The Scapular Contribution to the Amplitude of Shoulder External Rotation on Throwing Athletes - Technologies for Medical Sciences - page 224

the humeral head remains within the glenoid socket, creating a stable fulcrum for
rotation; this is known as the “ thrower´s paradox ”. With each pitch, the soft-tissue
envelope that surrounds the shoulder is loaded at levels that approach the ultimate
failure loads of the tissues, which are thus quite vulnerable to injury.

1.3 The “Throwing Shoulder”

Numerous studies have documented motion adaptations on the dominant shoulder of
throwers either by comparing shoulders bilaterally or with the dominant shoulder of
non-athletes [ 5 , 6 , 13 , 23 , 24 ]. One of the most visible and highlighted adaptations,
imposed by the repetitive throwing cycle at high velocities over time, includes
changes on shoulder rotational ROM pattern with increased external rotation
(external rotation gain) and limited internal rotation (glenohumeral internal rotation
deficit), while the range of the total arc of motion (external arc plus internal arc) is
kept unchanged [ 22 ].
In general, the shoulder rotational adaptation on the asymptomatic dominant
throwing shoulder of an elite-level athlete was described as an increased external
rotation arc and a correspondent decrease in the internal rotation arc, while the
amplitude of the total arc is kept unchanged, in a condition called the “ posterior
shift ”[ 11 , 25 - 27 ]. This adaptive pattern was mostly described through goniometric
studies [ 28 - 30 ] where the athletes were assessed in a supine or a sitting position
with the arm placed at 90 ı of abduction. The arm is then passively rotated from
the extreme position (end-range) internal rotation until the end-range of external
rotation, or vice-versa. Following this standard goniometry procedure, the shoulder
rotation end-range is determined by the examiner according to the sensation
of capsular end-feel, the scapular liftoff momentum or perceived pain. A few
studies described the changes on the rotational pattern using an active end-range
determination [ 31 , 32 ] and no studies to date have specifically investigated how
humeral rotational pattern is affected by active or passive end-range determination
in overhead throwing athletes.
The posterior shift in the total arc of motion is considered to be a physiological
adaptation of the shoulder joint to throwing. According to Wilk et al. [ 27 ]most
throwers exhibit an obvious motion disparity, whereby shoulder external rotation
(ER) is excessive and internal rotation (IR) is limited when measured at 90 ı of
abduction. This loss of IR on the throwing shoulder, referred to as “ glenohumeral
internal rotation deficit ”(GIRD)[ 8 , 33 , 34 ], is suggested to be caused by the
retraction of the posterior capsule induced by the increased amplitude of external
rotation in the late cocking phase. This allows hyper-external rotation as the
posterior capsule reaches maximum length while the anterior capsule still allows
for additional external rotation. Burkhart et al . [ 19 ] described the GIRD as an
alternative mechanism for primary progression of “ internal impingement-like ”
changes in the shoulder. The glenohumeral internal rotation deficit model is based
on the high prevalence of posterior capsular contractures and contractures of the