Background: Scapular dyskinesis (SD) is closely related to imbalance of the rotator cuff (RC) muscles. However, previous studies have only focused on isometric strength. To our knowledge, there has been no study examining potential differences in concentric and eccentric strength and functional strength ratio (FSR) of the RC muscles based on functional parameters related to throwing in with or without SD. Objects: This study aimed to determine whether there was a difference in the RC muscle strength and FSR between the dominant shoulder with SD and the non-dominant shoulder without SD in adolescent baseball players. Methods: Forty adolescent baseball players participated and classified types of SD based on movement patterns using the SD test by two examiners. The isokinetic concentric and eccentric peak torque of the internal rotation (IR) and external rotation (ER) were measured and quantified as peak torque to body weight (PT/BW). Also, the FSR was obtained by calculating the strength ratios of eccentric IR to concentric ER (IRecc/ERcon) and eccentric ER to concentric IR (ERecc/IRcon), respectively. Results: There was a significant decrease in the IR and ER PT/BW in the dominant shoulder with SD compared to the non-dominant shoulder without SD (p < 0.05), regardless of contraction types. However, no significant difference was observed in the FSR in both IRecc/ERcon and ERecc/IRcon ratios. Conclusion: The findings indicate that the isokinetic concentric and eccentric PT/BW of the IR and ER muscles were significantly lower in the dominant shoulder with SD than in the nonthrowing shoulder without SD. Therefore, when establishing a strategy for addressing RC muscle weakness in adolescent baseball players with SD, it is essential to consider an approach that accounts for scapular kinematic recovery.

Background: The external rotation (ER) exercise in performed at a 90° abduction of the shoulder joint is an effective to strengthen the infraspinatus. However, failure of the humeral head to control axial rotation during exercise can be increased the posterior deltoid over activity. Biofeedback training is an effective method of promoting motor learning and control it could look forward to activate the infraspinatus selectively by controlling the humeral head during exercise. Objects: The aim of this study was investigated that whether biofeedback for axial rotation was effective to activate selectively the infraspinatus during ER exercise. Methods: The 15 healthy males participated, and all subjects performed both ER exercise in a sitting position with shoulder abducted 90° under conditions with and without axial rotation biofeedback. Exercise was performed in a range of 90° ER, divided into three phases: concentric, isometric, and eccentric. The infraspinatus and posterior deltoid muscle activity were observed using surface electromyography. Results: Both infraspinatus activity (p < 0.01) and infraspinatus to posterior deltoid activity ratio (p = 0.01) were significantly higher with biofeedback however, posterior deltoid activity was significantly lower with biofeedback (p = 0.01). The infraspinatus muscle activity and muscle activity ratio were the highest in the isometric contraction type, and there were significant differences for all contraction types (p < 0.05). Whereas, the posterior deltoid activity was the lowest in the isometric contraction type, and showed a significant difference between isometric and other two contraction types (p < 0.05), but no significant different between concentric and eccentric contraction. Conclusion: Our results indicate that the axial rotation biofeedback during sitting ER exercise might be effective method to activating selective infraspinatus muscle and recommended to enhance the dynamic stability of the shoulder joint.