Evaluating Joint Position Sense With 3D Motion Capture System: The Effect of Muscle Activation Without a Reference Angle in Shoulder Movements.

BACKGROUND AND PURPOSE: Joint position sense (JPS) is frequently assessed using joint position reproduction (JPR); however, JPR has limitations, where reference angles are typically unavailable. This study aimed to examine the accuracy of achieving an angle corresponding to 50% of the elevation angle previously attained during shoulder flexion and abduction, without a reference angle.

METHODS

Participants first performed shoulder flexion to an absolute target angle (set at 90°), and the actual angle achieved was recorded. A relative target angle, corresponding to 50% of the achieved angle, was then set. Participants repeated shoulder flexion at this relative target angle under three conditions: passive (elevated by an examiner), active, and active with additional effort (elevated while holding a 1-kg weight). A similar protocol was applied to shoulder abduction.

RESULTS

Participants showed substantial deviation from the absolute 90° target in both motions. Between-trial reliability (ICC) increased with muscle activation in both flexion and abduction, whereas accuracy, as indicated by joint position error, improved only in shoulder flexion. Active with additional effort did not result in meaningful further improvement.

DISCUSSION: Without a reference angle, estimating target angles was challenging. The findings in this study provide preliminary insight into proprioceptive assessment of the upper extremity in clinical and sports settings, where muscle activation and JPS are relevant to rehabilitation and performance.