Accessory Inspiratory Muscles

Accessory inspiratory muscles are skeletal muscles that can supplement the principal muscles of inspiration when respiratory demand increases. During quiet breathing, inspiration is produced predominantly by the diaphragm, assisted by the external intercostal muscles. During deep, forceful, or labored inspiration, additional muscles can be recruited to increase expansion of the thoracic cavity.

Important accessory inspiratory muscles include the sternocleidomastoid, scalene muscles, pectoralis major, pectoralis minor, and serratus anterior. Depending on posture and fixation of their usual movable attachments, other muscles associated with the neck, shoulder girdle, and thoracic wall may also assist inspiration.

Role in Inspiration

Accessory inspiratory muscles increase thoracic volume primarily by elevating the sternum, upper ribs, or other components of the rib cage.

They become increasingly active when ventilation must rise substantially or when the normal mechanics of breathing are impaired.

Principal and Accessory Inspiratory Muscles

Group	Important Muscles	General Function
Principal inspiratory muscle	Diaphragm	Increases vertical dimension of thorax
Intercostal contribution	External intercostals and interchondral internal intercostals	Assist elevation and stabilization of ribs
Accessory inspiratory muscles	Sternocleidomastoid, scalenes, pectoral muscles, serratus anterior	Assist elevation and expansion of thoracic cage during increased respiratory effort

When Accessory Muscles Are Recruited

Accessory inspiratory muscles are recruited when inspiratory requirements exceed those of quiet resting ventilation.

Recruitment can occur during vigorous exercise, voluntary deep inspiration, respiratory distress, increased airway resistance, or other situations requiring increased inspiratory effort.

Sternocleidomastoid

The sternocleidomastoid is a prominent superficial muscle of the neck extending from the sternum and clavicle to the mastoid process and superior nuchal line.

Although its major functions involve movements of the head and cervical spine, it can act as an accessory inspiratory muscle when the head and neck are stabilized.

Respiratory Action of Sternocleidomastoid

With its cranial attachment stabilized, contraction of the sternocleidomastoid can elevate the sternum and medial clavicle.

This assists expansion of the superior thoracic cage during forceful inspiration.

Sternocleidomastoid Attachments

Feature	Anatomy
Inferior attachments	Manubrium of sternum and medial clavicle
Superior attachments	Mastoid process and lateral superior nuchal line
Respiratory effect	Assists elevation of sternum and upper thoracic cage

Scalene Muscles

The scalene muscles are a group of lateral cervical muscles extending between the cervical transverse processes and the upper ribs.

The group consists primarily of the anterior scalene, middle scalene, and posterior scalene muscles.

Anterior Scalene

The anterior scalene descends from cervical transverse processes to the first rib.

When the cervical spine is stabilized, contraction can elevate or stabilize the first rib during inspiration.

Middle Scalene

The middle scalene also extends from cervical transverse processes to the first rib.

Its contraction can assist elevation of the first rib and stabilize the superior thoracic aperture during increased inspiratory effort.

Posterior Scalene

The posterior scalene attaches inferiorly to the second rib.

When the cervical region is fixed, it can assist elevation of the second rib during inspiration.

Respiratory Actions of the Scalenes

Muscle	Inferior Attachment	Respiratory Contribution
Anterior scalene	First rib	Elevates or stabilizes first rib
Middle scalene	First rib	Elevates or stabilizes first rib
Posterior scalene	Second rib	Assists elevation of second rib

Fixation of the Upper Ribs

The scalene muscles are important not only because they can elevate the first two ribs but also because they can stabilize them.

A stable upper rib cage provides a mechanical foundation from which other inspiratory muscles can act on more inferior ribs.

Pectoralis Major

The pectoralis major is a large superficial muscle of the anterior thoracic wall. Its usual actions are primarily on the humerus.

When the upper limbs are fixed, however, the muscle can reverse its usual action and contribute to elevation of the thoracic cage during forceful inspiration.

Respiratory Action of Pectoralis Major

Fixation of the humerus allows the pectoralis major to act through its thoracic attachments rather than moving the upper limb.

This mechanism can assist expansion of the chest during labored breathing, particularly when a person braces the upper limbs against a stable surface.

Pectoralis Minor

The pectoralis minor lies deep to pectoralis major and extends from the upper ribs to the coracoid process of the scapula.

Its usual action involves stabilization and movement of the scapula, but its action can be reversed when the scapula is fixed.

Respiratory Action of Pectoralis Minor

With the scapula and coracoid process stabilized, pectoralis minor can exert an upward pull on its rib attachments.

This can assist elevation of the upper ribs during deep or labored inspiration.

Serratus Anterior

The serratus anterior extends between the lateral surfaces of the upper ribs and the medial border of the scapula.

Its principal actions involve protraction and stabilization of the scapula, but with the scapula fixed it can contribute to elevation of the ribs.

Respiratory Action of Serratus Anterior

When the shoulder girdle is stabilized, serratus anterior can act through its costal attachments to assist expansion of the thoracic cage.

This action is most relevant during forceful inspiration rather than normal quiet breathing.

Fixation of the Upper Limbs

Several accessory inspiratory muscles can assist respiration effectively only when their usual movable attachments are stabilized.

Fixing the arms or shoulder girdles allows muscles that normally move the upper limbs to reverse their action and instead elevate the ribs or thoracic wall.

Tripod Position

During significant respiratory effort, a person may support the upper limbs on the knees, a table, bed, or another stable surface.

This posture stabilizes the shoulder girdles and upper limbs, allowing muscles such as pectoralis major, pectoralis minor, and serratus anterior to contribute more effectively to thoracic expansion.

Thoracic Expansion

Accessory inspiratory muscles supplement the diaphragm by increasing dimensions of the rib cage.

Elevation of the ribs and sternum can increase both the anteroposterior and transverse dimensions of the thoracic cavity.

Pump-Handle Movement

Elevation of the upper ribs and sternum increases the anteroposterior diameter of the thorax.

This movement is commonly described as the pump-handle movement of the ribs and sternum.

Bucket-Handle Movement

Elevation of the more lateral portions of the ribs increases the transverse diameter of the thoracic cavity.

This movement is commonly described as the bucket-handle movement.

Superior Thoracic Aperture

The first and second ribs form important components of the superior thoracic framework.

Scalene contraction can stabilize or elevate these ribs, supporting expansion of the remainder of the thoracic cage during increased inspiratory effort.

Relationship to the Diaphragm

The diaphragm remains the dominant inspiratory muscle under most circumstances.

Accessory muscles do not replace normal diaphragmatic action in healthy quiet breathing. Instead, they supplement diaphragmatic contraction when greater thoracic expansion and inspiratory pressure are required.

Relationship to the External Intercostals

The external intercostal muscles help elevate and stabilize the ribs during inspiration.

Accessory inspiratory muscles can reinforce this rib-elevating action, particularly during deep inspiration or increased ventilatory demand.

Sequence During Forceful Inspiration

1. The diaphragm contracts and descends.
2. Inspiratory intercostal activity stabilizes and elevates the ribs.
3. Scalene muscles stabilize or elevate the first and second ribs.
4. Sternocleidomastoid can elevate the sternum and upper thoracic cage.
5. With the upper limbs fixed, pectoral and serratus muscles can assist rib elevation.
6. Thoracic volume increases.
7. Intrapleural pressure becomes more negative.
8. Alveolar pressure falls relative to atmospheric pressure.
9. Air enters the lungs.

Accessory Muscle Recruitment During Exercise

Ventilation increases substantially during vigorous physical activity.

Accessory inspiratory muscle recruitment can increase inspiratory flow and assist expansion of the thorax when the respiratory system must move larger volumes of air.

Deep Inspiration

A voluntary maximal inspiration requires greater thoracic expansion than quiet tidal breathing.

Accessory muscles can assist elevation of the ribs and sternum as the lungs approach larger inspiratory volumes.

Labored Breathing

Accessory muscle activity may become prominent when the work of breathing increases.

Visible contraction of the sternocleidomastoid and scalene muscles can accompany forceful inspiratory efforts when greater pressure must be generated to draw air into the lungs.

Increased Airway Resistance

Narrowing or obstruction of the airways increases the pressure required to generate airflow.

Recruitment of accessory inspiratory muscles can help generate greater negative intrathoracic pressure during inspiration.

Reduced Lung Compliance

When the lungs are more difficult to expand, inspiratory muscles must perform greater mechanical work.

Accessory muscle recruitment can supplement the diaphragm and intercostal muscles under these conditions.

Respiratory Distress

Prominent accessory muscle use is an important physical sign of increased work of breathing.

Contraction of cervical and upper thoracic muscles may become visible as the respiratory system recruits additional muscular force to maintain ventilation.

Sternocleidomastoid Innervation

The sternocleidomastoid receives motor innervation primarily from the spinal accessory nerve, with cervical spinal nerve contributions carrying proprioceptive and sensory fibers.

Its respiratory action depends on the same muscle being recruited in a different mechanical context from its usual head and neck functions.

Scalene Innervation

The scalene muscles receive motor innervation from branches of the anterior rami of cervical spinal nerves.

The exact segmental contributions vary among the individual scalene muscles.

Pectoralis Major Innervation

Pectoralis major is innervated by the lateral and medial pectoral nerves derived from the brachial plexus.

Its contribution to inspiration depends on stabilization of the upper limb.

Pectoralis Minor Innervation

Pectoralis minor receives innervation predominantly from the medial pectoral nerve.

When the scapula is stabilized, contraction can act on the rib attachments rather than primarily moving the scapula.

Serratus Anterior Innervation

Serratus anterior is supplied by the long thoracic nerve.

The muscle normally stabilizes and moves the scapula, but a fixed scapula allows its costal attachments to participate in respiratory mechanics.

Innervation Summary

Muscle	Principal Innervation
Sternocleidomastoid	Spinal accessory nerve with cervical contributions
Scalenes	Anterior rami of cervical spinal nerves
Pectoralis major	Medial and lateral pectoral nerves
Pectoralis minor	Predominantly medial pectoral nerve
Serratus anterior	Long thoracic nerve

Mechanical Reversal of Muscle Action

Many accessory inspiratory muscles normally move the head, neck, scapula, or upper limb rather than the ribs.

When one attachment is fixed, however, the direction of effective movement can reverse. This principle allows muscles attached to the thorax to act on the rib cage during respiratory effort.

Posture and Accessory Muscle Function

Posture can substantially influence the mechanical effectiveness of accessory inspiratory muscles.

Stabilization of the head, neck, shoulder girdle, or upper limbs can provide fixed points from which these muscles elevate the sternum or ribs.

Clinical Observation

Accessory muscle recruitment can often be observed during physical examination.

Prominent sternocleidomastoid contraction, scalene activity, elevation of the upper chest, or bracing of the upper limbs can indicate increased inspiratory effort.

Accessory Muscle Fatigue

Prolonged high respiratory workloads can fatigue respiratory muscles.

Because accessory muscles are recruited to supplement the principal inspiratory muscles, sustained prominent use can reflect substantial respiratory mechanical demand.

Neuromuscular Weakness

Weakness affecting accessory inspiratory muscles can reduce the reserve available for forceful inspiration.

The clinical effect depends heavily on the function of the diaphragm and other respiratory muscles, because accessory muscles normally provide supplementary rather than primary resting ventilation.

Cervical Spinal Injury

Injury involving cervical neural structures can affect the muscles of the neck and upper thorax as well as diaphragmatic function, depending on the neurological level and extent of injury.

The resulting respiratory impairment reflects the combination of muscles and neural pathways affected.

Comparison with Expiratory Muscles

Feature	Accessory Inspiratory Muscles	Active Expiratory Muscles
Primary effect	Increase thoracic volume	Decrease thoracic volume
Rib action	Generally elevate or stabilize ribs	Generally depress ribs
Examples	Sternocleidomastoid, scalenes, pectoral muscles, serratus anterior	Interosseous internal intercostals and abdominal muscles
Major recruitment	Deep or labored inspiration	Forced expiration

Key Anatomical Relationships

• The diaphragm is the principal muscle of inspiration.
• Accessory inspiratory muscles supplement normal inspiratory musculature when respiratory demand increases.
• Sternocleidomastoid can elevate the sternum when the head and neck are stabilized.
• Anterior and middle scalenes act on the first rib.
• Posterior scalene acts on the second rib.
• Pectoralis major can assist inspiration when the upper limb is fixed.
• Pectoralis minor can elevate ribs when the scapula is stabilized.
• Serratus anterior can assist rib elevation when the scapula is fixed.
• Fixation of the shoulder girdle can reverse the usual mechanical action of several muscles.
• Accessory muscle recruitment increases during exercise, deep inspiration, and labored breathing.
• Visible accessory muscle use can indicate increased work of breathing.
• Accessory inspiratory muscles primarily increase thoracic volume by assisting elevation of the ribs and sternum.

Clinical Importance

Accessory inspiratory muscles provide additional mechanical capacity when the diaphragm and ordinary inspiratory movements must be supplemented. By elevating or stabilizing the sternum and ribs, they increase the ability of the thoracic cage to expand during forceful inspiration.

Their activity is particularly important clinically because prominent recruitment can be visible during increased respiratory effort. Stabilization of the upper limbs, as occurs when a person braces the arms during labored breathing, can improve the mechanical effectiveness of several muscles that normally act on the shoulder girdle.

Understanding accessory inspiratory muscles provides the anatomical foundation for studying forced inspiration, thoracic mechanics, rib elevation, respiratory distress, accessory muscle recruitment, cervical and pectoral muscle anatomy, and the mechanics of ventilation.