Narrative Review
Reported mechanisms of shoulder injury
during the baseball throw
Craig A. Wassinger1, Joseph B. Myers2
Department of Physical Therapy, East Tennessee State University, Johnson City, TN, USA, 2Department of
Exercise and Sport Science, University of North Carolina at Chapel Hill, NC, USA
Published by Maney Publishing (c) W. S. Maney & Son Limited
1
Background: Shoulder complex injuries are common among overhand throwing athletes. These injuries
often manifest as a result of habitual sport performance and often lead to time loss injuries. The
mechanisms of these injuries are often non-traumatic and theories on how shoulder injuries manifest differ.
Objectives: To describe the proposed mechanisms of commonly reported shoulder injuries as they relate to
the phases of the throwing motion.
Major findings: Shoulder injuries commonly involve rotator cuff muscles and tendons, scapulothoracic
muscles, glenohumeral joint labrum, proximal humeral epiphysis, glenohumeral joint capsule, biceps
muscle and tendon, and subacromial bursa. The injuries found in these tissues and their purported
mechanisms of injury during the throw vary.
Conclusions: The late cocking and deceleration phases have been implicated with the largest number of
associated pathologies. Multiple injuries were theorized to occur at more than one phase of the throwing
motion. Consensus has not been achieved on the provocative events leading to shoulder injury during the
throwing motion.
Keywords: Biomechanics, Shoulder, Throwing injury
Introduction
Throwing athletes are prone to upper extremity
injuries secondary to repetitive end range movements
with high associated torques during sport participation. The dynamics of the throwing motion necessitate athletes to have sufficient range of motion,
strength, endurance, and neuromuscular control to
dissipate the stresses placed on the body. The upper
extremity, in particular the shoulder is the most
common injury location in throwing athletes.1–10
Furthermore, severe shoulder injuries are the
most likely to limit elite level baseball players’
participation.11 Shoulder injuries in throwers manifest in many tissues including rotator cuff muscles
and tendons, scapulothoracic muscles, glenohumeral
joint labrum, proximal humeral epiphysis, glenohumeral joint capsule, biceps muscle and tendon, and
subacromial bursa, among others. The majority of
throwing injuries are not incited by any one incident
but through repetitive end-range loading.12 The aim
of this narrative review is to describe common
shoulder injuries found in baseball players and their
theorized mechanisms of injury as they relate
to the biomechanics of the throwing motion. This
Correspondence to: C A Wassinger, Department of Physical Therapy,
East Tennessee State University, PO Box 70624, Johnson City, TN
37604, USA. Email: [email protected]
ß W. S. Maney & Son Ltd 2011
DOI 10.1179/1743288X11Y.0000000040
information will assist physical therapists and other
sports medicine professionals in injury prevention,
rehabilitation, and improvement of athletic performance in throwing athletes.
The throwing motion has been described as a
coordinated effort through the legs and trunk resulting
in a high velocity and forceful upper extremity
action.13 Exploiting this summation of energy yields
the ability to throw a baseball with velocities nearing
100 miles/hour but also places large physiological
stresses that have the potential to create many injuries.
The throwing motion has been described to have five
or six phases that are determined by kinematic time
points during the throw. The throwing motion (Fig. 1)
starts with the wind-up, progresses through early then
late cocking, acceleration, and deceleration, and
finishes with follow through.4
Reported Injury Mechanisms during Specific
Phases of the Throw
Phase 1: the wind-up
The wind-up starts from the initiation of movement.
The intention of this phase is to position the body in a
way that facilitates the rest of the throw. As such, there
is large individual variation in the kinematic pattern
and velocity of movement during the wind-up to
accommodate differing throwing styles (traditional
wind-up or the stretch).14 During this time, weight is
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Reported mechanisms of shoulder injury during the baseball throw
Figure 1 Phases of throw.
Published by Maney Publishing (c) W. S. Maney & Son Limited
shifted toward the plant (back) leg while the lead leg
elevates bringing the knee toward the chest.5 Wind-up
transitions into the early cocking phase once the lead
knee is at maximum height and the hands separate.4
No prior research has described specific injuries
occurring during this period given the relatively slow
kinematic velocities and low associated forces.
Phase 2: early cocking
The second phase is known as the cocking phase and is
often split into early and late portions. Early and late
cocking combined account for nearly 80% of the time
required for throwing which equals y1500 milliseconds.14
During early cocking, the trunk rotates toward the side of
the throwing arm and the lead leg positions the body
toward the target.5 In an attempt to locate the ball
posteriorly, the scapula is retracted while the humerus is
abducted to near 90u and is horizontally abducting while
externally rotating.15 The elbow is flexed to y90u.
Early cocking is the first phase during the throw
where specific biomechanically related injuries have
been reported. Pathological internal (posterior glenoid)
impingement has been linked with the early cocking
phase.16 Internal impingement is a condition in which
the superior aspect of the infraspinatus and/or the
posterior aspect of the supraspinatus tendons become
mechanically impinged between the humeral head and
the posterior superior glenohumeral labrum.17 Internal
impingement has been reported to be an expected and
normal phenomenon in throwing.18 Only when the
impingement leads to pain, inflammation, or damage of
the posterior/superior joint tissues (rotator cuff tendons, glenoid labrum, greater tuberosity, or superior
glenoid rim) does the condition become pathological. In
general, the mechanism of injury is described as arising
when the humerus repetitively horizontally abducts
beyond the plane of the scapula injuring the implicated
tissue(s) (Fig. 2).19,20 This positional fault may be exacerbated by several other factors including faulty scapular kinematics, anterior instability (described later),
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decreased humeral torsion, and poor throwing mechanics as well.15,21–23
Altered scapular position may predispose the posterior
tissues to internal impingement.21 Specifically, deficient
scapular external rotation during the cocking phase may
predispose internal impingement as the glenoid is
positioned more anteriorly.21 This malpositioned scapula
creates a more anteriorly oriented scapular plane. In
turn, internal impingement may occur as the humerus
horizontally abducts posterior to the anteriorly malpositioned scapular plane. Three-dimensional scapular kinematics measured during the throwing motion indicate
that maximal scapular external rotation occurs during
late cocking lending credence to altered scapular
kinematics as a mechanism of injury during this phase.15
Internal impingement has been described to occur in
both early and late cocking depending on individual
throwing characteristics.19,21,22,24,25 The phase of throwing during which internal impingement occurs will vary
based on predisposing factors including scapular kinematics, anterior glenohumeral restraints, and range of
motion utilized during the throw. The injurious kinematics involved, however, are similar.
Figure 2 Anatomical depiction of internal impingement.
Reprinted from the American Journal of Sports Medicine, 26/3,
Riand N, Levigne C, Renaud E, Walch G, Results of Derotational
Humeral Osteotomy in Posterosuperior Glenoid Impingement,
p. 454, Copyright 1998, with permission from Elsevier.
Wassinger and Myers
Published by Maney Publishing (c) W. S. Maney & Son Limited
Phase 3: late cocking
As the lead foot makes ground contact, the thrower
transitions from early cocking into the late cocking
phase. Late cocking places the humerus at its most
extreme range of motion and has been reported to
exhibit an anterior shear force equal to approximately
half the thrower’s body weight.26 During late cocking,
the entire body is moving toward the target location, the
torso is extending and rotating away from the throwing
side, the scapula retracts, elevates, and externally rotates
while the humerus is elevated and externally rotating.15
The elbow is flexing and the wrist will slightly extend.
The late cocking phase of the throw is associated
with assorted injuries secondary to the extreme range
of motion of the upper extremity during this time.
Superior labrum anterior-to-posterior (SLAP) lesions
have been reported to occur during the late cocking
phase of the throw via a ‘peel-back mechanism’.27,28
An SLAP lesion is a tearing of the superior aspect of
the glenohumeral labrum which often includes the
proximal insertion of the biceps tendon.29 The SLAP
lesion is theorized to occur via repetitive contraction of
the biceps with the humerus positioned in abduction
and maximal external rotation as in late cocking. This
humeral position creates a posteriorly oriented tensile
force through a rotationally torsioned long head of the
biceps. The extreme humeral position and corresponding angle of pull combined with the rotation through
the proximal bicep tendon is proposed to torsionally
stress the superior labrum. This may cause a compromise of the superior labrum or become pathological if
the labral attachment has been previously compromised.27,28 The peel-back injury mechanism to the
superior labrum theory has been supported through
cadaveric study.30
As skeletally immature throwers attempt to position
the humerus into maximal external rotation, the
humerus is susceptible to epiphyseal changes.31,32
Humeral retrotorsion, or posterior twisting of the
humeral head on the diaphysis, is a common finding
among throwing athletes as the epiphyseal plate is
weaker than the supportive ligaments.31,33–38 Humeral
retrotorsion is associated with alterations in humeral
rotation range of motion.34,36,39,40 Throwers often
exhibit increased external rotation and decreased
internal rotation at the glenohumeral joint associated
with the altered humeral shaft rotation.34,36,39 Similar to
internal impingement, humeral retrotorsion is thought
to be a normal osseous adaptation to repetitive
throwing that transpires prior to skeletal maturity.
However, humeral retrotorsion has also been implicated as a risk factor for shoulder injury.35,38
Proximal humeral epiphysiolysis, on the contrary, is
a pathologic condition at the proximal humeral
epiphysis noted by localized pain and radiographic
evidence of epiphyseal widening.31 This pathological
Reported mechanisms of shoulder injury during the baseball throw
condition has been referred to as: ‘Little Leaguer’s
shoulder’, proximal humeral epiphysis, proximal humeral epiphysiolysis, or a rotation stress fracture of the
proximal humeral epiphyseal plate.41 Proximal humeral epiphysiolysis is thought to occur when the rate or
frequency of loading into shoulder external rotation
(late cocking) causes growth plate damage.32 Rotational torques measured during throwing in youth
baseball players substantiate this injury mechanism.31
As the arm is placed in the late cocking position,
the anterior glenohumeral ligaments are placed under
tension.22 Movements toward terminal humeral
horizontal abduction and external rotation place
tensile stresses across the anterior ligaments which,
over time, can introduce tissue stretching creating
increased anterior humeral head shearing.22 In
addition to anterior ligament elongation pathologies
of the glenohumeral joint, tearing of the anterior
glenoid labrum and glenohumeral instability are also
implicated by such attenuation. This decreased
stability of the glenohumeral joint has been theorized
to create greater potential for impinging the posterior
rotator cuff tendons. This may be further exacerbated
with fatiguing throwing conditions.22
Phase 4: acceleration
Acceleration begins when the humerus initiates movement from maximal humeral external rotation into
internal rotation and ends with ball release. Acceleration has been reported to occur over 50 milliseconds
(2% of the throw) with reported humeral rotational
velocities averaging up to 7000u/second.26 It has also
been noted that up to 1100 N of distractive force occurs
on the humerus during acceleration.16 Kinematically,
the torso laterally bends and rotates away from the
throwing side while slightly flexing.42 The scapula
internally rotates and anterior tilts as the humerus
moves into horizontal adduction and internal rotation.15
The elbow continues to extend with wrist flexion and
pronation until ball release.22 At ball release, the lead
knee will be extending while planted on the ground.5
Glenoid labrum fraying has been noted to occur
during acceleration if active or passive humeral
stabilizers are ineffective. This instability can cause
the humeral head to articulate with the labrum as
opposed to the glenoid causing an abnormal compressive load with motion and thus labral tearing.7
The rotator cuff muscles are primarily responsible for
resisting the great distractive forces at the glenohumeral
joint. Labral fraying could follow if the humeral head is
not centered within the glenoid and the compressive
forces are repetitively misplaced onto the labrum. The
potential for increased contact with the labrum may
increase in the presence of abnormal capsuloligamentous laxity (described earlier) or muscle fatigue from
repeated throwing.
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Reported mechanisms of shoulder injury during the baseball throw
Phase 5: deceleration
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The deceleration phase attempts to dissipate the high
velocities of the upper extremity after ball release.
The angular deceleration has been reported to occur
over 50 milliseconds.14 Slowing the arm from the
high acceleration velocities imposes a distractive force
at the shoulder which has been demonstrated to
approximate body weight.26 Subsequently, the deceleration phase of throwing is implicated in the
majority of shoulder injuries as the body attempts
to slow the arm from exceptionally high rotational
velocities following ball release. Kinematically, during deceleration, the lead leg is in unilateral stance
and the torso continues to rotate away from the
throwing shoulder.42 The scapula continues to anteriorly tilt while internally and downwardly rotating.15
The humerus adducts, horizontally adducts, and
internally rotates with an elbow nearly full extended
and wrist toward end range of motion into flexion.14
SLAP lesions, which were described above to occur
during the late cocking phase, have also been reported
to occur via tensile forces during deceleration.2,16
The SLAP lesion occurs as the biceps is very active
proximally, in stabilizing the humeral head, and
distally, attempting to slow elbow extension.16 Elbow
extension velocities and maximal humeral distraction
torques occur at the same time interval requiring
eccentric biceps contraction to resist proximal and
distal loads. This large tensile load, acting through the
proximal attachment to the labrum is thought to be an
additional mechanism of SLAP tear. Accordingly,
tensile injuries to the biceps tendon have been
theorized to come about during deceleration.6,16 Inflammation and tearing secondary to the large eccentric strain placed on the biceps tendon implicate both
the tendon and the superior labrum.16
The deceleration phase has also been implicated in
tearing of the supraspinatus and infraspinatus.1 This
report, based on surgical data, indicated that the
majority of tears were noted in the mid-supraspinatus
to the mid-infraspinatus and resulted from tensile
failure as these muscles attempted to resist distraction, horizontal adduction, and internal rotation of
the humeral head.1 Supraspinatus and infraspinatus
intratendinous articular surface partial thickness
tears are also implicated with the large eccentric
forces of the rotator cuff during deceleration.43 This
area is reported to be at increased risk of tearing
due to the low ultimate stress to failure and hypovascularity relative to the bursal surface of the
tendons.44–46 The posterior shoulder musculature
including posterior rotator cuff (supraspinatus, infraspinatus, and teres minor) and posterior deltoid
insufficiency has also been implicated in posterior
shoulder tightness.8,9 These muscles are responsible
for slowing the horizontal adduction and internal
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rotation of the humerus. The associated large torques
during deceleration are thought to overcome the
muscle forces of the posterior rotator cuff and related
posterior joint capsule and ligaments creating tearing
and inflammation.10,18 Over time these tendons, along
with the intimately related posterior joint capsule,
become contracted as the body lays down collagen
tissue to help support the role of these muscles/
tendons.3 The accumulation of collagen tissue leads to
contracture of the posterior joint tissues causing a
condition known as posterior shoulder tightness or
posterior capsular tightness.10,18 The implications of
the contracted posterior shoulder tissues are far
reaching and beyond the scope of this manuscript.
Maximal humeral head superior shear forces of the
humerus occur during deceleration.16 This may contribute to subacromial impingement if the other
rotator cuff muscles are unable to resist the superior
shear force.16 The shear forces pose another risk to the
supraspinatus tendon and implicate the biceps tendon
and subacromial bursa as well. During this phase, the
scapula is pulled into internal rotation, anterior tilting,
and downward rotation thus minimizing the available
subacromial space.47 Concomitant with these scapular
motions, the humerus remains in a relatively elevated
and internally rotated position which furthers the risk
of mechanical irritation of the subacromial structures
by placing the greater tuberosity in this space.15
Aberrant scapular kinematics, therefore, can also play
a role in subacromial impingement during throwing.
Phase 6: follow through
The follow through and deceleration phases are
separated by maximum humeral internal rotation.
Follow through completes the throw cycle and
involves extension of the stride knee, continued hip
flexion, shoulder adduction and horizontal adduction, elbow flexion, and forearm supination.5 This
time period acts to decrease the loading rate on the
involved joints by increasing the time associated with
slowing body movements. Follow through of the
throw occurs over approximately 300 milliseconds,
which equates to y15% of the total throwing time.14
Some authors report injuries during follow through;
however, these primarily occur during deceleration
using the context described above, as these two
phases are occasionally not separated.
Conclusion
The intention of this manuscript was to provide an
overview of common shoulder injuries found in
throwing athletes. The late cocking and deceleration
phases have been implicated with the largest number of
associated pathologies. This review also attempted to
describe these injuries in relationship to the kinematic
and kinetic parameters associated with the throwing
motion. Many shoulder pathologies have multiple
Wassinger and Myers
proposed etiologies during the throwing motion suggesting the need for further research. By understanding
the characteristics of the throw and injuries that occur
during the respective phases, physical therapists and
other sports medicine clinicians will be well suited to
assist athletes with shoulder-specific training, injury
prevention, and rehabilitation programs.
23
24
25
26
Published by Maney Publishing (c) W. S. Maney & Son Limited
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