Lecture 5
Biomechanics of the Shoulder
Includes 4 articulations:
1. Glenohumeral
2. Acromioclavicular
3. Sternoclavicular
4. Scapulothoracic
First 3 are synovial joints – scapulothoracic is psuedo-articulation
What motions occur in the shoulder?
 Flexion/extension
 Abduction/adduction
 Internal/external rotation
 Horizontal abduction/adduction
 Scaption – elevation in the plane of the scapula (scapula is rotated 30-45
anterior to the frontal plane) – most function of all movements
Motion at all 4 joints is necessary for full functioning of the shoulder joint. Restriction in
any one joint can inhibit shoulder motion.
Glenohumeral Joint
Surface area of glenoid fossa is 1/3 to ¼ that of humeral head.
Allows for minimal bone to bone contact  lots of motion  susceptible to dislocation
Relies primarily on soft tissue structures for stability:
Anteriorly – superior, middle, and inferior g-h ligaments
Superiorly – coracohumeral ligament and acromion
Rotator cuff (supra/infraspinatus, subscapularis, teres minor) blend with the joint
capsule to provide ant, post, and inferior stability.
Stability Factors:
1. Size of glenoid fossa (75x57%)
2. Posterior tilt of glenoid fossa
3. Humeral head retroversion (posteriorily angled)
4. Intact capsule and glenoid labrum
5. Muscles – rotator cuff
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Which of the factors can we affect as physical therapists? And how?
Motion is primarily rotational with some gliding and rolling
Adheres to convex-concave rule. Convex moving on concave  move in opposite
directions
What are the most common injuries that occur at the G-H joint?
Acromioclavicular
Stability -Coracoclavicular ligament
This ligament permits scapula to move on clavicle about 3 axes:
 Conoid (coracoid to clavicle) – protraction /retraction (30 deg)
 Trapezoid (coracoid to clavicle) – rotation about transverse axis in frontal
plane (60 deg)
 Acomioclavicular ligament - rotation about transverse axis in sagittal plane
(30 deg)
Meniscus divides it into functional units
 Conoid – motion occurs between acromion and meniscus
 Trapezoid – hinging occurs between meniscus and clavicle
Sternoclavicular Joint – also has a meniscus creating 2 function units
Stability – costoclavicular ligament –1st rib to clavicle
Anterior/posterior gliding (protraction/retraction) – between sternum and meniscus
Superior/inferior gliding – between clavicle and meniscus
40 deg total movement (4:10 clavicular elevation: arm elevation) during arm elevation.
40 deg about long axis of clavicle
Motion at distal ends of clavicle is reciprocal except for long axis rotation
Scapulothoracic
Motions – protraction/retraction, elevation/depression, rotation
Serratus anerior holds scapula close to the wall.
What exercises can be used to strengthen the serratus anterior?
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Scapulothoracic Rhythm
Function:
1. Maximize ROM
2. Maintain glenoid fossa in optimal position
3. Maintain optimal length-tension relationship for muscles
Arm elevation: (flexion)120 deg G-H motion / 60 deg scapulothoracic
(abduction) 90 deg G-H motion / 60 deg scapulothoracic
1. Initial 60 flexion/30 abduction – inconsistent movement of S-T joint
(stabilizing phase)
2. Beyond this initial 60/30 – 1:1 ratio
3. Last 30 degress – 5:4 ratio G-H to S-T
SC and AC joints also contribute to the S-H movement.
The spine can influence the position of the scapula.
Upward rotation of the scapula:
1. Upper and lower trapezius
2. Serratus anterior
first 30 deg of S-H motion –motion occurs in axi
 Tension in coracoclavicular ligament ( conoid and trapezoid) tightens
preventing movement at AC joint
 30 deg of clavicular rotation upward (elevation is checked when
costoclavicular ligament becomes tight)
 10 deg tipping to maintain scapula –rib cage relationship
 90-100 deg total motion
last 30 deg of S-H motion
 costo- and coracoclavicular ligaments tighten  clavicle rotates about
longitudinal axis
 rotation about an A-P axis thru the acromion
 20 degrees of tipping
 40 degrees of winging
Horizontal = 60 deg GH / 30 SH about SC joint
Horizontal to vertical = 60 deg GH / 30 deg SH about AC joint
For the clavicle to rotate about its longitudinal axis both the SC and AC joints need to be
free to move.
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Muscles of the Shoulder
Pec Major –
Pec Minor – anterior rotation of scapula/ protraction
Teres Major – IR, adduct, extend
What is the best way to evaluate the strength of supraspinatus?
What is the best way to evaluate the strength of serratus anterior?
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