Abstract
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 five-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.
Keywords: Subacromial space, Shoulder, Dynamic ultrasound, Scapulohumeral rhythm, Functional test, Rehabilitation
Introduction
Subacromial impingement syndrome remains one of the most common causes of shoulder pain and functional limitation in both the general population and physically active individuals [1,2]. Despite its high prevalence, clinical assessment of subacromial space disorders still relies largely on pain-provocation tests and imaging examinations performed under static conditions. This approach often fails to reflect the dynamic nature of shoulder movement or the complex arthrokinematic mechanisms underlying the dysfunction. Traditional clinical tests for subacromial impingement primarily aim to reproduce pain during specific movements or resisted conditions [3,4]. Although these tests are useful screening tools, they provide limited information about the behavior of the humeral head and greater tuberosity in relation to the acromion during movement. Moreover, they do not allow dysfunction severity to be graded or functional changes over time to be objectively monitored, particularly in response to therapeutic interventions.
Musculoskeletal ultrasound offers unique opportunities for dynamic assessment of shoulder structures in real time and under functional loading conditions [5,6]. Ultrasound examination allows direct visualization of the relationships among the acromion, greater tuberosity of the humerus, supraspinatus tendon, and subacromial–subdeltoid bursa during passive and active movement. Despite the wide availability of this modality, its use in assessing subacromial impingement has long remained predominantly descriptive and focused on identifying structural changes, without a clear functional reference. Recent developments in musculoskeletal ultrasound have substantially expanded its clinical applications beyond static morphological assessment. High image resolution, real-time motion recording, and the increasing availability of modern ultrasound equipment have contributed to its growing use in functional diagnostics [5,6]. Ultrasound allows objective evaluation of spatial relationships between osseous and musculotendinous structures during movement and loading, which is particularly important for assessing functional shoulder disorders and monitoring therapeutic outcomes.
The subacromial space is a clinically relevant region in which movement mechanics determine functional integrity. Disturbances of scapulohumeral rhythm are recognized as key contributors to subacromial impingement and rotator cuff pathology [7,8]. Altered scapular kinematics and impaired scapula-humerus coordination modify spatial conditions within the subacromial space and promote mechanical conflict [9,10]. Importantly, subacromial dysfunction is not binary; impairment may appear only under load or during active or resisted movement, while passive motion remains preserved [9–12]. In advanced stages, restriction may also be present during passive movement, and further elevation may occur only through compensatory scapular rotation, a documented adaptive mechanism in scapulohumeral disorders that contributes to impingement development [7,8,13]. The aim of the present study was to propose a standardized functional ultrasound-based test and corresponding classification system for subacromial impingement, based on the conditions under which altered rolling of the greater tuberosity of the humerus beneath the acromion occurs. The proposed classification is continuous and reversible, allowing both cross-sectional assessment of the current dysfunction level and monitoring of functional changes over time. By defining dysfunction levels according to mechanical conditions rather than pain intensity alone, this approach seeks to provide a clinically useful and objective tool for assessing shoulder function. In the following sections, the proposed procedure is referred to as the Functional Impingement Subacromial Ultrasound (FISUN) test.
Materials and Methods
Compliance with Ethical Standards
The illustrative materials used in this paper, including ultrasound images and video recordings, are for demonstration purposes only and serve exclusively to present the proposed FISUN diagnostic procedure. These materials were obtained during routine clinical practice, and all information that could identify individuals was removed. Because of the conceptual and methodological nature of the paper and the absence of clinical data analysis, ethics committee approval was not required. Informed consent is not applicable.
Study Design
This paper is conceptual and methodological and presents a standardized procedure for functional ultrasound assessment of movement disturbances within the subacromial space during shoulder abduction. The proposed procedure, referred to as the FISUN test, is based on dynamic visualization of the relationship between the acromion and the greater tuberosity of the humerus under different functional loading conditions. Importantly, this test does not replace standard morphological B-mode ultrasound assessment of structures within the subacromial space, including the subacromial–subdeltoid bursa and rotator cuff tendons; rather, it is intended to complement that assessment.
The FISUN test combines real-time dynamic observation with a controlled three-stage movement protocol consisting of passive abduction, active abduction, and resisted active abduction. An integral component of the method is the proposed functional classification system (levels 0–IV), which enables determination of the dysfunction level and monitoring of functional changes over time, including responses to therapeutic interventions.
Clinical Indications
The FISUN test is intended for functional assessment of patients with suspected subacromial space disorders, particularly when clinical findings suggest functional subacromial impingement. Indications for the test include scapulohumeral rhythm disturbances, limitations in shoulder function, and pain associated with upper-limb elevation.
Although pain during upper-limb abduction is common, its presence is neither required for performing the test nor necessary for interpretation. The FISUN test focuses on objective assessment of the mechanical conditions of movement rather than solely on subjective pain perception. In some cases, impaired greater tuberosity rolling may occur without clear pain symptoms.
Patient Position
The examination is performed with the patient seated, which serves as the reference position for the entire FISUN procedure. The patient sits upright on a stable chair without back support, with both feet positioned symmetrically on the floor. The trunk is maintained in a neutral position, and the tested upper limb moves in the scapular plane.
The seated position helps limit the influence of anthropometric differences between the patient and examiner and facilitates stable control of the ultrasound transducer during dynamic abduction. This position also promotes a natural shoulder movement pattern and limits compensations resulting from postural instability.
Ultrasound Equipment and Probe Stabilization with Scapular Control
A high-frequency linear ultrasound transducer is used to perform the FISUN test. The transducer is positioned over the superolateral aspect of the shoulder so that a single imaging field includes the scapular acromion, greater tuberosity of the humerus, supraspinatus tendon, and subacromial–subdeltoid bursa (Fig. 1).
Fig. 1. Reference ultrasound image of the subacromial space structures in the Functional Impingement Subacromial Ultrasound (FISUN) test.
A, acromion; T, greater tuberosity of the humerus; S, supraspinatus tendon; D, deltoid muscle; arrow, anatomical location of the subacromial-subdeltoid bursa.
A key technical element of the test is stabilization of the ultrasound transducer over the acromion, which serves as the constant reference point or anchor point. The transducer must remain in an unchanged spatial relationship to the acromion throughout abduction until complete rolling of the greater tuberosity of the humerus beneath the acromial edge is reached.
The preferred stabilization method is a pinch grip between the thumb and index finger or a grip involving additional fingers. The thenar eminence and digits IV and V rest directly on the patient’s shoulder, creating a stable support point. This hand position allows precise control of transducer placement and effectively limits uncontrolled transducer displacement over the skin surface, or probe drift. Importantly, this hand position serves two stabilizing functions. In addition to maintaining a constant probe–acromion relationship, the examiner applies controlled, firm compression by anchoring the thenar eminence and digits IV and V over the scapular spine, a clearly palpable osseous landmark. This compression stabilizes the humeral head relative to the acromion while limiting excessive scapulothoracic elevation, or scapular shrugging, during abduction. The contact surface of the palm enables continuous tactile and palpatory monitoring of scapular motion, helping ensure that the observed interaction between the greater tuberosity and acromion primarily reflects glenohumeral mechanics rather than compensatory scapular substitution patterns (Fig. 2).
Fig. 2. Probe grip enabling probe and scapular stabilization, with a thumb–index pinch and thenar and digits IV–V support.
Transducer displacement before complete greater tuberosity rolling beneath the acromial edge constitutes a technical error and prevents reliable dynamic assessment.
Three-Stage FISUN Test Protocol
Passive abduction
Shoulder abduction is performed passively by the examiner while the patient’s upper limb remains fully relaxed. The patient’s forearm rests on the examiner’s forearm, and the examiner’s other hand stabilizes the patient’s arm above the elbow joint (Fig. 3). This grip allows the patient’s active neuromuscular control to be completely excluded.
Fig. 3. Passive abduction phase with examiner-supported arm positioning to allow complete muscle relaxation.
The passive phase serves as the reference condition for subsequent parts of the FISUN test and allows assessment of movement resulting solely from the structural properties of the osseous–articular system (Fig. 4).
Fig. 4. Passive stage of the three-stage Functional Impingement Subacromial Ultrasound (FISUN) test protocol.
Active abduction
During active abduction, the examiner steps back from the patient to enable free movement of the upper limb in the scapular in the anatomical position. Changing the examiner’s position is necessary to maintain a natural movement trajectory and limit compensations.
The patient performs abduction independently, and dynamic changes in the relationship between the acromion and greater tuberosity are assessed in comparison with passive conditions (Fig. 5).
Fig. 5. Active stage of the three-stage Functional Impingement Subacromial Ultrasound (FISUN) test protocol.
Resisted active abduction
During resisted active abduction, the examiner applies manual resistance while allowing abduction to be performed. Resistance is applied below the elbow joint on a longer lever arm, and the force is adjusted to the patient’s capabilities (Fig. 6).
Fig. 6. Resisted active stage of the three-stage Functional Impingement Subacromial Ultrasound (FISUN) test protocol.
The aim of applying resistance is not to assess maximal muscle strength but to increase functional loading during movement. If elbow discomfort occurs, the lever arm may be shortened by applying resistance to the distal part of the humerus.
The FISUN procedure, including passive, active, and resisted active shoulder abduction, is demonstrated in Video clip 1.
Technical Examination Conditions
The ultrasound system and transducer cable routing must not restrict upper-limb range of motion or the examiner’s freedom of movement. The equipment setup should allow the test to proceed without modifying the movement trajectory.
Spatial or technical limitations that lead to movement compensations constitute procedural errors in the FISUN test.
Assessed Structures and Observation Parameters
During all phases of the FISUN test, the dynamic relationship between the acromion and greater tuberosity of the humerus, displacement of the supraspinatus tendon, and behavior of the subacromial–subdeltoid bursa are qualitatively assessed. The presence of pain is recorded; however, except at the borderline level, pain is not the criterion used to determine the dysfunction level.
A clinical ultrasound image illustrating subacromial impingement is presented in Video clip 2.
Functional Ultrasound-Based Classification of Subacromial Impingement (0–IV) in the FISUN Test
The FISUN classification is based on the condition under which altered rolling of the greater tuberosity beneath the acromion occurs during the three-stage functional test.
Across levels 0–IV, grading is structured according to the loading condition under which this mechanical alteration becomes apparent. Assignment to levels II–IV is determined exclusively by sonographically confirmed loss of normal rolling under defined loading conditions (Fig. 7). The only sonographic marker defining a level transition is loss of smooth, complete greater tuberosity rolling beneath the acromial edge under a defined loading condition.
Fig. 7. Functional ultrasound-based classification of subacromial impingement in the Functional Impingement Subacromial Ultrasound (FISUN) test, levels 0–IV.
Pain contributes to the classification only at level I, where rolling of the greater tuberosity remains preserved despite symptom provocation during its passage beneath the acromion. At levels II–IV, pain may commonly coexist with mechanical impairment; however, it is not a criterion for differentiating the degree of dysfunction.
Level 0: Physiological state
In all variants of the functional test, normal, smooth rolling of the greater tuberosity of the humerus beneath the acromion is observed. No sonographic features of mechanical conflict or altered shoulder arthrokinematics are present. Passage of the greater tuberosity beneath the acromial edge does not provoke pain. No disturbances of scapulohumeral rhythm are observed.
Level I: Borderline level: pain without loss of mechanics
Rolling of the greater tuberosity beneath the acromion occurs normally in all variants of the functional test. Pain occurs during passage of the tuberosity beneath the acromial edge and may increase when resistance is applied during active abduction. No sonographically detectable alteration in arthrokinematics or loss of rolling is identified.
Level II: Load-dependent conflict
Sonographically confirmed impaired rolling of the greater tuberosity becomes apparent during resisted active abduction. In the passive variant and in active testing without resistance, normal rolling is preserved. Application of resistance provokes mechanical conflict within the subacromial space.
Level III: Active motion conflict
Sonographically confirmed loss of normal rolling is observed during active abduction without resistance. The passive variant still allows normal rolling of the greater tuberosity, indicating preserved structural movement capacity but insufficient active control. Resisted active testing further accentuates the arthrokinematic impairment. Disturbance of scapulohumeral rhythm may coexist.
Level IV: Full functional subacromial impingement
Loss of normal rolling is present in all variants of the functional test, including passive abduction. The greater tuberosity establishes structural contact with the acromion, preventing further physiological glenohumeral movement. Continuation of upper-limb abduction occurs through compensatory scapular rotation, without further increase in glenohumeral range.
Final remarks on the classification
The proposed classification represents a continuous and potentially reversible spectrum. Therapeutic progress may be objectified by a gradual shift in the loading condition under which impaired greater tuberosity rolling occurs: from conflict present in all test variants, through activity- and load-dependent conflict, to a fully physiological pattern.
The system enables both cross-sectional assessment of the dysfunction level and longitudinal monitoring of functional changes over time.
Discussion
The aim of the present study was to propose a standardized functional test based on dynamic ultrasonography, together with a corresponding classification system for subacromial space dysfunction defined by the conditions under which altered rolling of the greater tuberosity beneath the acromion occurs. In contrast to previously proposed approaches, the present method is not limited to qualitative movement observation but introduces a clearly defined functional criterion for grading dysfunction and monitoring changes over time.
In recent years, increasing efforts have been made to apply dynamic ultrasonography to functional assessment of the shoulder joint and subacromial space. Park et al. demonstrated that ultrasound examination performed during movement allows identification of dynamic impingement that is not visible on static imaging, highlighting the importance of real-time assessment of spatial relationships [14]. Van Holsbeeck and Introcaso [15] also emphasized the usefulness of dynamic ultrasonography in evaluating shoulder pathology and its potential for analyzing function and movement mechanics, although they did not introduce a standardized examination protocol or grading system for dysfunction.
Similarly, Bureau et al. [16] showed that dynamic ultrasonography may reveal functional features of subacromial impingement and changes occurring in response to therapeutic intervention. Nevertheless, the proposed approaches were descriptive and did not allow unambiguous classification of dysfunction severity [16]. Terslev et al. [17] highlighted the potential of ultrasonography as a tool for monitoring rehabilitation outcomes in musculoskeletal disorders and emphasized its value in objectifying functional changes, although without reference to specific standardized functional tests.
In addition to descriptive visualization of impingement phenomena, quantitative biomechanical analyses have expanded the understanding of subacromial mechanics. Lin et al. [18] analyzed the trajectory of the greater tuberosity and acromiohumeral spatial relationships during arm elevation, strengthening the kinematic foundation for dynamic assessment. Despite these important contributions, a loading-dependent functional grading model that translates mechanical observations into a structured clinical classification of dysfunction severity has not yet been established [18].
The FISUN test builds on these biomechanical and dynamic insights but shifts the interpretive focus toward functional mechanical capacity by defining dysfunction according to the specific loading condition under which rolling impairment becomes apparent. This transition from structural visualization to loading-dependent functional grading represents a conceptual extension of previous work rather than a contradiction of it.
In contrast to protocols based on provocative arm positions, such as flexion or internal rotation described by Bureau et al. [16] and Lin et al. [18], the FISUN test is performed in the scapular plane, along the axis of the supraspinatus tendon, without additional internal or external rotation of the glenohumeral joint. This positioning was intentionally selected to assess rolling mechanics under the most physiological conditions for the scapulohumeral complex, in which the glenohumeral joint achieves its greatest range of mobility. The objective is not to maximally provoke impingement through provocative positioning, but to identify mechanical impairment under the most functionally relevant loading condition, where it first becomes apparent during dynamic assessment [16,18].
A particularly relevant reference is the study by Wu et al. [19], in which the authors presented dynamic assessment of impaired rolling of the greater tuberosity beneath the acromion during passive shoulder abduction and its normalization after therapeutic intervention in patients with adhesive capsulitis. That study supports the clinical usefulness of dynamic ultrasonography for documenting functional changes in a before–after manner; however, similar to other reports, it does not provide a tool for grading dysfunction or monitoring gradual changes throughout the therapeutic process.
In this context, the functional test and classification system proposed in the present study complement and extend existing approaches. The introduction of a three-stage examination protocol that includes passive, active, and resisted active movement enables identification of impairments that become apparent at different levels of loading and movement control. A key feature of the proposed classification is its linkage of the dysfunction level to the specific functional condition in which altered rolling of the greater tuberosity occurs, allowing precise grading of impairment.
An important characteristic of the proposed system is its continuous and reversible nature. In contrast to binary assessments based on differentiating the presence from the absence of subacromial impingement, this classification allows gradual regression of dysfunction to be observed as movement control and scapulohumeral rhythm improve. This feature is particularly relevant in light of pathophysiological mechanisms described in biomechanical studies, which have shown that disturbances of scapulohumeral rhythm may manifest in a staged manner and may depend on loading level and motor control [7,9,10].
Importantly, the proposed test has diagnostic value and clear potential to guide therapeutic decision-making. It enables therapeutic progress to be documented through assessment of functional changes independently of subjective pain perception, which, according to the literature, does not always correlate directly with movement mechanics [2,3]. From this perspective, the test may support clinical decision-making and provide a basis for monitoring the effectiveness of interventions aimed at improving movement control and restoring normal scapulohumeral rhythm.
A limitation of the present study is the lack of validation analyses regarding the reliability and reproducibility of the proposed test, which reflects the conceptual and methodological nature of this paper. Correct performance of the examination requires experience in musculoskeletal ultrasonography and dynamic assessment, particularly with stabilization of the ultrasound transducer relative to the acromion. Future studies should focus on intra- and inter-rater reliability, clinical validation of the proposed classification, and evaluation of its usefulness for monitoring therapeutic outcomes, including the application of ultrasound-based biofeedback.
In summary, the proposed functional test and classification system address an important gap between descriptive ultrasonographic assessment and the need for an objective, graded diagnostic and therapeutic tool for evaluating the subacromial space. This method aligns with current trends in functional diagnostics and provides a foundation for further validation studies and clinical applications, including assessment of reliability and reproducibility and use of the test as a biofeedback tool during therapy.
Key points
The Functional Impingement Subacromial Ultrasound (FISUN) test introduces a standardized, reproducible framework for functional ultrasound assessment of subacromial space mechanics during shoulder abduction. A continuous, reversible classification system based on levels 0–IV enables functional grading of subacromial dysfunction according to mechanical conditions rather than pain alone. The proposed framework supports objective monitoring of shoulder mechanics and provides a foundation for future reliability, validation, and clinical studies.
Footnotes
Author Contributions
Conceptualization: Niewiadomy P, Szuścik-Niewiadomy K. Data acquisition: None. Data analysis or interpretation: Niewiadomy P. Drafting of the manuscript: Niewiadomy P, Szuścik-Niewiadomy K. Critical revision of the manuscript: Niewiadomy P, Szuścik-Niewiadomy K. Approval of the final version of the manuscript: all authors.
Conflict of Interest
No potential conflict of interest relevant to this article was reported.
Supplementary Material
Demonstration of the Functional Impingement Subacromial Ultrasound (FISUN) procedure, showing passive, active, and resisted active shoulder abduction, as well as the reference ultrasound image (https://doi.org/10.14366/usg.26040.v1).
Clinical ultrasound image of subacromial impingement (https://doi.org/10.14366/usg.26040.v2).
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Demonstration of the Functional Impingement Subacromial Ultrasound (FISUN) procedure, showing passive, active, and resisted active shoulder abduction, as well as the reference ultrasound image (https://doi.org/10.14366/usg.26040.v1).
Clinical ultrasound image of subacromial impingement (https://doi.org/10.14366/usg.26040.v2).