Ultrasound Evaluation of the Lower Trapezius in Adolescent Baseball Pitchers

Background: Biomechanical studies have demonstrated that the lower trapezius (LT) facilitates normal shoulder function, and given its potential role in causing and/or perpetuating dysfunction, clinicians have begun to evaluate and rehabilitate the LT in the context of comprehensive shoulder rehabilitation programs. Although some studies have compared side-to-side differences in LT thickness among nonthrowers, no studies have been performed to examine the normal ultrasound appearance of the LT in the adolescent throwing population. Objectives: To examine whether LT thickness and cross-sectional area (CSA) differ between dominant and nondominant sides in adolescent baseball pitchers and to determine the inter-rater and intrarater reliability of the sonographically measured parameters. Study Design: Observational, repeated measures study. Setting: Hospital-based outpatient practice. Participants: Thirty asymptomatic male baseball pitchers aged 13-18 years with at least 3 years of pitching experience from a convenience sample in the Rochester, Minnesota, area. Methods: LT thickness and CSA were measured at the T8 and T5 vertebral levels at rest, and thickness was measured again in active prone T- and Y-pose positions by multiple investigators. Side-to-side differences in thickness and CSA were examined, and inter-rater and intrarater measurement reliability coefficients were estimated. Main Outcome Measurements: Thickness and cross-sectional area were measured. For reliability, we calculated intraclass correlation coefficients along with the standard error of measurement and the minimal detectable change. Results: Resting LT thickness was 0.064 cm greater on the dominant side than on the nondominant side (P = .001). During contraction, LT thickness was 0.084 and 0.097 cm greater on the dominant side in the T and Y poses (P = .036 and P = .001, respectively). The CSA at the T5 level was 0.36 cm² greater on the dominant side than on the nondominant side (P < .001). Inter-rater and intrarater reliability coefficients exceeded 0.898 across all levels of measurement. Conclusion: Asymptomatic adolescent pitchers have greater resting and contracted LT thickness and resting CSA on the dominant versus the nondominant side, although the quantitative differences are small. These differences suggest that structural changes and asymmetry can manifest at young ages; however, clinically, the nondominant arm can be used as a control in the adolescent pitching population. Ultrasound provides a noninvasive way to reliably evaluate LT thickness and CSA in adolescent pitchers.