The Functional Impingement Subacromial Ultrasound (FISUN) test: a conceptual and methodological framework for functional assessment of the subacromial space.

The purpose of this study was to present a standardized technical and methodological framework for functional ultrasound assessment of the subacromial space. This framework, the Functional Impingement Subacromial Ultrasound (FISUN) test, was designed to evaluate dynamic relationships between the greater tuberosity of the humerus and the acromion during functional arm elevation. The FISUN test is performed using real-time musculoskeletal ultrasound during functional arm elevation in the scapular plane under three sequential conditions with progressively increasing functional demands. The assessment is based on dynamic analysis of the relationship between the greater tuberosity of the humerus and the acromion during passive arm elevation, active arm elevation, and active arm elevation against resistance. A four-level classification system (levels 0-IV) is proposed to describe patterns of functional subacromial space behavior. The FISUN test provides a reproducible, clinically applicable framework for functional ultrasound assessment beyond static imaging and pain-provocation tests. By integrating standardized movement conditions with real-time dynamic ultrasound, the test allows subacromial space behavior to be evaluated in the context of motor control and functional loading. This Technical Note presents the methodological foundations of the FISUN test and provides a basis for future reliability studies.