The primary muscle responsible for extending the head is the trapezius muscle, specifically its upper portion.
The trapezius is a large, paired surface muscle that extends longitudinally from the occipital bone to the lower thoracic vertebrae of the spine and laterally to the spine of the scapula (shoulder blade). It has several functions, including moving, rotating, and stabilizing the scapula (shoulder blade), as well as extending the head.
Here's a breakdown:
- Location: The trapezius is located in the upper back and neck.
- Upper Fibers: The superior or upper fibers of the trapezius are most active in extending the head at the neck. These fibers originate from the occipital bone (base of the skull) and insert onto the clavicle (collarbone) and acromion process (part of the scapula).
- Mechanism: When the upper trapezius fibers contract, they pull the back of the head upwards, causing extension at the atlanto-occipital joint (the joint between the skull and the first vertebra of the neck).
- Synergists: While the upper trapezius is the primary mover, other muscles assist in head extension, including:
- Splenius capitis: Located deep in the back of the neck.
- Semispinalis capitis: Also located deep in the back of the neck.
- Longissimus capitis: Another deep muscle in the neck.
In essence, while other muscles contribute, the upper portion of the trapezius muscle is the main muscle involved in extending the head.
