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BIOMECHANICS EXAM #1
Concepts(star slides)
Third Class Levers most common used lever in the musuloskeletal system
Advantages vs Disadvantages?
 Class 3 have an MA<1 but would produce larger area of motion distally pg 25
 Muscles are mostly designed to generate and optimize large forces producing significant
angular displacement of the distal segments
 Decreased mechanical advantage but increased distal segment arcs of movement
What is the effect of changing the force or weight arm length? Pg 19
Torque
 To alter torque there must be a change in
o Length of moment arm
o Amount of force applied to the moment arm
Calculate the change in torque in a class one lever system? Pg 20
Class 2 lever torque? Change weight arm? Change force arm? Pg 21-22
Class 2 lever have a MA> 1 but would produce smaller area of motion distally pg 25
Class 3 Lever torque? Change weight arm? Change force arm? Pg 23
Tensile Forces
Tension : two externally applied forces that are equal and act along the same line and in opposite
directions  results in stretching, separation, and elongation deformation
 A tensile force applied to an object will normally be resisted by internal tensile stresses within
the object (pg4)
Compression
Compression : two externally applied forces that are equal and act in a line toward each other on
opposite ends of a structure
 A compressive force applied to an object will normally be resisted by internal compressive
stresses within an object (pg 4)
Shear Forces (external)
Shear : two equally applied forces that are equal, parallel and applied in opposite directions, but are
not in line with each other (pg 5)
Stress
Stress : the internal reaction to an applied external force (load)
 A tensile load will create a tensile stress within a ligament, tendon, fascia
Loads Stresses
 Load forces are transmitted into structures (bones,lig,muscle) producing stresses with that
structure.
 Within structures, stresses develop to resist loading forces
 Compressive loads compressive stress
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

Tensile loads tensile stress
Shear loads shear stress
Tension Load on Tendon(pg 5)
1. Contraction of a muscle applies tension load to tendon
2. Tension forces are transmitted through the as TENSION STRESSES
3. Tendon also resists tension with compression stresses
4. Tension and compression stresses are forces acting in parallel and in opposite directions which
may result in the production of shear stresses
Mechanical Work
Work= Fmagnitude x S distance the object moves
*Time is not a factor
If the direction of the force is not perpendicular to the object  W=(F x cos0) x s
s= distance moved
Mechanical Power
Power= the rate at which a force does work
Power=work/time
KINEMATICS
Angular Motion : All points of the object move at the same angle but do not undergo the same linear
displacement, rotate around fixed axis (pg9)
 All particles of the body , except those on the axis of rotation, move along circular paths in
planes perpendicular to the of rotation
SHOULDER COMPLEX
Joints
 Sternoclavicular Joint
o Saddle Joint
 Anterior and Posterior surfaces are Concave/convex
 Superior/Inferior surfaces convex/concave
o Movements
 Anterior/Posterior Glide
 Elevation/Depression
 Clavicular Rotation
o Closed Packed Position
 Full abduction in frontal plane with full external humeral rotation
o Loose Packed Position
 20° of scapulohumeral abduction
 Acromioclavicular Joint
o Movements
 Anterior/Posterior Glide
 Elevation/Depression
 Rotation of Clavicle
o Closed Packed Position
 90° of abduction in the frontal plane
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o Loose Packed Position
 20° of scapulohumeral abduction
 Scapulothoracic Joint
o Movements
 Elevation/Depression
 Adduction or Retraction
 Abduction or Protraction
 Upward Rotation
 Downward Rotation
Scapular Rhythm
 Movement of the scapula relative to movement at GH joint
 Average rhythm is 2° of GH motion to 1° of scapular motion (2:1)
 For every 3° of Frontal Abduction or Sagittal Flexion there is 2° at GH joint and 1° is from
scapular upward rotation
Scapular Movement
 Phase 1: (0-90°)
o GH Abd/Flex
o Scapula elevation
o >60° upward rotation
o lateral clavicle elevation
o medial clavicle depression
 Phase2: (>90°)
o GH Abd/Flex
o Scapula upward rotation
o NO Clavicle/scapula elevation
o Clavicle rotation for GH Abd/Flex >120°
o Clavicle rotate posterior to overcome ligament to continue up ward rotation
o Increase compression
Glenohumeral Joint
Osteokinematic Movements
 Flexion/Extension= 0-180
 Hyperextension= 0-45/60
 Vertical adduction/abduction= 0-180
 Internal Rotation= 0-55/70
 External Rotation= 0-80/90
Force Couple
 2 Equal and Opposite forces acting simultaneously to rotate
 Upward
o Upper trapezius against Latissimus dorsi and serratus anterior
 Downward
o Levator scapulae with rhomboids against pectoralis minor
Suprascapular and Axillary Nerve Block
 Suprascapular nerve block
o Loss of supraspinatus muscle function but not the deltoid
 Supraspinatus is main abductor below 90°
o Loss results in 60% decease in abduction force from 0-60°
o Loss of 30% decrease in abduction above 60°
 Axillary Nerve Block
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o Loss of deltoid muscle
o Loss results in 35% decrease in abduction force from 0-60°
o Loss results in 60-80% in abduction force >90°
Shoulder Muscle Strength
 Adductors>internal rotators>extensors >flexors>abductors>external rotators
 At 90° Abduction
o Deltoid generates 8X weight of UE and GHJ 10X
 At 60° Abduction
o Rotator cuff generates a GHJ reaction of 9.6X UE
 Shoulder muscles especially the trapezius, supraspinatus and deltoid fatigue significantly more
rapidly with work above 90°
Shoulder (StarSlides)
Clavicle
Protraction Retraction
 As the clavicle retracts, the proximal clavicle rolls and slides posteriorly on the sternum (pg 3840)
Elevation Depression
 Clavicular Elevation inferior glide, superior roll
 Clavicular Depression superior glide, inferior roll
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5
ADDUCTION
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GHJ INTERNAL ROTATION
GHJ EXTERNAL ROTATION
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GHJ HYPEREXTENSION
SHOULDER ISOMETRIC STRENGTH



At 900 vertical abduction
o Deltoid generate force equal to 8x the weight of upper extremity
o Total joint reaction force is equal to 10x weight of upper extremity
At 600 of vertical abduction
o Rotator cuff muscles generate a force equal to 9.6x weight of upper extremity
Add weight to end of limb???
o Without weight, the joint reaction force of the deltoid is 10x weight of arm
o Add weight to end, the joint reaction force of the deltoid is 10x weight of arm plus the
amount of weight added
Why would trap, supraspinatus, and deltoid fatigue more rapidly with work above 900 when
compared to when the arms are at 450?
How would you use this information in an exercise program targeted for improving endurance of
these muscles?
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ELBOW COMPLEX
Joints
 Humeroulnar
o Hinge synovial joint
o Motions: flex/Ext
o Concave/convex
 Humeroradial
o Hinge synovial joint
o Motions: flex/Ext
o Convex/concave
 Proximal Radioulnar Joints
o Pivot Synovial Joint
o Motions: supination/pronation
 3 Joints but 1 capsule
Joint Stability
 Bony Limitations
o Trochlear notch of olecranon of ulna/trochlea
o Coronoid process of ulna/trochlea
o Radial head/capitulum
o Radial head/radial notch of humerus
 Ligamentous Support
o Medial Collateral Ligament (MCL)
 Strong ligament
 Superior Fibers
 Slack in flex
 Taut in Ext
 Inferior Fibers
 Taut in flex
 Slack in Ext
 Posterior Fibers
 Taut after 90 flex
 Most ADLs increase valgus strain at elbow which MCL resists
o Lateral Collateral Ligament (LCL)
 Weak ligament that attaches to annular ligament
 Provides slight resistance to tensile strain and varus strain
o Annular Ligament
 Strong circular lig that wraps around head of radius
 Holds radial head against the radial notch to allow smooth rotation during
supination and pronation
 Prevents dislocation
o Interosseous membrane
 Helps to maintain the position of the radial head in the radial notch
 Resists dislocation
Loads on Elbow
 Valgus
o “to turn inward” (Oh no Varus my pig?)= bow leg
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o
o
o
o
o
 Varus
o
o
o
o
o
o
TISSUE
MCL
LCL
Joint
Capsule
Bone
Resistance
Tenses MCL
Tenses medial capsule
Traction in HU jt
Compresses HR jt
Slacks lateral restraints
“to turn outward”
Tenses LCL
Tenses lateral tissue (lateral capsule and anconeus)
Distracts HR jt
Compresses HU Jt
Slacks MCL, medial tissue
VALGUS
VARUS
O Degrees
90 Degrees 0 Degrees
90 Degrees
31%
54%
None
None
None
None
14%
9%
38%
10%
31%
13%
DISTRACTION
0 Degrees
6%
5%
85%
90 Degrees
78%
10%
8%
31%
None
None
33%
55%
75%
Carrying Angle
 Lateral position of the elbow when elbow is at 0 degrees
o 5 degrees in males
o 10-15 degrees in females
 result of the medial end of trochlea being distal to capitellum which positions ulna and radius at
an angle lateral to the humerus
Closed Packed Position
 humeroulanr= full extension
 humeroradial= semiflexion with semipronation
 Radioulnar= full pronation or full supination
Loose Packed Position
 Humeroulnar= 70-90 flexion
 Humeroradial= 70 flexion/35 supination
 Radioulnar= 70 flexion/35 supination
MOVEMENT
Osteokinematic
 Flexion/Extension= 0-140/150
 Pronation= 0-80/90
 Supination= 0-80/90
For ADLs
 Flexion/Extension= 20-130
 Pronation= 50
 Supination= 50
Arthokinematics
 Humeroulnar Flexion
o Ulnar rotates anteriorly
o Glides anteriorly
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o Ulna adducts
 Humeroradial Flexion
o Radius rotates anteriorly
o Glides anterior
o Moves cranially on ulna
 Humeroulnar Extension
o Ulnar rotates posterior
o Glides posterior
o Ulna abducts
 Humeroradial Extension
o Radius rotates posterior
o Glides posterior
o Moves caudally on ulna
 Pronation
o Radius rotates medially
o Glides laterally
 Supination
o Radius rotates laterally
o Glides medially
MUSCLES
** Remember that muscle volume and length determine how much force a muscle can generate
Flexors
 Brachialis
o Work horse
o Mechanical advantage: greatest at 100° flexion
o Works at all speeds/resistance/positions
 Biceps
o Mechanical advantage: greatest at 80-90° flexion
o Least affective at full flexion with shoulder flexion
o Active with resisted elbow flexion/supination
o Produces glide of radial head above 100° of flexion
 Brachioradialis
o Produces large compression at elbow joint
o Not active during slow non-resisted motion/eccentrically
o Active when increased resistance to flexion
o Best position: mid supination/pronation when flexion is resisted and movement is at
moderate to high speed
o * longer muscles are designed to create more velocity but not as strong
 Other Flexors
o Extensor carpi radialis longus
o Pronator teres
o Flexor carpi Ulnaris
o Flexors carpi radialis
o Flexor Digitorum Superficialis
Extensors
 Long Head of triceps
o Affected by shoulder position
o Least affective: full elbow extension
o Greatest Activity: heavy resistance or quick extension
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
Medial and Lateral Heads of Triceps
o Not affected by shoulder position
o Medial Head active with resistance and non-resistance
 ** work horse
o Lateral head is active with resisted extension
o Triceps max torque at 90° flexion
 Other Extensors
o Anconeus
o Extensor carpi Ulnaris
Pronators
 Pronator Teres
o Active with rapid pronation and resisted movement
o Maintains position of radial head with capitellum
 Pronator Quadratus
o Work Horse
o Active during non-resisted and resisted pronation/ fast and slow
o Maintain compression of distal joints (stabilizer)
 Other Pronators
o Flexor carpi radialis
o Palmaris longus
Supinators
 Supinator
o Work Horse
o Active with resisted and non resisted supination/ fast and slow
o Maintains position of radial head with capitellum
 Biceps
o Active with resisted/fast supination
o Most effective at 90° flexion
 Other supinators
o Abductor pollicis longus
o Extensor pollicis Brevis
o Extensor Indicis
WRIST COMPLEX
Joints
 Distal Radioulnar
o Movement: supination/pronation
 Radiocarpal
o Movement: Flexion/Extension
 Ulnocarpal
o Movements: Flexion/Extension
 Midcarpal
o Movements: flexion/extension
o Radial and Ulnar Deviation
Ligaments
 Extrinsic
o Palmar extrinsic ligaments thicker and stronger than dorsal extrinsic ligaments
o Resist wrist extension more than flexion because palmar extrinsic ligaments are
stronger
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o Palmar and Dorsal Extrinsic Ligaments
 Radial collateral ligament
 Palmar Radiocarpal ligament
 Ulnar collateral ligament
 Palmar Ulnocarpal ligament
 Dorsal Radiocarpal ligament
o Intrinsic
 Dorsal and palmar intrinsic ligaments are named by the carpal bones to which
they attach
 Interosseous ligaments are between adjacent carpal bones
MOVEMENTS
Osteokinematic Movement
 Palmar flexion= 0-85/90°
 Dorsiflexion= 0-75/80°
 Radial deviation= 0-15/20°
 Ulnar deviation= 0-35/37°
 Pronation= 0-80/90°
 Supination= 0-80/90°
ADLs
 Most can be done with 10° of palmar flexion and 35° of dorsiflexion
Immobilization
 10-20° of dorsiflexion is best for ADL function
 greater than 20° of palmar flexion results in a marked decrease in hand flexion
Closed Packed position
 full wrist extension with radial deviation
Open Packed Position
 neutral with slight ulnar deviation
Arthokinematics Movement
 palmar flexion
o 60% of movement occurs at the midcarpal joint
o 40% of movement occurs at the Radiocarpal joint
 Dorsiflexion
o 60-70% of movement occurs at the Radiocarpal joint
o 30-40% of the movement occurs at the midcarpal joint
Fractures
 Radiocarpal Joints
o Palmar flexion ROM may still be good
o Dorsiflexion may be poor
 Midcarpal Joints
o Palmar Flexion may be poor
o Dorsiflexion may be good
Sequential Movements During Wrist Extension from Neutral
 Distal carpal row moves dorsally and the proximal row moves palmary until about 60° of
extension
 At ~60° of extension
o The hamate/capitate/trapezoid/scaphoid lie in a closed position and form a rigid mass
o This position of these 4 carpal bones also produces radial deviation
 The ridge mass moves dorsally as a unit on the Triquetrum and lunate
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 **Roll dorsal and glide palmar
 Triquetrum and lunate move palmary
 At full extension, wrist is in a close packed position
 During extension the pisiform moves distally and the radius moves proximally on ulna
Sequential Movements during Flexion
 ****The distal carpal row moves palmary and the proximal row moves dorsally
o during about the last 30° of wrist flexion movement occurs mainly at the midcarpal jt
with distal row moving palmary
Sequential Movements during Radial Deviation
 movement mainly at mid-carpal jt
 distal carpal row moves radially
 scaphoid and lunate move palmar to make room for the trapezium
 radial deviation is limited when the wrist is fully flexed
o scaphoid and lunate dorsally which blocks palmar movement of these carpals
 radial deviation is limited when the wrist is fully extended
o close packed position decreases intercarpal movement at the midcarpal jt
o decreased midcarpal movement blocks radial deviation at the distal row
Sequential Movement during Ulnar Deviation
 * distal carpal row moves ulnarly and the hamate is pulled proximally
 most say that the proximal row moves radially
 scaphoid and lunate move dorsally
 ulnar deviation is extensive when the wrist is fully flexed
o during wrist flexion the scaphoid and lunate move dorsally so they are already in a
dorsal position for ulnar deviation
o during wrist flexion the midcarpal joint is not blocked as during wrist extension and so
the distal row can move ulnarly to produce ulnar deviation
 ulnar deviation is limited when the wrist is fully extended
o the close packed position blocks intercarpal movement at the midcarpal joint
o blockage of midcarpal movement blocks the distal carpal row from moving ulnarly for
ulnar deviation
Full Pronation to Full Supination
 the distal radius rotates laterally over the head of the ulna
 the ulnar head glides palmarly as it moves from a dorsal position at full pronation to a palmar
position at full supination
Full Supination to Full Pronation
 the distal radius rotates medially over the head of the ulna
 the ulnar head glides dorsally as it moves form a palmar position at full supination to a dorsal
position at full pronation
Loading of Wrist
 the wrist is designed to withstand compression
 compression is transmitted mainly through the lunate/scaphoid to the radius
o about 80% of the compression forces at the wrist are transmitted to the large distal
radius
o about 20% of the compression forces by disc
 During a Fall: A person breaks the fall with the arm, wrist, and hand extended
o Ground force is transmitted via 3rd metacarpal to capitate
o From capitate force transmits to the lunate and scaphoid
o From the lunate and scaphoid force transmitted to the radius
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o * If the forces are high, the distal radius or scaphoid can be fractured or the lunate
dislocated
MUSCLES
Extensors
 extensor carpi radialis longus
 extensor carpi radialis Brevis
 extensor carpi Ulnaris
o *** all 3 of these muscles lie in the periphery so the longer force arm and thus
 Mechanical advantage over the other extensors
 Other assistors:
o Extensor Digitorum
o Extensor Indicis
o Extensor digiti minimi
Flexors
 Flexor carpi radialis
 Flexor carpi Ulnaris
 Other Muscles
o Flexor Digitorum Superficialis
o Palmaris Longus
Ulnar Deviation
 Extensor carpi Ulnaris
o With wrist in neutral or extension
 Flexor Carpi Ulnaris
o With the wrist neutral and flexion
Radial Deviation
 Extensor carpi radialis longus
o With the wrist in neutral or extension
 Extensor carpi radialis Brevis
o Only slight radial deviation with the wrist in neutral or extension
 Flexor Carpi Radialis
o With wrist in neutral or flexion
 Other Muscles
o Abductor pollicis longus
o Extensor pollicis Brevis
Dynamic Lateral Stability
 Extensor carpi Ulnaris
o Stabilize the ulnar side of the wrist with the wrist in neutral or in flexion
 Flexor Carpi Ulnaris
o Stabilize the ulnar side of the wrist with the wrist in neutral or in extension
 Extensor Carpi Radialis Longus
o Will stabilize the radial side of the wrist with the wrest in neutral and flexion
 Flexor Carpi Radialis
o Stabilize the radial side of the wrist with the wrist in neutral or in extension
 Abductor pollicis longus
o Stabilize the radial side of the wrist with the wrist in flexion
 Extensor Pollicis Brevis
o Stabilize the radial side of the wrist with the wrist in flexion
Wrist Position and Hand Function
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
Wrist Extension
o Stretches the finger flexors to take up elastic component of the muscles
o Slackens the finger extensors which decreases extension force
o Tightness of the finger flexors and looseness of the finger extensors make it difficult to
fully extend the fingers
 Wrist Flexion
 Stretches the finger extensors
 Slackens the finger flexors
 Tightness of the finger extensors and looseness of the finger flexors make it difficult to fully flex
the fingers
Grip Strength
 Greatest with wrist in ~20° of extension
 When grip strength is greatest, the wrist is extended and in ulnar deviation
 Least with wrist ~40° of flexion
Thumb Position
 With wrist flexed and hand relaxed
o the tip of the thumb reaches the level of the DIP of index finger which makes it easier to
pick up objects between the tip of the thumb and the tip of the index finger
 Wrist extended and hand relaxed,
o the tip of the thumb lies short of the PIP joint of the index finger which make it difficult
to pick up objects between the tip of the thumb and tip of the index finger
HAND COMPLEX
Arches of the Hand
 Proximal transverse arch
o Distal carpal row and carpometacarpal joints
 Distal Transverse Arch
o Heads of the meta =carpals at the MCP joints
 Longitudinal Arch
o Center of the hand from the proximal carpal row though the 4 fingers
 Arches are maintained by the intrinsic muscles
o A flat hand results form a collapse of these arches because of paralysis of the intrinsic
hand muscles
MOVEMENTS
Osteokinematics
JOINT
TYPE OF MOVEMENT DEGREES OF MOVEMENT
CMC of thumb
Flexion/Extension
0-45/0-15
Abduction/Adduction
0-70/0-80
CMC index, middle fingers
Flexion/Extension
Slight to none
Abduction/Adduction
Slight to none
CMC ring finger
Flexion/Extension
0-15
CMC little Finger
Flexion/Extension
0-30
Abduction/Adduction
0-10/0-20
MCP of thumb
Flexion/Extension
0-50
Abduction/Adduction
slight
MCP index, middle, ring, little
Flexion/Extension
0-90
fingers
Abduction/Adduction
0-45/0-45
IP Thumb
Flexion/Extension
0-90
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PIP index, middle, ring, little finger
DIP index, middle, ring, little finger

Flexion/Extension
Flexion/Extension
0-100
0-90/0-10
Movement is very limited at the CMC of the index and middle finger
o Because flexor carpi radialis/extensor carpi radialis longus/extensor carpi radialis
Brevis attach here and need a stable attachment site to move the wrist
o Also provides a stable base against which the thumb can compress and hold objects
Arthokinematics
PIP/DIP of fingers and IP of thumb
 Flexion: palmar rotation & palmar translation
 Extension: dorsal rotation & dorsal translation
 Close packed: 70° flexion
 Least packed: slight flexion
MCP joints of fingers
 Flexion: palmar rotation & palmar translation
 Extension: dorsal rotation & dorsal translation
 Abduction/Adduction: occur in reference to the middle finger
Abduction
 Index and Middle finger
o Radial rotation & translation
 Middle, ring, little finger
o Ulnar rotation & translation
Adduction
 Index and middle finger
o Ulnar rotation & translation
 Middle, Ring, Little Finger
o Radial rotation & translation
CMC Joint of Thumb
 Flexion: ulnar rotation & translation
 Extension: radial rotation & translation
 Abduction: palmar rotation & dorsal translation
 Adduction: dorsal rotation & palmar translation
Extrinsic Muscles
MUSCLES OF THE HAND
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Extrinsic Muscles of Thumb
Ranking of the Extrinsic Muscles by Force Production
Intrinsic Muscles of Hand
 Abductor Digiti Minimi
o Abducts CMC (slight) and MCP of little finger
 Flexor digiti minimi
o Flexes CMC and MCP of little finger
 Opponens Digiti Minimi
o Rotates 5th metacarpal at CMC toward the thumb
 Lumbricals
o Flexes MCP if index, middle, ring and little fingers
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o Extends PIP and DIP of index middle ring and little fingers through the extensor
assembly
 Dorsal Interossei
o Flexes the MCP of index middle and ring fingers
o Extends the PIP and DIP of index middle and ring fingers via extensor assembly
 Palmar Interossei
o Flexes the MCP of the index, middle, ring and little fingers
o Extends the PIP and DIP of index ring and little fingers via extensor assembly
o Adducts the MCP of the index ring and little fingers
Intrinsic Muscles of Thumb
 Abductor Pollicis Brevis
o Abducts CMC and MCP of thumb
 Flexor Pollicis Brevis
o Flexes CMC and MCP of thumb


Opponens Pollicis
o Rotates 1st metacarpal at CMC toward the little finger
Adductor Pollicis
o Adducts CMC of thumb
NOTE:
 Notice that the force output of the flexor Digitorum Superficialis and Profundus is much greater
then extensor Digitorum
 Passive component of flexors is greater than contraction component of extensors
 Alone the extensors cannot fully extend the fingers
 To Fully extend fingers:
o Contraction of interossei, Lumbricals, and extensor Digitorum is needed
 If ulnar nerve is damaged
o The 4 fingers will lack full extension because all interossei are and Lumbricals 3&4 will
be involved
o Greater loss of full extension in the ring and little fingers because both interossei and
Lumbricals are involved
 In index and middle finger only the interossei were involved not Lumbricals
 If median nerve is damaged
o Only the Lumbricals to the middle and index fingers are involved
 Middle and index fingers will have a loss of full extension but not the ring and
little fingers which will still fully extend
 When either the interossei or Lumbricals are involved, the extensor Digitorum itself cannot
overcome the passive component of finger flexors so fingers cant be fully extended
Ligaments
 Collateral ligaments
 Palmar plate
 Flexor tendon sheath
Deformities
 Claw Hand
o Intrinsic hand muscles become weak because of ulnar and median nerve damage
o Imbalance results in flexion of PIP and DIP especially at the ring and little fingers
o Index and middle fingers aren’t quite as involved because the lumbricals are innervated
by the median nerve
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
Boutonniere Deformity
o Result of RA
o Dorsal synovitis and distension of the PIP dorsal capsule
 Distension results in dorsal subluxation of the PIP joint with lateral bands
displaced palmarly
o Palmar shift of the lateral bands hold the PIP in flexion as the bands lie palmar to joint
axis at PIP
o The terminal tendon remains dorsal to the axis of the DIP producing DIP hyperextension
o Characteristic of Deformity
 MCP hyperextension/PIP flexion/DIP Hyperextension
 Swan Neck Deformity
o Result from RA
o Palmar synovitis and distension of the PIP palmar capsule
 Distension results in palmar subluxation of PIP with dorsal displacement of
lateral bands
o Dorsal shift of lateral bands hold the PIP in hyperextension as the bands lie dorsal to the
joint axis of the PIP
o Dorsal shift of lateral bands decreases tension on the terminal tendon allowing flexion at
the DIP by passive flexor pull of the flexor Digitorum Profundus
o Characteristic of Deformity
 MCP flexion/PIP Hyperextension/DIP Flexion
Hand Grips and Finger Pinches
Grips
 Power grips
o Fingers are flexed at the MCP, PIP, DIP
o Thumb is adducted acting as clamp
o Examples
 Cylindrical grip
 Hammer grip
 Fist
 Jar opening
o * stability for large forces but little precision
 Precision grips
o Fingers are semi-flexed
o Thumb is abducted and opposed
 More of a holding than a squeezing
o Examples
 Spherical grip
 Open jar lid with fingertips
 Screwing light bulb
o * needed for control
Pinches
 Dynamic Tripod
o The thumb index and middle fingers hold the object and ring and little finger provide
support and stability
o Writing position
o Cutting with scissors
 Tip Pinch
o Tip of thumb presses against tip of index finger or other finger
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o Picking up a button or coin from table
 Palmar Pinch
o Palmar surface of thumb presses against the palmar surface of the index finger or
another finger
o Holding a sheet of paper
 Lateral (key) Pinch
o Palmar surface of the thumb presses against the radial surface of the index finger
o Holding a key while putting it into a lock
Forces During Grip and Pinch
 Compressive forces on finger joints
o Least at DIP
o PIP and MCP vary with function
 Tip inch forces are greater at the PIP than MCP
 Lateral pinch forces are grater at MCP than the PIP
 Opening big jars, greater at MCP than PIP
 Holding a glass, Greater at PIP than MCP
 Compressive forces at thumb
o Forces at IP= 2-3X force
o Forces at MCP= 5-6X force
o Forces at CMC= 6-12X force
 During normal pinch and grip
o Forces on tendons of the extrinsic muscles are 4-5X force
 During Normal pinch and grip
o Tensile forces on the tendons of intrinsic muscles are 1 ½ to 3X force
MECHANICAL ADVANTAGE
Fixed Pulleys
 Can be used to change direction of force but not magnitude
 Many fixed pulleys in body
 Force is equal in each strand
 Force in each strand is equal to the force of resistance
Movable Pulleys
 Changes the direction and magnitude of the force
 Forces on the supporting strands of the system are less that the resistive force can be used in
combo with fixed pulleys
Levers
 3 classes
 5 parts
o fulcrum
o weight
o force
o weight arm
o force arm
 Class 1 Lever
o See Saw
o Examples
 Triceps acting on elbow
 Soleus on the ankle
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

 Erector spinae acting on spine
Class 2 Lever
o Wheelbarrow
o Examples
 Toe raises with axis at metatarsophalangeal joints
Class 3 Lever
o Example
 Biceps acting on elbow
 Quads acting on knee
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