Reliability Study of Weight-Bearing Upper Extremity Sway Test Performed on a Force Plate in the One-Handed Plank Position

(1) Background: The aim of this study is to determine the intra-rater and test–retest reliability of an upper extremity sway test performed in the one-handed plank position on a force plate and to establish if there are any between-limb differences in assessment with either open or closed eyes. (2) Methods: Fifteen males participated in this study (23.1 ± 2.3 years; 71.2 ± 7.4 kg; 175 ± 4 cm). The sway test (ST) on the force plate was performed during two separate visits, with a one-week interval between tests performed with eyes open and closed, separately for the left and right upper limbs. (3) Results: Intra-rater reliability for measurements with eyes open and closed on both the left and right sides was good to excellent for almost all variables (ICC = 0.84–0.96). Moderate reliability was observed only for the center of pressure (COP) area. Test–retest reliability was somewhat lower. With eyes open and closed, good to excellent reliability was observed for COP length and velocity in the M-L direction and for the total COP length (ICC = 0.77–0.95). However, for COP length and velocity in the A-P direction, as well as for COP area, reliability was moderate (ICC = 0.58–0.70). No significant differences were found between the left and right sides in any variable. (4) Conclusions: The results of this study indicate that the upper limb sway test performed on a force plate in the one-handed plank position, both with eyes open and closed, is a reliable measurement option and has practical applications in clinical settings, including for baseline assessments, return-to-play protocols, and injury prevention strategies.