The sternocleidomastoid muscle (SCM) is a prominent muscle located on each side of the neck. It extends from behind the ear down towards the collarbone and breastbone. This muscle plays a role in various head and neck movements, contributing to both stability and mobility.
Understanding the Sternocleidomastoid Muscle
The SCM is visible as two thick cords on the front and sides of the neck. Its name provides a direct clue to its anatomical attachments: “sterno” refers to the sternum (breastbone), “cleido” refers to the clavicle (collarbone), and “mastoid” refers to the mastoid process, a bony prominence behind the ear. This muscle is paired, with one SCM on each side of the neck.
Anatomical Basis of Movement
The SCM originates from two points in the upper chest. One head attaches to the manubrium, the uppermost part of the sternum. The second head originates from the medial (inner) third of the clavicle. These two heads then merge as they ascend the neck.
The muscle inserts onto the mastoid process of the temporal bone, behind the ear. When the muscle contracts, it pulls its insertion point towards its origin, producing head and neck movement.
The Unilateral Action
The SCM’s unilateral action occurs when only one of the paired muscles contracts. This single-sided contraction produces a combined movement of the head. For example, when the right SCM contracts, it pulls the head in two distinct ways simultaneously.
One primary action is the rotation of the head to the opposite side. If the right SCM contracts, the head will turn to the left. This occurs because the mastoid process is pulled, causing the face to rotate away from the contracting muscle. This rotational movement is important for visual scanning and orienting the head.
Simultaneously, unilateral SCM contraction also causes lateral flexion of the head to the same side. When the right SCM contracts, the head will tilt towards the right shoulder. This action occurs as the muscle shortens and pulls the mastoid process closer to its origin. Therefore, a single SCM contraction results in the head rotating to the opposite side while simultaneously tilting towards the contracting muscle.
Functional Importance of Unilateral SCM Activity
The SCM’s unilateral action is important for many everyday movements. It allows individuals to look over their shoulder, turn their head to listen, or position their head to answer a phone. These combined movements enable precise head positioning.
Its unilateral action is also important for maintaining balance and adjusting posture. The SCM helps to stabilize the head during movements of the body, ensuring that visual and auditory input remains consistent. Any imbalance or restriction in the unilateral function of the SCM can impact an individual’s ability to perform these common tasks efficiently.