Multilevel Determinants of Acromiohumeral Distance Within a Systems-Based Functional Morphology Framework and Their Implications for Personalized Neuromusculoskeletal Modelling

Background: The acromiohumeral distance (AHD) is a widely used imaging parameter for evaluating the subacromial region and shoulder function. Traditionally, it has been interpreted as a static anatomical measurement reflecting the available subacromial space and associated with rotator cuff pathology, superior humeral migration, and subacromial impingement. However, inconsistent relationships between AHD, symptoms, and function suggest that purely structural interpretations may be insufficient. Objectives: To propose a systems-based functional morphology framework that reinterprets AHD as an emergent functional state variable reflecting the instantaneous state of the glenohumeral system. Methods: A conceptual synthesis integrated evidence from shoulder biomechanics, functional morphology, movement science, connective tissue research, systems theory, and personalized neuromusculoskeletal modelling. Structural, connective tissue, neuromuscular, and mechanical determinants of AHD were integrated within a multilevel systems perspective. Results: The proposed framework conceptualizes AHD as emerging from the dynamic interaction of four organizational domains: structural constraints, connective tissue properties, neuromuscular regulation, and mechanical context. Rather than representing a fixed anatomical space, AHD is interpreted as a potential systems-level functional biomarker whose value varies according to the functional state of the glenohumeral system. This perspective explains the variability observed across imaging studies and the frequent discordance between structural findings, symptoms, and functional outcomes. Conclusions: Reframing AHD as an emergent functional state variable provides a theoretical basis for integrating anatomy, biomechanics, movement behavior, and neuromuscular control within personalized shoulder assessment. This systems-based perspective may help bridge the traditional gap between structural imaging and clinical presentation while supporting future developments in precision rehabilitation and subject-specific computational modelling.