The middle trapezius is stretched with protraction of the scapula at the scapulocostal (scapulothoracic) joint.
Note: Medial rotation of the arm at the shoulder (glenohumeral ) joint can, via scapulohumeral rhythm, create increased scapular protraction, thereby increasing the efficacy of the stretch.

The middle trapezius is stretched with protraction of the scapula at the scapulocostal (scapulothoracic) joint.