ABSTRACT
The muscles of breathing comprise three main groups: the diaphragm, intercostal/accessory muscles, and the abdominal wall group. These muscles are unique from the other skeletal muscles in three aspects: (1) they are both involuntary and voluntary; (2) they overcome resistive and elastic loads rather than inertial loads; and (3) they must contract regularly without prolonged rest throughout life.1 The respiratory muscles work as either prime movers or stabilizers of the chest wall to promote inspiration and exhalation. The inspiratory muscles of breathing function in a coordinated effort to enlarge the thoracic cavity, creating a negative intrapleural pressure and inflating the lung. The
diaphragm is the primary muscle responsible for inspiration during quiet breathing (tidal breathing). Exhalation during quiet breathing is largely passive. The inspiratory action of the diaphragm is dependent on its configuration and the presence of abdominal resistance. In patients with pulmonary compromise, the diaphragm may flatten, weakening its ability to inspire. The diaphragm is innervated at cervical level 3, 4, and 5 (Figure 41.1). The accessory muscles include the scalenes, sternocleidomastoids, and trapezius; these muscles contribute minimally to inhalation during tidal breathing. DeTroyer et al.2 speculated that the scalenes may play an active role by stabilizing the rib cage during quiet breathing. In cases of cervical paralysis, such as that seen in those with tetraplegia, the pectoralis major has been shown to participate in exspiration.3
