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Biomechanics of the Shoulder in Youth Baseball Pitchers

Implications for the Development of Proximal Humeral Epiphysiolysis and Humeral Retrotorsion

Michelle B. Sabick, PhD^*,, Young-Kyu Kim, MD, Michael R. Torry, PhD^*,, Michael A. Keirns, PhD, ATC^ll and Richard J. Hawkins, MD, FRCS(C)^*,¶

From the ^* Steadman-Hawkins Sports Medicine Foundation, Vail, Colorado, the Center for Orthopaedic & Biomechanics Research and Department of Mechanical Engineering, Boise State University, Boise, Idaho, the Gachon Medical School, Gil Medical Center, Inchon, South Korea, ^|| Regis University, Denver, Colorado, and the ^¶ Steadman-Hawkins Clinic, Vail, Colorado

Address correspondence to Michael R. Torry, PhD, Steadman-Hawkins Sports Medicine Foundation, 181 West Meadow Drive, Suite 1000, Vail, CO 81657 (e-mail: Mike.Torry{at}shsmf.org).

Background: The effects of repetitive throwing on the shoulders of developing athletes are not well understood because of the paucity of data describing the biomechanics of youth pitchers and the plasticity of the developing skeleton.

Hypothesis: The direction and magnitude of the stresses that exist at the proximal humeral physis during the fastball pitching motion are consistent with the development of proximal humeral epiphysiolysis (Little League shoulder) and/or humeral retro-torsion.

Study Design: Descriptive laboratory study.

Methods: A total of 14 elite youth baseball pitchers (mean age, 12.1 ± 0.4 years) were filmed from the front and dominant side while throwing fastballs in a simulated game. The net force and torque acting on the humerus throughout the throwing motion were calculated using standard biomechanical techniques.

Results: The external rotation torque about the long axis of the humerus reached a peak value of 17.7 ± 3.5 N·m (2.7% ± 0.3% body weight x height) just before maximum shoulder external rotation. A shoulder distraction force of 214.7 ± 47.2 N (49.8% ± 8.3% body weight) occurred at, or just after, ball release.

Conclusion: Shear stress arising from the high torque late in the arm-cocking phase is large enough to lead to deformation of the weak proximal humeral epiphyseal cartilage, causing either humeral retrotorsion or proximal humeral epiphysiolysis over time. The stresses generated by the external rotation torque are much greater than those caused by distraction forces generated during the pitching motion of youth baseball pitchers.

Clinical Relevance: The motion of throwing fastballs by youth baseball pitchers results in force components consistent with proposed mechanisms for 2 clinical entities.

Key Words: pitching mechanics • humerus • retrotorsion • proximal humeral epiphysiolysis • shoulder

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