Muscle Activity Relationships During Isometric Shoulder Internal and External Rotation Using the ForceFrame Dynamometer and Athletic Shoulder Tests in Baseball Athletes

Ben Ashworth¹Mikulas Hank^2*Petr Miřátský²Ferdia Falon Verbruggen²Frantisek Zahalka²Tomas Maly²

• ¹School of Health and Sport Sciences, Liverpood Hope University, Liverpool, United Kingdom
• ²Sport Research Center, Faculty of Physical Education and Sport, Charles University, Prague, Prague, Czechia

Background: Optimal shoulder and trunk neuromuscular coordination significantly contributes to performance and injury prevention in overhead athletes. Although isolated shoulder rotation tests are often discussed in research, they may not fully evaluate the complex muscle synergies necessary for functional thrower positions or force deficiencies. Despite the increasing use of the Athletic Shoulder (ASH) test in elite sports, evidence is lacking on the relationship between muscle activation patterns in this multi-joint test and traditional isolated assessments. This study aimed to examine muscle activity relationships during shoulder strength assessment of isometric rotation and the ASH test in overhead throwing athletes. Methods: Surface electromyography was used to analyse muscle activation of nine shoulder and trunk muscles during maximal voluntary contractions in seventeen male national team baseball athletes. Normalized values were used in muscle activation relationship analysis between internal/external rotation and ASH test's three shoulder positions in dominant arm (ASH I, ASH Y, ASH T).Results: Analysis between internal rotation, external rotation, and ASH I, ASH Y, ASH T test positions showed significant differences in muscle activity (p < 0.05). Infraspinatus, anterior and posterior deltoid and upper trapezius showed higher activity in the shoulder rotation tests, while external obliques and pectoralis major indicated higher activity during ASH tests. Moderate (ρ = 0.40-0.69) correlations were found between muscles in tests, while strong (ρ = 0.70-0.89) to very strong (ρ = 0.90-1.00) correlations were found between tests for individual muscles.The ASH and shoulder rotational tests resulted in different shoulder and trunk neuromuscular activation levels, demonstrating a low relationship between prime movers for ASH positions and muscles activated during shoulder rotation. This implies that muscle synergies vary significantly depending on joint positioning and test mechanics. It also emphasizes the importance of multiple test batteries to capture force production deficits and abnormalities relevant to overhead athletes. Results support a rehabilitation progression starting with infraspinatus strengthening in neutral shoulder abduction, followed by scapular stabilizer training at wider abduction angles to restore functional overhead capacity. Acute or chronic performance may be monitored weekly for further training optimization, return-to-sport decisions, and injury prevention in elite overhead throwing athletes.