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The Biomechanics of Throwing: Simplified and Cogent Alexander E. Weber, MD,* Andreas Kontaxis, PhD,w Stephen J. O’Brien, MD, MBA, z and Asheesh Bedi, MD*

Abstract: The majority of shoulder injuries occur due to repetitive overhead movements, with baseball pitching being the most common mechanism for overuse injury. Before studying the treatment of these shoulder injuries, it is paramount that the health professional have an understanding of the etiology of and the underlying mechanisms for shoulder pathology. The act of overhead throwing is an eloquent full-body motion that requires tremendous coordination from the time of force generation to follow-through. The shoulder complex is a crucial component of the upper body kinetic chain as it transmits force created in the lower body to the arm and hand to produce velocity and accuracy with ball release. The focus of this article is on the biomechanics of the throwing motion, with emphasis on the kinematics of the shoulder. The established phases of the throwing motion will be reviewed in a stepwise manner and the contributions of osseous and soft-tissue structures to the successful completion of each phase will be discussed. Key Words: throwing phases, pitching, shoulder, biomechanics, kinetics, kinematics

(Sports Med Arthrosc Rev 2014;22:72–79)

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o appreciate shoulder pathology, it is paramount that the surgeon has a firm understanding of the etiology of shoulder injuries. The bulk of shoulder injuries, especially in sports, are caused by repetitive overhead activities leading to overuse injuries rather than a single traumatic mechanism.1,2 Several aspects of the biomechanics of the throwing motion have been studied as a model for understanding the forces imposed on the shoulder that lead to injury.3,4 The objective of this article is to review the biomechanics of overhand throwing in a simple and cogent manner that will provide the orthopedic surgeon with a clear understanding of the kinematics (motions) and kinetics (forces and torques) of the throwing motion. Overhand throwing is a complex motion and has often been divided into phases and events in many biomechanical studies. The biomechanics of each of the most established phases will be reviewed and discussed separately in this work. Special emphasis will be placed on the biomechanics of the shoulder during the throwing motion. Repetitive overhead motions in sports are known to cause shoulder injuries. The overhand pitching motion in baseball has been studied extensively and is the sport in From the *Department of Orthopaedic Surgery, University of Michigan, Ann Arbor, MI; wLeon Root, M.D. Motion Analysis Laboratory; and zSports Medicine and Shoulder Service, Hospital for Special Surgery, New York, NY. Disclosure: The authors declare no conflict of interest. Reprints: Asheesh Bedi, MD, Department of Orthopaedic Surgery, University of Michigan, MedSport, 24 Frank Lloyd Wright Drive, Lobby A, Ann Arbor, MI 48106. Copyright r 2014 by Lippincott Williams & Wilkins

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which the greatest shoulder angular velocities are generated.5 In addition, baseball is the sport with the greatest incidence of sports-related shoulder injuries.1–6 Numerous other sporting activities require the athlete to overhand throw or strike an object overhead (football, tennis, volleyball, swimming, etc.); however, given the high degree of similarity between these athletic motions and the baseball pitch we will focus predominately on the baseball pitching motion.7–10

THE PHASES AND MECHANICS OF OVERHAND THROWING The main objective of the pitching motion is to create velocity and accuracy as the ball is released from the pitcher’s hand. This is achieved by creating force and energy in the “kinetic chain” of the athlete. At the initiation of the throwing motion, energy is created in the legs and trunk of the throwing athlete. As the phases of the throwing motion progress, the energy created in the lower body and the trunk (proximal kinetic chain) is transferred “downstream” to the shoulder, elbow, and hand (distal kinetic chain), and ultimately imparted onto the ball.11–14 Studies have illustrated that muscle imbalances or muscle weakness in the proximal kinetic chain may cause increased demands on the distal kinetic chain.15–20 As we discuss the phases of the throwing motion, we will highlight the more common weaknesses in the proximal kinetic chain that lead to increased demand and ultimately potential for upper extremity injuries. The complete baseball throwing motion can take place in...