What is the action of the trapezius muscle in the lifting phase of a standing dumbbell lateral raise?

Concentric humeral abduction is the role of the middle deltoid and supraspinatus; the trapezius acts on the scapula, not directly on the humerus.

While the upper trapezius can produce scapular elevation, the functionally dominant trapezius action during a lateral raise is upward scapular rotation, which involves coordinated upper and lower fiber activity.

Eccentric adduction of the humerus describes the deltoid's role during the lowering phase of a lateral raise, not a trapezius action, and the trapezius does not adduct the humerus.

As the humerus abducts during a lateral raise, scapulohumeral rhythm requires the scapula to upwardly rotate so the glenoid fossa continually faces the moving humeral head and subacromial impingement is avoided. The middle and lower trapezius fibers, working with the serratus anterior, concentrically contract to produce this upward scapular rotation. This is the trapezius's primary functional contribution during lateral and overhead arm elevation movements.