Chronic Effects of Pitching on Muscle Thickness and Strength of the Scapular Stabilizers in Professional Baseball Players.

Background: The posterior scapular muscles eccentrically contract to disperse the high forces observed in the deceleration phase of pitching. Muscular adaptations often occur following chronic eccentric loading, however, no study has evaluated the adaptations of the posterior scapular muscles with regard to throwing and their relationship with humeral retroversion (HR) in professional pitchers.

Hypothesis: Significant chronic adaptations in muscle thickness (MT) and strength of the trapezius and rhomboids would be observed in healthy professional baseball pitchers, and there would be a significant relationship between humeral adaptations (ie, HR) and posterior scapular muscle adaptations (ie, strength and MT).

Study Design: Cross-sectional; Level 3.

Methods: A total of 28 healthy male professional baseball pitchers (age, 22 ± 2 years; mass, 95 ± 17 kg; height, 190 ± 7 cm) were included in the study. Bilateral isometric muscle strength of the upper trapezius (UT), middle trapezius, lower trapezius (LT), and rhomboids was measured during a maximum voluntary isometric contraction. Diagnostic ultrasound images of the UT, middle trapezius, LT, rhomboid major, and rhomboid minor muscles were collected bilaterally to measure MT. HR was also quantified bilaterally with ultrasound. Paired sample tests were used to compare dominant and nondominant strength and MT. Pearson correlation coefficients were used to assess the relationship between HR, isometric strength, and MT.

Results: A significantly increased MT of the LT was found on the dominant arm compared with the nondominant arm (5.4 ± 1.1 mm vs 4.4 ± 1.5 mm; = 0.00). The Pearson correlation coefficient demonstrated a significant weak negative relationship between HR and rhomboid major MT ( = 0.03; = -0.36), and a significant weak negative correlation between HR and middle trapezius isometric strength ( = 0.03; = -0.37).

Conclusion: LT thickness was greater in the throwing arm compared with the nonthrowing arm of pitchers, suggesting a positive adaptation of the LT. Interestingly, there was a weak negative relationship between HR and both rhomboid major MT and middle trapezius isometric strength. This negative relationship suggests that since increased HR leads to decreased internal rotation range of motion during deceleration, the scapula may be forced into anterior tilt and protraction, which can place excessive eccentric load on the rhomboid major and middle trapezius.