The Throwing Shoulder



The Throwing Shoulder


James P. Sostak II

Carlos A. Guanche



INTRODUCTION


Throwing Motion



  • Throwing in baseball has been analyzed extensively, with the specific maneuvers being broken down into 6 phases (6). While the analysis of the baseball throw is the best understood and most studied, other throwing, racquet, and overhand sports have also been evaluated. The mechanics of a baseball throw are transferable, for the most part, to other sports with some modifications depending on the size of the ball or the maneuver being analyzed. The baseball throw, by virtue of its speed and frequency, however, is the most traumatic to the shoulder.


  • The phases of throwing are summarized as follows:



    • Wind up



      • □ Readying phase


      • □ Minimal shoulder stress


      • □ Ground, legs, and trunk are force generators


    • Early cocking


    • Late cocking



      • □ Scapular retraction for stable throwing base


      • □ Maximal external rotation


      • □ Posterior translation of the humeral head as a result of abduction/external rotation (ABER)


      • □ Shear force across anterior shoulder of 400 newtons


      • □ Compressive force of 650 newtons generated by cuff


    • Acceleration



      • □ Transition from eccentric to concentric forces anteriorly (vice versa posteriorly)


      • □ Rotation occurs at 7,000-9,000 degrees per second


      • □ Only one-third of the kinetic energy leaves with the ball (the remainder is dissipated through the extremity)


    • Deceleration



      • □ Most violent phase (responsible for dissipation of energy not imparted to ball)


      • □ Distractive forces generated by the acceleration phase are countered by violent contraction of posterior rotator cuff and scapular stabilizers


      • □ Largest joint loads



        • Posterior shear force of 400 newtons


        • Inferior shear forces of > 300 newtons


        • Compressive forces of > 1000 newtons


        • Adduction torque > 80 newton-meter; horizontal abduction torque of 100 newton-meter


    • Follow-through



      • □ Rebalancing phase


      • □ Compressive forces of 400 newtons


      • □ Inferior shear of 200 newtons


  • The entire motion takes less than 2 seconds with most of the time (1.5 seconds) taken up by the early phases (wind up and cocking).



    • Three critical points in the motion:



      • □ Cocking: Point in process where full external rotation/abduction is achieved, shear force on labrum is maximum, biceps vector shifts posteriorly, and the energy generated by trunk/legs is transferred to shoulder; leads to potential injury situation for the shoulder at risk (2,3).


      • □ Acceleration: The body falls ahead of the shoulder while the internal rotators are maximally contracting and the angular velocity exceeds 7,000 degrees per second.


      • □ Deceleration: Scapular stabilizers and posterior rotator cuff contract violently to counter the distractive force of acceleration and lessen the load on the posterior inferior glenohumeral ligament (PIGHL).


Kinetic Chain of Throwing



  • The forces needed to propel the ball and generate the velocity of the throw require contributions from all body segments (4). These contributions help to minimize joint stress and allow for the forces to be passed to distal segments as the motion progresses.


  • The kinetic chain includes



    • Force generators – ground, legs, trunk


    • Force regulator – shoulder


    • Force delivery – arm



Dead Arm



  • Any pathologic shoulder condition in which the thrower is unable to throw with preinjury velocity and control because of a combination of pain and subjective unease in the shoulder (4).


Glenohumeral Internal Rotation Deficit (GIRD)



  • Basic definition: Loss of glenohumeral internal rotation in the throwing shoulder compared to the nonthrowing shoulder.


  • What constitutes a clinically significant amount of GIRD is debated.


  • Acceptable level of GIRD, as defined by Burkhart et al. (2,3,4), is less than 20 degrees or less than 10% of the total rotation measured in the nonthrowing shoulder.


  • Clinical significance may occur when internal rotation loss exceeds external rotation gain in the throwing athlete.


  • GIRD can also be seen in asymptomatic throwers and, in this case, may be related to increased humeral retroversion (21).


May 22, 2016 | Posted by in SPORT MEDICINE | Comments Off on The Throwing Shoulder
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