: An understanding of normal joint kinematics is essential to develop treatments that restore normal joint mechanics ; however, reports of kinematics during dynamic motion are rare. This study thus investigated glenohumeral GH translation in vivo during loaded and unloaded shoulder abduction. Nine healthy shoulders were studied from our patient cohort average age, 31 years . We created 3D models of the scapula and humerus using computed tomography CT images and recorded uoroscopic images during active abduction in neutral rotation in the plane of the scapula using a hand-held 3kg weight or no additional load. 3D motions were determined using model-based 3D-to-2D registration. Glenohumeral translation was determined by nding the location on the humeral head with the smallest separation from the plane of the glenoid. The humerus moved an average of 2 mm during arm abduction, from inferior to the center of the glenoid. There were no statistically signi cant differences between the unloaded and loaded conditions. Variability in humeral translation decreased with abduction using both 3-kg and 0-kg conditions, with signi cantly lower variability showing above a 70 GH abduction. We showed that humeral translation to the center of the glenoid maximizes joint congruency for optimal shoulder function and joint longevity. This data should lead to better strategies for shoulder injury prevention, enhanced rehabilitation, and improved surgical treatments.