Ultrasonographic measurements of acromion-greater tuberosity (AGT) distance have shown to be reliable and valid in the assessment of glenohumeral subluxation (GHS) in patients with stroke. The primary aim of this study was to investigate the effect of arm positions on ultrasonographic measurements of AGT distance. The secondary aim was to assess the intra-rater reliability of AGT distance in different arm positions. Sixteen healthy individuals with a mean age 28 standard deviation 11 years who gave informed written consent were recruited. Four clinically relevant arm positions for patients with stroke were selected: (1) arm hanging freely by the side; (2) forearm on a pillow placed on participants lap with the elbows at 90° flexion and the forearms in pronation; (3) as in position 2, but with the elbow supported and (4) shoulder in 10° of abduction and 60° of flexion with the arm resting on a pillow placed on a table. Repeated measures analysis of variance showed a statistically significant effect of arm positions on mean AGT measurements for the right (F (3, 45) = 51.2666, p < 0.001, effect size = 0.774) and left (F (3, 45) = 51.883, p < 0.001, effect size = 0.776) shoulders. AGT distance was significantly reduced in positions 3 and 4. Within-session intra-class correlation coefficients and the minimum detectable change values ranged from 0.94 to 0.97 and 0.1 to 0.2 cm, respectively, for all four positions. This preliminary study corroborates that supported arm position does reduce the AGT distance in healthy people suggesting ultrasonographic technique has potential to provide objective measurements in clinical management of patients with GHS.