Function of the Sternocleidomastoid Muscle
The sternocleidomastoid muscle functions primarily as an accessory muscle of respiration during respiratory distress and as a motor for head and neck movements, including contralateral rotation, ipsilateral flexion, neck flexion, and head stabilization.
Primary Respiratory Function
The sternocleidomastoid serves as a critical accessory muscle of respiration when increased ventilatory effort is required:
During respiratory distress, the bilateral contraction and relaxation of the sternocleidomastoid (along with scalene muscles) generates more negative intrapleural pressures to maintain adequate tidal volumes when lung compliance is reduced 1.
This accessory muscle recruitment produces visible clinical signs including head nodding (in young children with limited head control) and tracheal tugging, both resulting from sternocleidomastoid contraction during inspiration 1.
The muscle becomes particularly important in conditions with "stiff" lungs (low compliance), such as pneumonia, where greater inspiratory force must be generated 1.
In sports contexts, higher sternocleidomastoid muscle activity during jumping headers helps stabilize the head-neck complex on impact 1.
Motor Functions for Head and Neck Movement
The sternocleidomastoid produces multiple head and neck movements through various contraction patterns:
Unilateral contraction produces contralateral (opposite side) rotation of the head 2.
Unilateral contraction also produces ipsilateral (same side) lateral flexion of the neck 2.
Bilateral contraction produces neck flexion and head protraction (forward movement) 2.
The muscle is active during head extension movements as well 2.
Anatomical Basis
Understanding the muscle's anatomy clarifies its functional capabilities:
The sternocleidomastoid connects the manubrium sterni and clavicle proximally to the mastoid process and occipital bone distally 3.
The accessory nerve (CN XI) supplies the sternocleidomastoid and upper trapezius muscle 4, 5.
The muscle consists of sternal and clavicular heads that merge together 3, 6.
Clinical Relevance
Dysfunction or injury to the sternocleidomastoid or its nerve supply produces significant clinical consequences:
CN XI injury causes weakness and atrophy of the sternocleidomastoid, which can be painful and disabling, affecting quality of life 5.
In chronic neck pain, the sternocleidomastoid becomes overactive, and targeted stretching and massage of this muscle reduces pain and disability while increasing range of motion and endurance 7.
The muscle's role in respiratory distress makes its contraction patterns important clinical signs for assessing severity of respiratory illness, particularly in children 1.