thus, diaphragmatic excursion is reduced, and breathing is less efficient, which necessitates recruitment of the accessory muscles of inspiration.

The SCM muscles elevate the sternum to increase the anteroposterior (AP) diameter of the thorax. In patients with weakness of the diaphragm, the SCM muscles are required to act as primary muscles of inspiration. The upper trapezius muscles elevate the shoulders and, indirectly, the rib cage during labored inspiration. They also fixate the neck so the scalenes have a stable attachment. The pectoralis major muscles can act to elevate the rib cage and contribute to inspiration when the arms are overhead.

Expiration

Expiration is a passive process when a person is at rest. When the diaphragm relaxes after a contraction, the diaphragm rises and the ribs drop. The elastic recoil of tissues decreases the intrathoracic area and increases intrathoracic pressure, which causes exhalation. During active expiration, which can be controlled, forced, or prolonged, several accessory muscles groups are active.

Abdominals. The rectus abdominis, the internal and external obliques, and the transversus abdominis contract to force down the thoracic cage and force the abdominal contents superiorly into the diaphragm. When the abdominals contract, the intrathoracic pressure increases and air is forced out of the lungs. A strong contraction of the abdominals also is necessary for a strong cough. The abdominals are innervated by spinal cord levels T10 to T12.

Other accessory muscles of expiration include the pectoralis major muscles (when the distal insertion is inferior to the clavicle and the arm is fixed in position), the quadratus lumborum, because of its attachment to the twelfth rib, which enables it to act to stabilize the diaphragm during phonation, and possibly the internal intercostals, which may act to depress the rib cage.

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