The trapezius or “traps” muscle is a large muscle that is shaped like a trapezoid. It is located on the back side of the neck and mid back. It is attached to the base of the skull, out to the scapula (shoulder blade), then down to the lower thoracic spine (mid back). It also has a small insertion to the front part of the shoulder at the clavicle (collarbone). Because of its size, the trapezius muscle is involved in multiple actions of the neck, mid back, and shoulders. Although it is one muscle, it is typically categorized into three sections: upper trapezius, middle trapezius, and lower trapezius. It is the most superficial muscle on the upper back and neck so it is easily felt by palpating (touching, pushing) the upper back and shoulders.
(Red area above shows location of muscle)
The trapezius helps to rotate and side bend the head and neck. It extends the head and neck back when both sides are contracted. It helps to shrug the shoulders, and moves the shoulder blade up, down, and inward (towards the spine).
It is very common to have issues with the trapezius muscles with neck pain. It is a common spot to hold “tension” when your body feels stressed. Common reasons for increased tightness, tenderness, or the formation of trigger points are: head forward posture, sitting for long periods at a desk, rounding shoulders forward, weakness in other neck and mid back muscles, or injury to the neck.
Check out this link to basic neck stretches that will help to decrease tightness in the trapezius muscles, https://structuralhealth.net/2020/04/basic-neck-stretches/. Doing strengthening exercises to your “deep” neck flexor muscles, mid back and shoulders can help the trapezius muscle to function better.