Background: Various functional tests such as upper quarter Y-balance test (UQYBT) are used to evaluate shoulder stability and mobility in clinical or sports fields. Previous studies have been conducted to determine the correlation between the scapular or trunk muscle and UQYBT. However, the correlation between UQYBT and hip flexor, which can be considered as a core muscle, has not been confirmed.
Objects: To verify the relationship between the UQYBT and scapular muscle (scapular protractor and lower trapezius [LT]), trunk muscle, and hip flexor strengths in healthy male participants.
Methods: A total of 37 healthy male participants were recruited and underwent UQYBT in the push-up posture. The isometric strength of the scapular protractor, LT, trunk flexor and extensor, and hip flexors were measured using a smart KEMA strength sensor (KOREATECH Inc.).
Results: The superolateral direction of the UQYBT was moderately to strongly related to trunk extensor (r = 0.443, p < 0.01), scapular protractor (r = 0.412, p < 0.05), LT (r = 0.436, p < 0.01), and both sides of the hip flexors (supporting-side: r = 0.669, p < 0.01; non-supporting- side: r = 0.641, p < 0.01). The inferolateral direction of the UQYBT was moderately related to the scapular protractor (r = 0.429, p < 0.01), LT (r = 0.511, p < 0.01), and both sides of hip flexors (supporting-side: r = 0.481, p < 0.01; non-supporting-side: r = 0.521, p < 0.01). The medial direction of the UQYBT was moderately to strongly related with the scapular protractor (r = 0.522, p < 0.01), LT (r = 0.541, p < 0.01), and both sides of hip flexors (supporting-side: r = 0.605, p < 0.01; non-supporting-side: r = 0.561, p < 0.01).
Conclusion: This study showed that the strength of the scapular muscles, trunk muscles, and hip flexor muscles correlated to the UQYBT. Therefore, the strength of not only the scapular and trunk muscles but also the hip flexor muscles should be considered to improve the UQYBT.

INTRODUCTION
MATERIALS AND METHODS
    1. Participants
    2. Instrumentation
    3. Procedures
    4. Statistical Analysis
RESULTS
DISCUSSION
FUNDING
ACKNOWLEDGEMENTS
CONFLICTS OF INTEREST
AUTHOR CONTRIBUTION
ORCID
REFERENCES

• Joo-young Jeon(Department of Physical Therapy, The Graduate School, Yonsei University)
• Jun-hee Kim(Kinetic Ergocise Based on Movement Analysis Laboratory)
• Oh-yun Kwon(Kinetic Ergocise Based on Movement Analysis Laboratory, Department of Physical Therapy, College of Software and Digital Healthcare Convergence, Yonsei University) Corresponding author