Sports Medicine: Just the Facts

• The phases of throwing are summarized as follows:
  •Wind-up
  a. Readying phase
  b.Minimal shoulder stress
  
  
  • Early cocking
  
  
  • Late cocking
    a. Scapular retraction for stable throwing base
    b.Maximal external rotation
    c. Posterior translation of the humeral head as a
    result of abduction/external rotation
    d. Shear force across anterior shoulder of 400 N
    e. Compressive force of 650 N generated by cuff
    •Acceleration
    a. Transition from eccentric to concentric forces
    anteriorly (vice-versa posteriorly)
    b.Rotation occurs at 7000°–9000°/s
    c. Only 1/3 of the kinetic energy leaves with the ball
    (the remainder is dissipated through the extrem-
    ity)
    •Deceleration
    a. Most violent phase (responsible for dissipation of
    energy not imparted to ball)
    b.Largest joint loads
    
    
    1. Posterior shear force of 400 N
    
    
    2. Inferior shear forces of greater than 300 N
    
    
    3. Compressive forces of greater than 1000 N
    
    
    4. Adduction torque >80 N-m; horizontal abduc-
       tion torque 100 N-m
       •Follow-through
       a. Rebalancing phase
       b.Compressive forces 400 N
       c. Inferior shear of 200 N
    
    
    
    
  
  
  • The entire motion takes less than 2 s with most of
    the time (1.5 s) taken up by the early phases (wind-
    up and cocking)
    •Two critical points in the motion
    a. Cocking: Full external rotation maximizes ante-
    rior shear forces and applies the highest torque to
    the shoulder.
    b.Acceleration: The body falls ahead of the shoulder
    while the internal rotators are maximally contract-
    ing and the angular velocity exceeds 7000°/s.
  
  
  
  

PATHOPHYSIOLOGY

ROTATOR CUFF

• Supraspinatus, infraspinatus, and teres minor fire in
  late cocking to move to maximal external rotation,
  followed by eccentric firing in deceleration.
  •Tensile stress developed in tissues that may speed
  normal degeneration. Factors of stressful loading, dis-
  traction, excessive internal, and external rotation can

cause acute inflammatory responses early (leading to

impingement) or tendon failure in the later stages

(rotator cuff tears).

LABRAL TEARS (SUPERIOR LABRUM ANTERIOR

TO POSTERIOR(SLAP))

TRACTIONMECHANISM

• During deceleration, biceps muscle contraction is
  strong as both elbow extension and glenohumeral dis-
  tractions occur. The biceps muscle has been shown to
  be essential to limiting torsional forces to the shoulder
  in the abducted, externally rotated position (Rodosky,
  Harner, and Fu, 1994). By this mechanism, the effect
  on the superior labrum would be one of failure either
  by tension or direct compression.

PEELBACKMECHANISM

•Tension overload develops in the abducted/externally

rotated extremity (Morgan et al, 1998) based on three

observations:

1. A type II SLAP (posterosuperior) lesion can cause
   anterior pseudolaxity.
   2.Abduction/External Rotation(ABER) causes peel-
   back of posterosuperior labrum.
   3.Posterior inferior capsule tightens in overhand throw-
   ers. Axis of rotation of humeral head is shifted pos-
   terosuperiorly and increases internal impingement.

CAPSULAR

EXTERNALROTATIONEXCESS

• Excessive external rotation leading to soft tissue adap-
  tive changes and subsequent instability (Kvitne and
  Jobe, 1993).
  •With failure of ligamentous restraints, coracoacromial
  arch impingement may result (secondary impinge-
  ment).


• Secondary posterior capsular tightness also occurs.

INTERNALIMPINGEMENT

• Rotator cuff impinges on the posterosuperior rim of
  the glenoid in ABER (Jobe and Sidles, 1992).


• Causes pain in ABER position and correlates with
  positive apprehension and relocation maneuvers
  (Walch et al, 1992).


• Etiology: Two theories
  
  
  1. Physiologic phenomenon causing labral and cuff
     tearing with repetitive activity
  
  
  2. Secondary internal impingement as a result of
     excessive external rotation developing with repeti-
     tive throwing
  
  
  
  

288 SECTION 4 • MUSCULOSKELETAL PROBLEMS IN THE ATHLETE