Lecture 6
Objectives:
Biomechanic of the Shoulder
Stability of G-H Joint
Glenoid fossa and humeral head  incongruent
 No pure rotation  combined with rolling and gliding
 Follow concave-convex rule
Static Stabilization
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Arm at side  superior joint capsule and coracohumeral ligament provide
translatory component vs. Gravity
Rotary component toward the joint  compression
Supraspinatus becomes active with heavier loads
G-H subluxation is common in post-CVA patients secondary to loss of rotator
cuff function (most notably supraspinatus)
Muscles:
1. Move the humerus
2. Provide intra-articular gliding
3. Maintain apposition of joint
Dynamic Stabilization
Prime movers (abduction)
 Deltoid and supraspinatus
Prime movers (flexion)
 Anterior deltoid
Deltoid – most of force translates humerus superiorly – small proportion  rotation
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Unopposed the deltoid cause impaction of the humeral head in the
coracoaromial arch  rotator cuff is the opposing force
Rotator Cuff (SITS – supraspinatus, infraspinatus, teres minor, and subscapularis)
 Tendon blends with G-H joint and reinforces capsule
 Provides rotary force that provides compression/stabilizing force
(infraspinatus, subscapularis, TM)
 Inferior translatory force (infraspinatus, subscapularis, TM) offsets superior
translatory force of deltoid
 Supraspinatus (steerer)– significant abductor and along with gravity produces
downward sliding of humeral head on glenoid.  vertical steerers
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Stability
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Other RTC muscles are steerers (horizontal and vertical) – subscapularis
offsets anterior dislocating forces
Anterior  subscapularis
Posteriorly  infraspinatus and TM
Equilibrium is a function of :
1. Force of prime movers
2. Force of gravity
3. Force of compressors and steerers
4. Joint reaction force / friction force
Supraspinatus – susceptible to degenerative changes secondary to:
1. Elevated subacromial bursal pressure
2. Poor vascularization
 tendon tear with trauma
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Supraspinatus is always working
Rotator cuff lesions  painful arc 60-120 degrees
Disruption of strength balance  excessive mobility of humeral head
Predisposition to dislocations
1. Anterior tilt of glenoid fossa
2. Excessive retrotorsion of humeral head
3. Weakened RTC
Biceps tendon
 Reinforces anterior G-H joint
 Wearing of bicipital tendon sheath – transverse humeral ligament
 Poorly vascularized.
Musculature
Since G-H is unstable, muscles exerting force on the humerus must act in synergy with
other muscles to avoid dislocating.
Methods of testing:
EMG – what is EMG? – analyze electrical activity
Nerve block – look at kinematics
Nerve stimulation – look at kinematics
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Elevation
Deltoid
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Resting position when arm is at side
Maintenance of optimal length-tension is dependent on scapula motion.
Based on orientation – best suited for superior translation
With counterforce to the superior translation:
 Anterior and middle deltoids are effective prime movers
 Posterior deltoid  small moment arm  joint compression
At the G-H joint, > elevation  > joint compression
RTC (oblique – infraspinatus, supraspinatus, & teres minor) combines with deltoid to
form a force couple that produces elevation of the UE.
Has demonstrated that the deltoids (middle) peaks at ~ 90 degrees of abduction – anterior
deltoids peak later during flexion.
EMG activity increases with elevation even though the MA increases and the effect of
gravity decreases. Why?
Active insufficiency – muscle is in a shortened position – requires more motor units to be
recruited for same magnitude of force.
NOTE: Axillary n. innervates deltoid
Scapula plays major role in determining/maintaining the proper length/tension
relationship for the deltoids.
Without the RTC  deltoid mainly translates the humerus (40 degrees abduction)
Supraspinatus (suprascapular n.)
Major abductor and flexor
Can bring G-H joint through full ROM without assistance of deltoid.
Functions:
1. Compresses G-H joint
2. Steers humeral head vertically
3. Maintains stability in the dependent arm
4. Prime mover for abduction and flexion
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Infraspinatus (suprascapular n.), teres minor (axillary n.), subscapularis (subscapular n.)
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Greater activity in flexion than abduction
Assists in depressing the humeral head initially  later externally rotates
humerus which is necessary for elevation
Subscapularis
 Medial rotation
 Compresses joint
 Stabilizes
Trapezius and Serratus Anterior
Two force couples:
1. Upper segments of traps and SA
2. Lower segments of traps and SA
With weakness of the SA and full trapezius function  FULL ABDUCTION
With trapezius weakness and full SA function  75 degrees of elevation
 Actually get a downward rotation of scapula
Note:
SA is more critical during shoulder flexion and conversely the trapezius is more critical
during shoulder abduction.
e.g. with SA weakness – 130-140 degrees of flexion  only get 20 degrees of upward
scapula rotation as opposed to the normal 60 degrees.
Middle traps and rhomboids  synergists for scapula stability – work eccentrically to
control upward rotation.
SEE GRAPHS OF ABDUCTION AND EXTERNAL ROTATION
Depression
 Pull-downs – free UE
 Pull-ups – fixed UE
Latissimus dorsi
 Adduction, medial rotation, extension, depression
 Seated push-ups, crutch walking
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Pectoralis major
 Sternal portion parallels the latissimus to depress the shoulder complex
Pectoralis minor
 Depresses and rotates scapula downward
Teres major
 Adduction, medial rotation, extension, depression – active only in static
positions of the humerus
Rhomboids
 Counteract pull of teres major and contribute to depression
SHOW EMG LINEAR ENVELOPES FOR ABDUCTION AND FLEXION (NORDIN)
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