During the concentric phase of a chin-up (with the forearms in a supinated grip) the glenohumeral joint is moving through ______ in the ______ plane.

Abduction in the frontal plane describes moving the arm laterally away from the midline, as in a lateral raise, which is not the motion occurring at the glenohumeral joint during a chin-up.

In the starting position of a chin-up, the shoulder is in a flexed position with the arms overhead. As the body is pulled upward during the concentric phase, the humerus moves posteriorly and inferiorly relative to the torso, which constitutes glenohumeral extension. This motion occurs in the sagittal plane, which divides the body into left and right halves and governs anterior-posterior limb movements.

Flexion in the sagittal plane would mean moving the arm forward and upward from a neutral position, which is the opposite of the concentric chin-up movement.

Adduction in the frontal plane describes bringing the arm toward the midline in a lateral arc, which is not the dominant glenohumeral motion in a narrow supinated-grip chin-up.