Chapter 7:
The Biomechanics of the
Human Upper Extremity
Basic Biomechanics, Susan J. Hall
Objectives
• Explain how anatomical structure affects
movement capabilities on upper extremity
articulations.
• Identify factors influencing the relative mobility
and stability of upper extremity movements
• Identify muscles that are active during specific
upper extremity movements
• Describe the biomechanical contributions to
common injuries of the upper extremity.
Structure of the Shoulder
• Most complex joint in body
• Separate articulations:
– Sternoclavicular Joint
– Acromioclavicular Joint
– Coracoclavicular Joint
– Glenohumeral Joint
– Scapulothoracic Joint
– Also: Bursae
Sternoclavicular Joint
• Provides major axis of rotation for movement
of clavicle and scapula
• Freely permitted frontal and transverse plane
motion.
• Allows some forward and backward sagittal
plane rotation.
• Rotation
Acromioclavicular Joint
• Irregular diarthrodial joint between the
acromion process of the scapula and the
distal clavicle.
– allows limited motions in all three planes.
• Rotation occurs during arm elevation
• Close-packed position with humerus
abducted to 90 degrees
Coracoclavicular Joint
• A syndesmosis with coracoid process of
scapula
– bound to the inferior clavicle by the
coracoclavicular ligament.
• Permits little movement
Glenohumeral Joint
• Most freely moving joint in human body
• Glenoid Labrum composed of:
– Joint capsule
– Tendon of long head of biceps brachii
– Glenohumeral ligaments
• Rotator Cuff
• Rotator Cuff Muscles
• Most stable in close-packed position, when the
humerus is abducted and laterally rotated.
Scapulothoracic Joint
• Region between the anterior scapula and
thoracic wall.
• Functions of muscles attaching to scapula:
– Contract to stabilize shoulder region
– Facilitate movements of the upper
extremity through appropriate positioning of
the glenohumeral joint.
Bursae
• Small fibrous sacs that secrete synovial fluid
internally to lessen friction between soft
tissues around joints.
• Shoulder contains:
– Subcoracoid bursa
– Subscapularis bursa
– Subacromial bursa
Movements of the Shoulder
Complex
• Humerus movement usually involves some
movement at all three shoulder joints
– Positioning further facilitated by motions of
spine
– Scapulohumeral Rhythm
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
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.
Muscles of Glenohumeral
Joint
• Many muscles involved, some contribute
more than others.
• Large ROM can complicate tension
development with 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.
Flexion at Glenohumeral Joint
• Prime flexors:
– Anterior deltoid
– Pectoralis major: clavicular portion
• Assistant flexors:
– Coracobrachialis
– Biceps brachii: short head
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
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
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
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
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
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
Common Shoulder Injuries
• Dislocations
• Rotator Cuff Damage
– Impingement Theory
• Subscapular Neuropathy
• Rotational Injuries
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
Structure of the Elbow
• Humeroulnar Joint
• Humeroradial Joint
• Proximal Radioulnar Joint
Segments at the Elbow
• Flexion and Extension
– Muscles crossing anterior side of elbow are
the flexors:
• Brachialis, biceps brachii, brachioradialis
– Muscles crossing posterior side of elbow
are the extensors:
• Triceps, anconeus muscle
Segments at the Elbow
• Pronation and Supination
– Involves rotation of radius around ulna
– Articulations:
• Proximal and distal radioulnar joints
(both pivot joints)
• Middle radioulnar joint (syndesmosis)
• Pronator quadratus
• Supinator
Loads on the Elbow
• Large loads generate by muscles that cross elbow
during forceful pitching/throwing
– Also in weight lifting, gymnastics
• Extensor moment arm shorter flexor moment arm
– Tricep attachment to ulna closer to elbow joint
center than those of the brachialis on ulna an
biceps on radius
• Moment arm also varies with position of elbow
Common Injuries to Elbow
• Sprains
• Dislocations
– “nursemaid’s elbow” or “pulled elbow”
• Overuse Injuries
– Lateral Epicondylitis = “tennis elbow”
– Medial Epicondylitis = “Little Leaguer’s
Elbow”
• Elbow injuries are more chronic than acute
Structure of the Wrist
• Radiocarpal joint
– Reinforced by: volar radiocarpal, dorsal
radiocarpal, radial collateral and ulnar
collateral ligaments
• Retinacula
– Form protective passageways for tendons,
nerves and blood vessel to pass through
Movements of the Wrist
• Sagittal and frontal plane movements
• Rotary motion
• Flexion
• Extension and Hyperextension
• Radial Deviation
• Ulnar Deviation
Joint Structure of the Hand
• Carpometacarpal (CM)
• Metacarpophalangeal (MP)
• Interphalangeal (IP)
Movements of the Hand
• CM Joints allow large ROM because similar
to ball and socket joint
– Digits 2-4 constrained by ligaments
• MP joints allow flexion, extension, abduction,
adduction and circumduction for digits 2-5
• IP joints allow flexion and extension
• Extrinsic Muscles
• Intrinsic Muscles
Common Injuries of the Wrist
and Hand
• Sprains and strains fairly common, due to
breaking a fall on hyperextended wrist
• Certain injuries characteristic of sport type
– Metacarpal fractures and football
– Ulnar collateral ligament and hockey
– Wrist fracture and skate/snowboarding
– Wrist in non-dominant hand for golfers
• Carpal Tunnel Syndrome
Summary
• Shoulder is the most complex joint in the
human body.
• Movements of the shoulder girdle contribute
to optimal positioning of the glenohumeral
joint for different humeral movements.
• Humeroulnar articulation controls flexion and
extension at the elbow
• Pronation and supination of forearm occur at
proximal and distal radioulnar joints.
