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7/28/2019 14205119 Muscular Control
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SPS211/FSRUiTM
Skeletal Muscle: Structure & Function
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Skeletal Muscle:Structure & Function
• The human body contain over 400 skeletal muscles, whichconstitute 40-50% of the total body weight.
• 3 functions:
– Force generation for locomotion & breathing– Force generation for postural support
– Heat production during periods of cold stress
• Skeletal muscles are attached to bones through a connectivetissue called tendon
• One end of the muscle is attached to the bone that does notmove (origin), while the opposite end in fixed to a bone(insertion) that is move during muscular contraction.
• Muscles that decrease joint angle are call flexors, while musclethat increase joint angle are called extensors
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Structure of Skeletal Muscle
• Skeletal muscle is composed of several kind of tissues,include muscle cells, nerve, blood and various type of connective tissue
• Individual muscle are separate from each other and in
position by a connective tissue call Fascia• There are 3 separate layer:
– Epimysium (outer layer)
– Perimysium (middle layer)
– Endomysium (inner layer)
• The cell membrane surrounding the muscle cell is calledthe Sarcolemma.
• Beneath the sarcolemma lies the Sarcoplasm (cytoplasm)which contain myofibrils.
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Structure of Skeletal Muscle
• In general, myofibrils composed of two type of proteinfilaments:– Myosin (thick filament)
– Actin (thin filament)
• Both play an important role in the regulation of contractileprocess.
• Myofibrils can further subdivided into individual segmentcalled sarcomeres.
• Sarcomere are divided from each other by a thin sheet of
structural protein called a Z line• Myosin filaments are located primarily in the dark portion
of sarcomere (A band ), while actin filament occur in thelight portion (I band). In the center of the sarcomere thereis a portion of the myosin filament with no overlap of actin,
called H zone.
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Structure of Skeletal Muscle
• Within a sarcoplasm, there is a membranous channel called
sarcoplasmic reticulum (SR)
• SR – storage site for calcium• Another set of membranous channel is transverse tubules (t
tubules)
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Muscular Contraction
• The process of muscular contraction is best explained
by the sliding filament model of contraction
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Sliding Filament ModelRelaxed Muscle Actin & myosin filaments in relaxed muscle
and contracted muscle are the same length
Contracting
muscle
During contraction, actin at each end of the
sarcomeres slide past the myosin toward
each other. Z disk are closer & sarcomeresshorten.
Contracting
muscle
As the actin slide over the myosin, the H
zone and the I band narrow. The A nab do
not narrowFully contracted
muscle
In a fully contracted muscle the end of the
actin overlap and the H zone disappears
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Fiber Types
• Muscle fibers can be classified into 2 categories:
– Type I - Slow twitch (ST) (slow oxidative)
– Type II - Fast twitch (FT)
• Type IIa (fast oxidative glycolytic)
• Type IIb or IIx (fast glycolytic)
Max Short
velocity
Type I Type IIa Type IIb/IIx
0
4
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Characteristics of Fiber Types
Fast Fibers Slow Fibers
Characteristic Type IIb / IIx Type IIa Type 1
Number of
mitochondria
Low High/Mod High
Resistance to
fatigue
Low High/Mod High
Predominant energy
system
Anaerobic Combination Aerobic
ATPase activity Highest High Low
Vmax (speed of
shortening)
Highest Intermediate Low
Efficiency Low Moderate High
Specific tension High High Moderate
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Fiber Type & Performance
Sports % Slow Fibers
(Type 1)
% Fast Fibers
(Type Ila & Ilb)Distance runner 70-80 20-30
Track Sprinters 25-30 70-75
Non athletes 47-53 47-53
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Alteration of Muscle Fiber Type
by Exercise Training
• In the past, researchers have concluded that endurancetraining does not result in the conversion of FT to STfiber.
• By contrast, recent investigations using improvedtechnique have shown that rigorous exercise trainingresults in alteration of muscle fiber types.
• Long duration exercise training (90 min/day; 10 week)
is capable of promoting a Type II to Type I fiber• However, the resistance training –induced changes in
fiber type are often small & do not result in completeconversion to Type IIb to Type I fiber
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Age Related Changes in
Skeletal Muscle Aging associated with a loss of muscle mass (atrophy)
• 25 to 50 years – 10% loss
• 50 to 80 years – additional 40% loss
• Aging results in loss of FT fibers (esp. Type IIb)
• Loss of muscle in lower limb is more pronounced in
older adults
• However, regular exercise (PRT) can improve muscular
endurance and strength but cannot completely
eliminate the age related loss in muscle mass
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Muscle Actions
3 type of actions:• Concentric
– Shortening of muscle
– Dynamic action
• Static
– Muscle generating force but it length remains
static (unchanged)
– Isometric action• Eccentric
– Lengthening of muscle
– Dynamic action
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Generation of Force
Depends on:
– The number of motor units activated• More motor unit – more force
– The type of motor units activated• FT generate more force
– The size of the muscle• Larger muscle generate more force
– The muscle initial length when activated
• Increasing or decreasing the muscle length beyond 20% reduce forceproduction
• However, in general, the amount of power generated by a musclegroup increases as a function of movement velocity.
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Generation of Force
Depends on:
– The angle of the joint• E.g., biceps brachii – best joint angle is 1000 to lift 100lb force
– The muscle’s speed of action (force-velocity/ power-velocity)
• During concentric actions, maximal force decreases at higher speed.
• Fast eccentric actions allow max application of force
• Higher % FT – velocity or speed is greater• However, in general, the amount of power generated by a muscle
group increases as a function of movement velocity.
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Receptors in Muscle
• Chemoreceptor– Send info to CNS in response to changes in
muscle pH, [potassium], changes in O2 & CO2 tensions.
• Golgi Tendon Organs– Provide CNS with feedback concerning the
tension developed by the muscle
– Serve as safety devices that help preventexcessive force during muscle contraction
• Muscle Spindles– Provide sensory information concerning relative
muscle length
– “Length Detector”