Accessory Muscles of Respiration
The accessory muscles of respiration include the scalenes, sternocleidomastoids, pectoral muscles, and abdominal muscles, which are recruited during increased ventilatory demand beyond quiet breathing. 1
Primary Accessory Muscles
Inspiratory Accessory Muscles
- Sternocleidomastoid (SCM): Neck muscle that elevates the sternum and upper ribs
- Scalenes: Anterior, middle, and posterior scalene muscles that elevate the first two ribs
- Pectoral muscles: Assist in rib cage elevation when the upper extremities are fixed
Expiratory Accessory Muscles
- Abdominal muscles: External oblique, internal oblique, rectus abdominis, and transversus abdominis
- These muscles increase intra-abdominal pressure and force the diaphragm upward during active expiration
Clinical Significance
Palpable phasic contraction of scalenes, sternocleidomastoids, pectoral muscles, or abdominal muscles during quiet breathing indicates abnormally elevated respiratory motor output. 1 This is a critical clinical sign that the respiratory system is under stress, whether from:
- Respiratory muscle weakness
- Increased ventilatory demand
- Airway obstruction
- Restrictive lung disease
Functional Context
During normal quiet breathing, the diaphragm and intercostal muscles handle ventilation without accessory muscle recruitment. The accessory muscles are reserved for:
- Exercise and increased metabolic demand
- Respiratory distress or failure
- Compensation for primary respiratory muscle weakness 1
The recruitment pattern is hierarchical—as ventilatory demand increases, the nervous system progressively activates these accessory muscles to maintain adequate ventilation and arterial blood gases. 2, 3
Additional Muscles in Extreme Conditions
In severe respiratory compromise, additional muscles may contribute:
- Upper trapezius
- Latissimus dorsi
- Serratus anterior
These are typically recruited only during maximal respiratory efforts or in pathological states such as spinal cord injury where compensatory mechanisms are maximized. 4