Research Paper: Comparing the Isometric Strength of the Shoulder and Scapulothoracic Muscles in Volleyball and Futsal AthletesPurpose: This study aimed to evaluate the symmetry of the isometric strength of the shoulder and scapulothoracic muscles in volleyball and futsal athletes. Methods:In this case-control study, the strength of the scapulothoracic muscles and rotation of the shoulders in 20 adult volleyball athletes and 20 futsal adult athletes, who were randomly selected from sports clubs around Tehran, were measured using a manual dynamometer. The obtained data between the different groups and within the individual groups were compared by Independent and paired t tests, respectively. Results:The isometric strength of the external rotator muscles, internal rotator muscles, upper trapezius and serratus anterior of the dominant side of volleyball and futsal athletes were different (P<0.05). The isometric strength of the same muscles in the dominant side of the volleyball athletes was more than the non-dominant side (P<0.05). However, the isometric strength of the middle trapezius and rhomboid muscles were less. The ratio of agonist to antagonist, in all muscle groups between two groups, in the dominant side as well as between the dominant and non-dominant side, were different in volleyball athletes. Conclusion:In volleyball athletes, disturbances in the strength ratio of the agonist and antagonist muscles of the scapula, and weaknesses in these muscles, impair the dynamic stability of the scapula and shoulder joints and disturbances in the rhythm of the scapulohumeral muscles. It is recommended that during training, assessment and treatment of these athletes strengthening these muscles be considered.