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1
Skeletal Muscles and Functions
Huei-Ming Chai, PT, Ph.D.
School of Physical Therapy
National Taiwan University
Classification of Muscles
• striated muscles – skeletal muscles: voluntary contraction
– cardiac muscles
• non-striated (smooth) muscles
2
Skeletal Muscle Statistics
• the most abundant tissue in the body,
accounting for 40-45% BW
• > 430 skeletal muscles
• Most movements are completed by < 80 pairs
of skeletal muscles
Skeletal Muscle and Function
Structure of skeletal muscle
• Muscle contraction
• Muscle coordination
3
Properties of Skeletal Muscle
• Muscle fiber – extensibility
– elasticity
– contractility
• tendon or aponeurosis – viscoelasticity
– non-contractility
Organization of Skeletal Muscle
muscle fasciculus
muscle epimysium
perimysium
muscle fiber endomysium
muscle fibril
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Muscle Fiber
• a long cylindrical cell with hundreds of nuclei – 10-100 m in diameter
– 1-30 cm in length
• Contractile component: myofabril
• Non-contractile component: endomyosium
Organization of Muscle Fiber
muscle fibril
endomysium muscle fiber
myosin filament actin filament
sarcomere
cross section
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Contractile Component
-- Sarcomere
actin myosin crossbridge
Z line A band
I band H band
Types of Muscle Fibers
• slow twitch fiber (Type I) – red in color
– slow to peak when contracted
– fatigue resistant
• fast twitch fiber (Type IIA) – white in color
– fast to peak when contracted
– easy fatigue
• intermediate (Type IIB)
Smith, p.88
6
Fiber Architecture
• parallel fiber arrangement: parallel to the longitudinal axis of the muscle – longitudinal: sartorius – quadrate or quadralateral: rhomboid – triangular or fan-shaped: pectoralis major – fusiform or spindle-shaped: biceps brachii
• pennate fiber arrangement: at an angle to the longitudinal axis of the muscle, – unipenniform: extnesor digitorum longous – bipenniform: flexor hallucis longus – multipenniform: middle deltoid
Effect of Pennation
• pennation angle effective force
transmitted to the tendon
• tension in the muscle fibers pennation
angle
• pennate arrangement:
to allow packing of
more fibers
given the same space.
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Skeletal Muscle and Function
• Structure of skeletal muscle
Muscle contraction
• Muscle coordination
Functions of Skeletal Muscle
• To move the body limb by creating motion
• To provide strength by generating active force
• To protect joints by absorbing shock
• specific functions of connective tissues within muscle – To provide gross structure to muscle
– To generate passive tension against stretch
– To transmit force to the bone and across the joint
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Sliding Filament Mechanism
• AF Huxley & HE Huxley, 1964
• active shortening of sacromere, resulting from
the relative movement of actin and myosin
filaments with retaining its original length
• force of contraction is developed by the
crossbridges of myosin
Movement of Cross Bridges
shortening Lengths of myosin and actin
keep the same
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Types Based on Changes in Length
concentric
(shortening)
resting
eccentric
(lengthening)
isometric
(static)
Abdominal Muscle Contraction
• concentric contraction – to create trunk flexion (resisting gravity)
• eccentric contraction – to control trunk extension (checking gravity)
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Triceps Brachii Action
• downward motion
– elbow flexion
– elbow extensors (antagonist) eccentric contraction
• upward motion
– elbow extension
– elbow extensors (agonist)
concentric contraction
Abdominal Muscle Actions
concentric
eccentric
direction of motion muscle length
concentric gravity-resisted shortening
eccentric gravity-assisted lengthening
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Elbow Flexion at 90 of Shoulder ABD
elbow flexor, concentric elbow extensor, eccentric
elbow flexor, eccentric elbow extensor, concentric
open
kinematic
chain
closed
kinematic
chain
Quadriceps Actions
motion? gravity? muscle contraction? muscle length?
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Types Based on
Tension Development
• isotonic contraction
• isometric contraction
• isokinetic contraction
Isotonic Contraction
• iso = equal; tonus = tension
• defined by muscle physiologists as a kind of muscle
contraction that develops constant tension throughout
the whole muscle excursion as isotonic contraction – seldom seen in the living body
– clinically refer to a muscle contraction that causes a joint to
move through some range of motion
• Even though the resistance remains the same, the
tension generated by the muscle is not equal tension
because – moment arm to the joint axis changing throughout the motion
– resistance with respect to the gravity changing throughout the
motion
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Slight Squatting
• quadriceps action
• calf action
• shoulder flexor
• elbow flexor
Isokinetic Contraction
• iso = equal; kinetos = move
• first introduced by Hislop and Perrine in 1967
• definition: one kind of muscle contraction that
occurs when the rate of movement is constant
• not occur in the living body without using
special machine (isokinetic dynamometer)
• equal motion speed with
accommodating resistance
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Isokinetic Testing
Isokinetic Testing System
• Cybex: torque
• Kin-Com: force
Kin-Com:
load cell
angle
am
pli
tud
e ext.
flex.
Cybex:
dynamometer
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Isokinetic Contraction
joint angle
joint moment
Isokinetic
F
mg
M
Isotonic
dynamometer
Comparison of Muscle Contraction
• isotonic contraction – varying tension
– varying length
– varying speed
• Isometric contraction – varying tension
– equal length
– zero speed
• isokinetic contraction – accommodating resistance (various tension)
– varying length
– equal speed
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Tension Developed by
A Single Muscle Fiber Contraction
tensio
n
length
active
tension
resting
length
Mechanical Model of
Musculotendinous Unit
• Keele, Neil, Joels, 1982
parallel elastic component
contractile component
series
elastic
component
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Length-Tension Curve
-- maximum isometric contraction
tensio
n
length
total tension
active
tension
passive
tension
resting
length
Force-Velocity Curve
Hill’s model
0
contraction velocity
forc
e
eccentric concentric
isometric
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Skeletal Muscle and Function
• Structure of skeletal muscle
• Muscle contraction
Muscle coordination
Muscle Activities During Motion
• focal muscle – agonist (prime mover)
– antagonist
– synergist
• stabilizer
• neutralizer
• postural muscle – anticipatory postural adjustment (APA)
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Agonist
• the principal muscle that produces a joint
motion or maintains a static posture
• can be concentric, isometric, or eccentric
Antagonist
• 拮抗肌
• the muscle that contracts in the opposite
direction of the agonist
• passively elongates or shortens to allow
motion acted by agonist
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Synergist
• Syn = together; ergon = work
• the muscle that contracts together with the
agonist – stabilizer: to stabilize the proximal component of
the joint involved
– neutralizer: to rule out unwanted motions
Stabilizer
teres minor
• scapular muscles stabilize the scapula
deltoid can elevate the arm
teres minor can rotate the arm externally
22
Neutralizer
scapula
adduction
upper trapezius
lower trapezius neutralize
Cocontraction
co-contraction joint approximation
co-contraction: agonist and antagonist
contract simultaneously
23
Single- vs. Multi-Joint Muscle
• single-joint muscle: a muscle that cross one
joint only, e.g. the brachialis, the short head
of the biceps brachii
• two-joint muscle: a muscle that cross two
joints, e.g. the long-head of the biceps brachii,
the grastrocnemius, etc.
• multi-joint muscle: a muscle that cross more
than one joint e.g. the long finger flexors, the
long finger extensors, etc.
Action of Two-Joint Muscle
• active insufficiency – unable to reach the contraction force because of
the limit of muscle length
– e.g. make a fist as wrist extended vs. that as wrist flexed
• passive insufficiency – unable to reach full range of motion because of
the limit of muscle length
– e.g. open the hand as wrist extended vs. that wrist flexed
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Active Insufficiency
• unable to reach the contraction force because
of the limit of muscle length
• examples:
– Making a fist with the wrist extended is stronger
than that with the wrist flexed
– the strength of the elbow flexor decreases as the
shoulder joint is more flexed
Mechanism of Active Insufficiency
• the contractile tension of the agonist is
markedly weak when a multi-joint muscle is
attempt to contract at a shortened position
25
Passive Insufficiency
• unable to reach full range of motion because
of the limit of muscle length
• examples:
– automatically open the hand as wrist flexed
– difficult to reach the toes with the knee extended
as compared to that with the knee flexed
• Even though the agonist may contract
strongly, motion may be limited because of
the lack of excursion of the antagonist
Practice of Two Joint Muscles
• biceps brachii
• hamstring
• gastrocnemius
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