title: Increased horizontal shoulder abduction is associated with an increase in shoulder joint load in baseball pitching.
abstract: BACKGROUND
  Soft tissues of the shoulder undergoes substantial stresses due to humeral head movement, and this may contribute to throwing shoulder injuries in baseball pitchers. Prevention and management of throwing shoulder injuries critically rely on reduction of shear force at the shoulder joint. However, the amount and direction of the force applied to the shoulder during the throwing motion have not been clarified. The purpose of this study was to analyze forces applied to the shoulder during a baseball pitch.
  
  
  METHODS
  We performed biomechanical analysis of 213 baseball pitchers of various ages and skill levels. Throwing motion was analyzed with a 3-dimensional motion capture system. The Euler angle sequence was adopted to describe angular values of the upper arm relative to the trunk for shoulder rotation, and inverse dynamics was used to estimate the resultant joint forces at the shoulder.
  
  
  RESULTS
  There was a significant relation between horizontal abduction/adduction angle and resultant anterior/posterior force at the point of maximum external rotation (MER) (r = -0.63, P < .01), whereby increased horizontal abduction was associated with increased resultant anterior force. There was a significant but weak correlation between abduction/adduction angle and superior/inferior force at MER (r = 0.24, P < .01). Comparison among the groups with variable ages and skill levels showed larger horizontal abduction and smaller external rotation angles at MER in the adult amateur player group, whereas normalized compression force and internal rotation torque values at MER were smaller in the junior high school- and elementary school-aged groups.
  
  
  CONCLUSIONS
  These results suggest that excessive horizontal abduction at MER increases anterior shear force in the shoulder and may lead to shoulder injuries. Focusing on reducing horizontal abduction at MER in the throwing motion may be key to preventing and managing shoulder injuries in baseball pitchers.
authors.affiliation: Department of Orthopaedic Surgery, Hyogo College of Medicine, Nishinomiya, Japan. Electronic address: y.takgi126@gmail.com.
authors.forename: Yohei
authors.initials: Y
authors.lastname: Takagi
authors.affiliation: Department of Orthopaedic Surgery, Hyogo College of Medicine, Nishinomiya, Japan.
authors.forename: Takanori
authors.initials: T
authors.lastname: Oi
authors.affiliation: Nobuhara Hospital and Institute of Biomechanics, Tatsuno, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Tanaka
authors.affiliation: Nobuhara Hospital and Institute of Biomechanics, Tatsuno, Japan.
authors.forename: Hiroaki
authors.initials: H
authors.lastname: Inui
authors.affiliation: Department of Physical Therapy, Hyogo University of Health Sciences School of Rehabilitation, Kobe, Japan.
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Fujioka
authors.affiliation: Department of Orthopaedic Surgery, Hyogo College of Medicine, Nishinomiya, Japan.
authors.forename: Juichi
authors.initials: J
authors.lastname: Tanaka
authors.affiliation: Department of Orthopaedic Surgery, Hyogo College of Medicine, Nishinomiya, Japan.
authors.forename: Shinichi
authors.initials: S
authors.lastname: Yoshiya
authors.affiliation: Nobuhara Hospital and Institute of Biomechanics, Tatsuno, Japan.
authors.forename: Katsuya
authors.initials: K
authors.lastname: Nobuhara
mesh_terms: D000293:Adolescent; D000328:Adult; D001484:Baseball / Q000502:physiology*; D001696:Biomechanical Phenomena; D002648:Child; D006801:Humans; D008297:Male; D016059:Range of Motion, Articular; D012399:Rotation; D012782:Shoulder / Q000502:physiology*; D012785:Shoulder Joint / Q000502:physiology*; D055815:Young Adult
keywords: 3-dimensional motion analysis; Throwing shoulder injuries; baseball pitch; horizontal shoulder abduction; joint load; maximum external rotation; motion capture