Does the deltoid hyperextend the arm?
The answer to your first question depends partly on what you mean by hyperextend. If you mean extend past the frontal plane, then yes, the posterior fibers of the deltoid will hyperextend the humerus at the glenohumeral joint (shoulder). The trapezius muscle does not have an attachment site on the humerus, so it does not act at the glenohumeral joint. The trapezius muscle inserts onto the clavicle and the scapula. It does not directly move the humerus at the shoulder joint. Rather the trapezius muscle moves the shoulder girdle (scapula and clavicle). The actions of the trapezius muscle are adduction and upward rotation of the shoulder girdle. The upper trap elevates the shoulder girdle, while the lower trap depresses the shoulder girdle. However, full movement of the arm requires movement at both the shoulder girdle and the glenohumeral joint. When you lift your arm straight up to the side, about 1/3 of that movement occurs with upward rotation of the shoulder girdle and 2/3 is due to
