What action occurs at the glenohumeral joint during the concentric phase of the dumbbell fly?

During the concentric phase of the dumbbell fly, the primary action at the glenohumeral joint is horizontal adduction. This movement occurs as the arms are brought together in front of the body while lying on the bench, effectively engaging the pectoral muscles.

Horizontal adduction involves the movement of the arm towards the midline of the body in a horizontal plane, which is exactly what happens as the dumbbells are raised from the sides toward the center. This motion contrasts with horizontal abduction, which would involve the arms moving away from the body, and would not apply in this phase of the exercise.

Flexion and extension are terms typically associated with movements in a vertical plane, primarily affecting frontal movements (like raising arms overhead or moving them back down), and are not relevant to the horizontal component of the dumbbell fly. Thus, the correct identification of the action occurring at the glenohumeral joint during this part of the exercise is horizontal adduction.