Background: Subacromial impingement syndrome (SIS) is a common cause of shoulder pain and functional limitation. Biomechanical factors, including uncontrolled humeral head motion, rotator cuff dysfunction, and reduced glenohumeral internal rotation (IR) range of motion (ROM), contribute to narrowing of the subacromial space. Excessive anterior and superior humeral head translation is a risk factor for impingement. Quantifying olecranon marker displacement during shoulder rotation at 90° of abduction provides an indirect and clinically feasible approach for assessing humeral rotational control. However, few studies have simultaneously compared shoulder rotational strength, ROM, and humeral rotational control between individuals with and without SIS. Objects: Using olecranon marker displacement, we aimed to compare humeral head rotational control during shoulder IR and external rotation (ER), as well as shoulder IR/ER strength and ROM, between individuals with and without SIS. Methods: Forty male participants were recruited and classified into the SIS and non-SIS groups. Shoulder IR and ER ROM, isometric IR and ER strength, and humeral rotational control were assessed on the dominant side. Rotational control was evaluated by measuring olecranon marker displacement during shoulder IR and ER at 90° of shoulder abduction using a two-dimensional motion-tracking system. Independent t-tests were used for between-group comparisons, with statistical significance set at p < 0.05. Results: Compared with the non-SIS group, the SIS group demonstrated significantly reduced shoulder IR ROM and greater vertical displacement of the olecranon marker during IR (p < 0.05). No significant between-group differences were observed in ER ROM, IR or ER strength, or horizontal displacement of the olecranon marker during IR or ER (p > 0.05). Conclusion: Individuals with SIS exhibited reduced shoulder IR ROM and altered humeral rotational control during IR. Clinically, assessment of shoulder IR ROM and humeral rotational control should be considered when evaluating and managing individuals with SIS.

INTRODUCTION
MATERIALS AND METHODS
    1. Research Design
    2. Participants
    3. Inclusion Criteria
    4. Exclusion Criteria
    5. Procedure
    6. Instrumentation
    7. Measurements
    8. Statistical Analysis
RESULTS
DISCUSSION
CONCLUSIONS
FUNDING
ACKNOWLEDGEMENTS
CONFLICTS OF INTEREST
AUTHOR CONTRIBUTIONS
ORCID
REFERENCES

• Ju-hee Park(Department of Physical Therapy, The Graduate School, Yonsei University)
• Yul-ji Won(Department of Physical Therapy, The Graduate School, Yonsei University)
• Oh-yun Kwon(Department of Physical Therapy, College of Software and Digital Healthcare Convergence, Laboratory of Kinetic Ergocise Based on Movement Analysis, Yonsei University, Wonju, Korea) Corresponding author