1) The document discusses the biomechanics of the shoulder complex, including the muscles that control movements like flexion, extension, abduction, and rotation. 2) It also examines common shoulder injuries like dislocations, rotator cuff damage, and neurological issues. 3) Loads on the shoulder are described, along with the typical anatomical orientation of the shoulder bones.

1. The Biomechanics of the
Human Upper Extremity
Biomechanics and ergonomics 1
3rd semester
Part c

2. Movements of the Shoulder Complex
• Muscles of the Scapula
• Muscles of the Glenohumeral Joint
– Flexion
– Extension
– Abduction
– Adduction
• Medial and Lateral Rotation of the Humerus
• Horizontal Adduction and Abduction at the Glenohumeral
Joint
• Load on shoulder
• Common injuries of shoulder

3. Muscles of the Scapula
• Functions:
– 1) stabilize the scapula when shoulder complex is
loaded
– 2) move and position the scapula to facilitate
movement at glenohumeral joint
• Are:
– Levator scapula, rhomboids, serratus anterior,
pectoralis minor, subclavius, and four parts to
trapezius.

4. Muscles of Glenohumeral Joint
• Many muscles involved, some contribute
more than one action of the humerus
• Large ROM can complicate tension
development witsh orientation of humerus.
• Tension development in one shoulder muscle
is frequently accompanied by development of
tension in an antagonist to prevent dislocation
of the humeral head.

5. Flexion at Glenohumeral Joint
• Prime flexors:
– Anterior deltoid
– Pectoralis major: clavicular portion
• Assistant flexors:
– Coracobrachialis
– Biceps brachii: short head

6. Extension at Glenohumeral Joint
• Gravitational force is primary mover when
shoulder extension isn’t resisted.
– Control by eccentric contraction of flexors
• With resistance there is contraction of
muscles posterior to the glenohumeral joint
• Assisted by:
– Posterior deltoid
– Biceps brachii: long head

7. Abduction at Glenohumeral Joint
• Major abductors of humerus:
– Supraspinatus
• Initiates abduction
• Active for first 110 degrees of abduction
– Middle deltoid
• Active 90-180 degrees of abduction
• Superior dislocating component neutralized by
infraspinatus, subscapularis, and teres minor

8. Adduction of Glenohumeral Joint
• Primary adductors:
– Latissimus dorsi
– Teres major
– Sternocostal pectoralis
• Minor assistance:
– Biceps brachii: short head
– Triceps brachii: long head
– Above 90 degrees- coracobrachialis and subscapularis

9. Medial and Lateral Rotation of
Humerus
• Due to action of:
– Subscapularis
• Has greatest mechanical advantage for medial rotation
– Teres major
• Assisted by:
– Primarily: pectoralis major
– Also: anterior deltoid, latissimus dorsi and short
head of biceps brachii

10. Horizontal Adduction and
Abduction at the Glenohumeral
Joint
• Anterior to joint:
– Pectoralis major (both heads), anterior deltoid,
coracobrachialis
– Assisted by short head of biceps brachi
• Posterior to joint:
– Middle and posterior deltoid, infraspinatus,
teres minor
– Assisted by teres major, latissimus dorsi

11. Loads on the Shoulder
• Arm segment moment arm:
– Perpendicular distance between weight vector
and shoulder.
• With elbow flexion, upper arm and
forearm/hand segments must be analyzed
separately.
• Large torques from extended moment arms
countered by shoulder muscles.
– Load reduced by half with maximal elbow flexion

14. Common Shoulder Injuries
• Dislocations
• Rotator Cuff Damage
– Impingement Theory
• Subscapular Neuropathy
• Rotational Injuries

15. Dislocation
• May occur in anterior , posterior and inferior
direction .
• Coracohumeral ligament prevent displacement
in superior direction
• Typically occur when the humerus is abducted
and externally rotated
• Factors that predispose the joint to dislocation
include
1- inadequate size of glenoid fossa
2- anterior tilt of glenoid fossa

16. 3- inadequate retroversion of humeral head
4- deficit in rotator cuff muscle
Gleno humeral joint laxity may be present due
to genetic factor
In wrestrel and football player separation of
acromioclavicular joint is most common

17. Anatomical Orientation of the
Shoulder Complex
• Clavicle - 20 degrees posterior to the frontal plane
• Scapula – 35 degrees anterior to the frontal plane
(scapular plane)
• Humerus – 30 degrees posterior to the medial-lateral axis
line

18. • Mechanical compression and
irritation of the soft tissues(rotator
cuff and subacromial bursa) in the
suprahumeral space is called
impingement syndrome and is the
most common cause of shoulder
pain
• Symptoms that derive from
impingement are usually brought on
with excessive or repetitive
overhead activities that load
• the shoulder joint, particularly in
the mid-range.

20. Muscle Weakness Secondary to Neuropathy
• Muscle weakness may be related to nerve
involvement.
• Long thoracic nerve palsy has been identified
as a cause of faulty scapular mechanics
resulting from serratus anterior muscle
weakness, leading to impingement in the
suprahumeral region

21. Subscapular neuropathy
• Competitive volley ball players
• Denervation of infraspinatus muscle with loss
of strength during external rotation
Or
Repetitive stretching of nerve during serving
motion

22. • Muscle spasm may lead to a faulty
deltoid–rotator cuff mechanism and
scapulohumeral rhythm when the
patient attempts abduction The head
of the humerus may be held in a
cranial position in the joint, making it
difficult and/or painful to abduct the
shoulder because the greater
tuberosity impinges on the
coracoacromial arch.
• In this case, repositioning the head of
the humerus with a caudal glide is
necessary before proceeding with any
other form of shoulder exercise.

23. Rotational Injuries
• Tears of labrum
– Mostly in anterior-superior region
• Tears of rotator cuff muscles
– Primarily of supraspinatus
• Tears of biceps brachii tendon
• Due to forceful rotational movements
– Also: calcification of soft tissues, degenerative
changes in articular surfaces, bursitis