The trapezius muscle, also known as the traps, is a large, diamond-shaped muscle in the upper back and neck. It extends from the base of the skull and the cervical and thoracic vertebrae down to the scapula (shoulder blade) and clavicle (collarbone). The trapezius muscle is responsible for several movements of the shoulder girdle, including shrugging the shoulders, pulling the shoulders down, and rotating the scapula. The muscle is also involved in neck movements, such as extension and lateral flexion of the neck. The trapezius muscle is innervated by the accessory nerve (cranial nerve XI) and works in conjunction with other muscles of the neck and shoulder girdle to produce efficient movements during various activities, such as lifting, carrying, or reaching overhead. Dysfunction or weakness of the trapezius muscle can lead to difficulties with shoulder and neck movements and may contribute to neck pain, headaches, or postural imbalances.