If you look in the mirror and tighten the muscles in your neck, you might notice two long cords standing out on either side of the midline. These are called the sternocleidomastoid muscles. They are among the most prominent and important muscles of the neck, controlling much of its movement and posture.

Anatomy and Location

The sternocleidomastoid (often abbreviated SCM) runs diagonally across the side of the neck. Each person has two of them, one on the left and one on the right.
Each muscle starts at two points:

• The sternum (the breastbone)
• The clavicle (the collarbone)
  From there, it travels upward and backward to attach behind the ear at a bony spot called the mastoid process of the skull. The name “sternocleidomastoid” literally comes from those three attachment points: sterno (sternum), cleido (clavicle), and mastoid.

What They Do

The sternocleidomastoids are responsible for several key movements:

• Turning your head side to side: When one contracts, it rotates your head to the opposite side.
• Tilting your head forward or sideways: When both contract together, they pull your head forward toward your chest.
• Stabilizing the head: They work constantly to keep your head upright and balanced, especially when you move your shoulders or change position.

Why They Sometimes Stand Out

These muscles become visible and cord-like when they are tensed, such as when you turn your head sharply, sing forcefully, or lift heavy weights. In lean or athletic individuals, the SCMs can be clearly defined, forming the two long vertical ridges seen in the front of the neck.

Common Issues

Because of their constant activity, the sternocleidomastoids can become tight, strained, or trigger painful knots. Poor posture, long hours at a desk, or stress-related clenching can overwork them. Tight SCMs can even cause headaches, jaw tension, or a sensation of ear fullness because of their close relationship to nerves and vessels in the neck.

Keeping Them Healthy

Gentle stretching, massage, and maintaining good posture can help keep these muscles relaxed and balanced. Turning your head slowly from side to side, tilting the ear toward each shoulder, and rolling your shoulders backward are simple ways to ease tension.

In Summary

The two long “chords” you feel or see in the front of your neck are the sternocleidomastoid muscles. They connect your head, collarbone, and sternum, guiding your head’s movement and stability. Though often unnoticed until they tighten or stand out, they play a crucial role in every glance, nod, and rotation your head makes.