Final answer:
The trapezius muscle is located in the upper back, extending from the neck to the spine and shoulders. Its functions include moving and stabilizing the neck and shoulders, controlling head extension, and contributing to respiratory movements. Strength testing of this muscle involves shoulder shrugs to check for asymmetry and nerve function.
Step-by-step explanation:
The trapezius is a large, triangular-shaped muscle located in the upper back. It extends from the neck to the middle of the spine and out to the shoulder. This muscle has multiple functions, including controlling movements of the neck and shoulders. The trapezius is responsible for shrugging the shoulders, stabilizing and moving the scapula, and aiding in the extension and rotation of the neck.
When both sternocleidomastoid muscles contract, they flex the head forward, and when they contract individually, they rotate the head to the opposite side. The trapezius acts as an antagonist to these movements by causing extension and hyperextension of the neck. Additionally, the trapezius elevates the scapula and clavicle when the arms move or the shoulders shrug. This muscle is innervated primarily by the spinal accessory nerve, which works in conjunction with cervical spinal nerves to control the muscle's movements.
Testing of the trapezius muscle's strength is commonly performed by having the individual perform shoulder shrugs and observing for any asymmetry, which would indicate potential nerve damage or muscle dysfunction.