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The International Journal of Sports Physical Therapy | Volume 6, Number 1 | March 2011 | Page 51
ABSTRACT
Subacromial impingement is a frequent and painful condition among athletes, particularly those involved
in overhead sports such as baseball and swimming. There are generally two types of subacromial impinge-
ment: structural and functional. While structural impingement is caused by a physical loss of area in the
subacromial space due to bony growth or inflammation, functional impingement is a relative loss of sub-
acromial space secondary to altered scapulohumeral mechanics resulting from glenohumeral instability
and muscle imbalance. The purpose of this review is to describe the role of muscle imbalance in subacro-
mial impingement in order to guide sports physical therapy evaluation and interventions.
IJSPT
CLINICAL SUGGESTION
SHOULDER MUSCLE IMBALANCE AND SUBACROMIAL
IMPINGEMENT SYNDROME IN OVERHEAD ATHLETES
Phil Page, PhD, PT, ATC, LAT, CSCS, FACSM
CORRESPONDING AUTHOR
Phil Page, PhD, PT, ATC, LAT, CSCS, FACSM
Baton Rouge, Louisiana USA
Email: ppage100@gmail.com
The International Journal of Sports Physical Therapy | Volume 6, Number 1 | March 2011 | Page 52
INTRODUCTION
According to the late neurologist Vladimir Janda
MD, there are 2 approaches to classification of mus-
culoskeletal pathologies: structural and functional.1
The structural approach focuses on actual damage to
musculoskeletal structures such as rotator cuff ten-
donitis or a ligament injury. The functional approach
examines factors that contribute to structural lesions.
This approach is most useful for physical therapy
management of chronic ‘dysfunctions’ such as per-
sistent joint pain and tendonitis.
Shoulder impingement accounts for 44 to 65% of
shoulder complaints during physician visits.2-3 First
described by Neer,4 shoulder impingement has been
classified into two main categories: structural and
functional. Subacromial impingement can be caused
by narrowing of the subacromial space (SAS) result-
ing from a reduction in the space due to bony growth
or soft-tissue inflammation, (“structural”) or supe-
rior migration of the humeral head caused by weak-
ness and/or muscle imbalance (“functional”).5-8 It is
possible that some subacromial impingement results
from a combination of both structural and functional
factors.
Subacromial impingement occurs when the struc-
tures in the SAS (rotator cuff, biceps tendon long head,
and subacromial bursa) become compressed and
inflamed under the coracoacromial ligament.9 The
suprasinatus tendon in particular is at highest risk for
irritation and subsequent injury because it is the most
likely to contact the acromion when the humerus is
abducted to 90° and internally rotated 45°.10
Patients with impingement have significantly less
(–68%, p < .05) SAS during shoulder elevation com-
pared to the asymptomatic side when measured
using MR imaging,11 even though their SAS is not sig-
nificantly different from healthy shoulders in the
resting anatomical position.12 When compared to
normal subjects, patients with impingement demon-
strate more proximal translation of the humeral
head during abduction, thus reducing the SAS.6, 13
Functional impingement is related to glenohumeral
instability14 and is sometimes described as “func-
tional instability,” occurring mostly in overhead ath-
letes less than 35 years of age.15 The act of throwing
may cause tissues below the coracoacromial arch to
be subjected to subtle microtrauma, leading to
inflammation and tendinitis.16-17
The shoulder complex relies on muscles to provide
dynamic stability during its large range of mobility.
Proper balance of the muscles surrounding the shoul-
der complex is also necessary for flexibility and
strength; a deficit in flexibility or strength in an ago-
nistic muscle must be compensated for by the antago-
nist muscle, leading to dysfunction. These muscular
imbalances lead to changes in arthrokinematics and
movement impairments, which may ultimately cause
structural damage. Dr. Janda suggested that subacro-
mial impingement results from a characteristic pat-
tern of muscle imbalance including weakness of the
lower and middle trapezius, serratus anterior, infra-
spinatus, and deltoid, coupled with tightness of the
upper trapezius, pectorals and levator scapula.1 This
pattern is often referred to as part of Janda’s “Upper
Crossed Syndrome.” (See Figure 1)
While structural impingement sometimes requires
surgery to alleviate pain, functional instability requires
the implementation of precise therapeutic exercises
Figure 1. Janda’s Upper Crossed Syndrome. Reprinted, with
permission, from Page et al, 2010, Assessment and Treatment
of Muscle Imbalance: The Janda Approach (Champaign, IL:
Human Kinetics).
The International Journal of Sports Physical Therapy | Volume 6, Number 1 | March 2011 | Page 53
Imbalances or deficits in muscular strength and
activation levels can lead to functional impinge-
ment. Both glenohumeral and scapulothoracic mus-
cle imbalances can contribute to shoulder complex
dysfunction. The pathomechanics of functional
impingement may involve one or both of the shoul-
der force couples: deltoid/rotator cuff and scapular
rotators. (Figures 2-3) Because of the lack of prospec-
tive studies, researchers have not determined
whether muscle imbalance is a contributor to or
result of impingement.
GLENOHUMERAL IMBALANCES
Alterations in deltoid and rotator cuff co-activation
and rotator cuff imbalances have been described in
patients with impingement.30-34 The deltoid plays an
important role in the pathomechanics of impinge-
ment due to its ability to offer upwardly directed force
which must be balanced by the synchronous function
of the rotator cuff musculature. Muscle imbalances in
within the deltoid and rotator cuff force couple can
cause compression within the SAS.7, 35 The deltoid has
been found to be atrophied and infiltrated with
with the goal of restoration of normal neuromuscular
function. It is important for clinicians to understand
the pathomechanics of functional impingement in
order to guide appropriate examination, assessment,
and intervention, as well as to consider prevention.
The purpose of this clinical suggestion is to describe
muscle imbalances associated with functional impinge-
ment in overhead athletes and to offer suggestions to
guide intervention choices and prevention strategies.
PATHOMECHANICS OF MUSCLE
IMBALANCE IN SUBACROMIAL
IMPINGEMENT
Muscle tightness has been implicated in subacromial
impingement. In particular, during elevation, ante-
rior shoulder girdle muscle tension may affect the
tension on the leading edge of the coracoacromial
ligament, predisposing it to tightness ultimately
leading to structural impingement.16 Tightness of
the pectoralis major creates an anterior force on the
glenohumeral joint with a consequent decrease in
stability.18 A tight pectoralis minor limits scapular
upward rotation, external rotation, and posterior tilt,
thereby reducing SAS.19 This alteration in scapular
kinematics occurs in three separate planes of move-
ment and differs from scapular kinematics of those
with normal muscle length.20-21
Imbalances in glenohumeral rotation range of
motion may also contribute to altered shoulder kine-
matics. Specifically, excessive external rotation leads
to increased anterior and inferior translation of the
humerus, leading to anterior instability.22 In con-
trast, a lack of external rotation due to anterior mus-
cular tightness alters the scapulohumeral rhythm
and decreases posterior scapular tilt.23 Posterior cap-
sular tightness, often demonstrated by a loss of inter-
nal rotation, may lead to more superior and anterior
translation of the humeral head.23-25 This loss of
internal rotation is known as glenohumeral internal
rotation deficit, or “GIRD,” and is defined as a loss of
internal rotation greater than or equal to 20° com-
pared to the contralateral side.26 GIRD is a relatively
new concept in the literature that requires more
research regarding its incidence and effects in
normal, athletic, and injured populations. Recent
evidence suggests that overhead athletes with impinge-
ment often display signs and symptoms of GIRD.27-29
Figure 2. Rotator Cuff / Deltoid Force Couple. Reprinted,
with permission, from Page et al, 2010, Assessment and Treat-
ment of Muscle Imbalance: The Janda Approach (Cham-
paign, IL: Human Kinetics).
The International Journal of Sports Physical Therapy | Volume 6, Number 1 | March 2011 | Page 54
connective tissue in patients with shoulder impinge-
ment, 36-37 and it exhibits lower levels of EMG activa-
tion in patients with impingement.33, 38 While it is
assumed that these effects on the deltoid are caused
by impingement, it is unclear if the deltoid pathology
precedes or is a result of impingement.
The rotator cuff is important in maintaining normal
humeral head position in the glenoid during elevation
(flexion and abduction) movements. The compressive
forces of the rotator cuff stabilize the humerus against
the glenoid, thereby providing dynamic stabilization
of the glenohumeral joint.39-40 Weakness of the infra-
spinatus reduces this compressive force, promoting
instability.18 This instability may lead to functional
impingement.
When the dynamic stabilizing forces of the rotator
cuff are removed from the glenohumeral joint in a
cadaver model, there is a significant increase in
superior and anterior migration of the humeral head
during elevation, which would lead to impinge-
ment.35, 41-42 Downward compressive forces of the
inferior rotator cuff are necessary to neutralize
the upwardly directed shear forces of the deltoid.35
Without rotator cuff stabilization in cadaveric mod-
els, the humeral head migrated 1.7 mm vs. 0.7 mm
with rotator cuff stabilization at 60° of abduction,
and 2.1 mm vs. 1.4 mm at 90° of abduction.42 Clearly,
while cadaveric models do not accurately reflect the
effect of dynamic neuromuscular activation (muscle
activation and timing) of glenohumeral and scapulo-
thoracic muscles during glenohumeral kinematics,
they may offer some insight into the role of the rota-
tor cuff.
Decreased rotator cuff EMG activity may also con-
tribute to humeral head superior translation during
early abduction, leading to impingement.33 Experi-
mentally-induced fatigue of the rotator cuff leads to
superior migration of the humeral head at the initia-
tion of abduction,43-44 however, the effects of fatigue
experienced after actually participating in an activ-
ity (such as repeated throwing) have not been inves-
tigated. Since these two studies only assessed
scapular plane elevation, it is possible that other
muscles may compensate for upward migration of
the humeral head during functional activities that
occur in planes other than the scapular plane. Few
studies have assessed simultaneous rotator cuff
EMG and glenohumeral kinematics in patients with
impingement, leaving many questions unanswered
regarding the exact pathomechanics of impingement.
SCAPULOTHORACIC IMBALANCES
Scapular rotation force couple imbalance leads to
altered muscular activation patterns. When studying
patients with impingement, most researchers describe
an increase in upper trapezius EMG activation cou-
pled with a decrease in activation of the middle trape-
zius, lower trapezius, and the serratus anterior.45-51 In
contrast, other researchers have reported increased
EMG activation in both the upper and lower trapezius
in patients with impingement when compared to nor-
mal subjects.48 Ludewig and Cook hypothesized that
the increased lower trapezius activation was an
attempt to compensate for decreased serratus ante-
rior activation. Interestingly, Lin et al studied subjects
with various types of shoulder dysfunction and found
decreased serratus anterior activity and increased
upper trapezius activity without a change in lower
trapezius activity when compared to normals.52
Figure 3. Trapezius / Serratus Force Couple.
The International Journal of Sports Physical Therapy | Volume 6, Number 1 | March 2011 | Page 55
have a significantly higher upper trapezius activation
compared to normal subjects, a significant decrease
in lower and middle trapezius activation, and altered
trapezius muscle balance (See Table 1).
Overhead athletes with impingement have delayed
onset of middle and lower trapezius fibers in response
to a sudden downward movement.46 If the lower tra-
pezius reacts too slowly when compared to the upper
trapezius, the upper trapezius may become overac-
tive, leading to scapular elevation rather than upward
rotation.46 Freestyle swimmers with impingement
are reported to have increased variability in timing of
the onset of scapular rotators compared to healthy
swimmers.50 These alterations in activation patterns
are often seen bilaterally in patients with chronic
tendinosis, 31, 46, 50, 59-60 supporting a neuromuscular
mechanism. Since both painful and non-painful
shoulders exhibit altered activation patterns, it is pos-
sible that the dysfunction is related to a faulty motor
program within the central nervous system (CNS).
CONCLUSIONS
In summary, functional impingement may be asso-
ciated with muscle imbalance; therefore, careful
examination of flexibility and strength of important
muscles about the shoulder complex is vital to under-
standing the root cause of impingement and pre-
scribing effective treatment. Janda’s approach to
muscle imbalance suggests a possible neuromuscu-
lar component to functional impingement due to
the predisposition of certain muscles to be tight or
The lower trapezius may play the most important role
in the scapular rotation force couple because it acts
primarily as a scapular stabilizer.53-54 Decreased activa-
tion of the lower trapezius or increased activation of
the upper trapezius may lead to an alteration of scapu-
lar rotation position which in turn leads to an upward
migration of the axis of rotation of the glenohumeral
joint, thus causing impingement. It is assumed that
the lower trapezius demonstrates increased activity if
the humeral head migrates upward during shoulder
elevation,53 however, research has not verified this
notion. Researchers have simultaneously measured
trapezius EMG and 3-dimensional kinematics in
patients with shoulder dysfunction.48, 52 These studies
found no significant change in humeral elevation, and
either no change52 or an increase48 in lower trapezius
activation. Ludewig and Cook48 reported small but sig-
nificant increases in anterior-posterior translation of
the humerus, possibly leading to decreased SAS.
Several authors have studied athletes with shoulder
pain and have described altered EMG patterns and
patterns of muscle imbalance.50, 55-57 Overhead ath-
letes with shoulder dysfunction typically have
increased upper trapezius activation,51 as well as
decreased activation levels of the serratus anterior,47
and lower trapezius,47, 51 supporting Janda’s belief that
the lower trapezius and serratus are most prone to
weakness.58 Researchers have compared the EMG
activity of the trapezius in normal individuals,
overhead athletes, and those with impingement.45-46, 59
Cools et al51 reported that athletes with impingement
Table 1. EMG activation of subjects with and without impingement during isokinetic abduction at 120°/s
reproduced from Cools et al, 2007.
The International Journal of Sports Physical Therapy | Volume 6, Number 1 | March 2011 | Page 56
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weak. The literature substantiates that imbalances
in the glenohumeral and scapulothoracic muscula-
ture are present in patients with subacromial
impingement.
Most believe that functional impingement is best
managed with conservative treatment. While
structural impingement sometimes requires surgi-
cal intervention, surgery for functional impinge-
ment may make patients worse. Successful treatment
of functional impingement related to muscle imbal-
ance is often accomplished by addressing the cause
of the problem rather than symptomatic treatment
of the pain. By understanding muscle imbalances
associated with functional impingement, physical
therapists can prescribe appropriate exercises for
both treatment and prevention.
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