Chapter 3
Muscular Considerations for
Movement
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Characteristics of Muscle
• Irritability
– Ability to respond to stimulation
• Contractility
– Ability to shorten when it receives sufficient stimulation
– Unique to muscle tissue
• Extensibility
– Ability to stretch/lengthen beyond resting length
– Protective mechanism
• Elasticity
– Ability to return to resting length after being stretched
– Protective mechanism
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Functions of Muscle
• Produce movement
• Maintain postures and positions
• Stabilize joints
• Other functions
– Support and protect visceral organs
– Alter and control cavity pressure
– Maintain body temperature
– Control entrances/exits to the body
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Groups of Muscles
• Muscles typically act in unison, not individually
• Fascia
– Sheet of fibrous tissue
– Compartmentalizes groups of muscles
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Groups of Muscles (cont.)
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Muscle Architecture
• Parallel
– Flat, fusiform, strap, radiate (convergent) circular
• Pennate
– Unipennate, bipennate, multipennate
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Fiber Organization
• Fusiform
– Parallel fibers and fascicles
– High speed of contract, force production
– ACS = PCS
• Anatomical cross section (ACS)
• Physiological cross section (PCS)
– Sartorius, biceps brachii, brachialis
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Fiber Organization (cont.)
• Penniform
–
3 types
• Unipennate
• Off one side of tendon
• Semimembranosus
• Bipennate
• Off both sides of tendon
• Gastrocnemius
• Multipennate
• Both varieties
• Deltoid
–
PCS > ACS
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Fiber Type
• Type I
– Slow twitch, oxidative
– Red (because of high myoglobin content)
– Endurance athletes
• Type IIa
– Intermediate fast twitch, oxidative-glycolytic
• Type IIb
– Fast twitch, glycolytic
– White
– Sprinters, jumpers
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Individual Muscle Organization
• Belly
–
Thick central portion
• Epimysium
–
Outside covering of a muscle
• Fascicles
–
Bundles of muscle fibers
• Perimysium
–
Dense connective sheath covering a fascicle
• Fibers
–
Cells of a skeletal muscle
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Individual Muscle Organization (cont.)
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Individual Muscle Organization (cont.)
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Individual Muscle Organization (cont.)
• Endomysium
– Very fine sheath covering individual fibers
• Sarcolemma
– Thin plasma membrane branching into muscle
• Myofibrils
– Rod-like strands of contractile filaments
– Many sarcomeres in series
• Sarcoplasma
– Cytoplasm of muscle cell
• Sarcoplasmic reticulum
– Specialized endoplasmic reticulum of muscle cells
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Individual Muscle Organization (cont.)
• T-tubules
– Extension of sarcolemma that protrudes into muscle cell
– Also called, transverse tubule
• Myosin
– Thick, dark filament
• Actin
– Thin, light filament
• Sarcomere
– Unit of myosin and actin
– Contractile unit of muscle
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Motor Unit
• Group of muscles innervated by the same motor neuron
• From 4 to 2,000 muscle fibers per motor unit
• Action potential
– Signal to contract from motor neuron
• Neuromuscular junction
– Also called end plate
– Where action potential from neuron meets muscle fiber
• Conduction velocity
– Velocity at which action potential is propagated along
membrane
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Muscle Contraction
• Resting potential
– Voltage across the plasma membrane in a resting state
• Excitation-Contraction Coupling
– Transmission of action potential along sarcolemma
• Twitch
– Rise and fall reaction from a single action potential
• Tetanus
– Sustained muscle contraction from high-frequency
stimulation
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Muscle Twitch and Tetanus
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Muscle Contraction (cont.)
• Depolarization
– Loss of polarity
• Repolarization
– Movement to the initial resting (polarized) state
• Hyperpolarization
– State before repolarization
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Sliding Filament Theory
• A.F. Huxley
• Seeks to explain production of tension in muscle
• Myosin and actin
– Create cross-bridges
– Slide past one another
– Cause the sarcomere to contract
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Sliding Filament Theory (cont.)
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Muscle Attachment
• 3 ways muscle attaches to bone
–
Directly
–
Via a tendon
–
Via an aponeurosis
• Tendon
–
Inelastic bundle of collagen fibers
• Aponeurosis
–
Sheath of fibrous tissue
• Origin
–
More proximal attachment
• Insertion
–
More distal attachment
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Muscle Attachment (cont.)
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Characteristics of a Tendon
• Transmits muscle force to associated bone
• Can withstand high tensile loads
• Viscoelastic stress-strain response
• Myotendinous junction
– Where tendon and muscle join
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Mechanical Model of Muscle
• A.V. Hill
• Three-Component Model
– Contractile (CC)
• Converts stimulation into force
– Parallel elastic (PEC)
• Allows the muscle to be stretched
• Associated with fascia surrounding muscle
– Series elastic (SEC)
• Transfers muscle force to bone
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Hill Muscle Model
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Role of Muscle
• Origin
– Attachment closer to the midline or more proximal
• Insertion
– Attachment further to the midline or more distal
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Attachment Sites
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Muscle Role vs. Angle of Attachment
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Muscle Role vs. Angle of Attachment
(cont.)
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Role of Muscle
• Prime mover
–
Muscle(s) primarily responsible for a given movement
• Assistant mover
–
Other muscles contributing to movement
• Agonist
–
Muscle creating same joint movement
• Antagonist
–
Muscle opposing joint movement
• Stabilizer
–
Holds one segment still so a specific movement in an adjacent
segment can occur
• Neutralizer
–
Muscle working to eliminate undesired joint movement of
another muscle
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Net Muscle Actions
• Isometric
– Tension produced without visible change in joint angle
• Holding arms out to sides
• Concentric
– Muscle visibly shortens while producing tension
• Up phase of a sit-up
• Eccentric
– Muscle visibly lengthens while producing tension
• Lowering phase of squat
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Muscle Actions
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One- and Two-Jointed Muscles
• Muscles can cross one or two joints
• One-jointed muscles
• Brachialis, pectoralis major
• Two-jointed muscles (biarticulate)
– Save energy
• Gastrocnemius, hamstrings, biceps brachii
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One- and Two-Joint Muscles
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Two-Joint Muscles
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Factors Influencing Muscle Force
• Angle of attachment
• Force-time characteristics
– Force increases nonlinearly due to elastic
components
• Length-tension relationship
• Force-velocity relationship
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Force-Velocity Relationship
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Force-Length Relationship
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Stretch-Shortening Cycle
• Prestretch
– Quick lengthening of a muscle before contraction
– Generates greater force than contraction alone
– Utilizes elastic component of muscle
• Prestretch and Fiber Type
– Type I
• Slower prestretch best because of slow cross-bridging
– Type II
• Faster prestretch best because of fast cross-bridging
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Plyometrics
• Conditioning protocol that uses prestretching
– Single-leg bounds, depth jumps, stair hopping
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Stretch-Shortening Cycle and Plyometric
Exercise
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Muscle Fatigue
• Fatigue results from:
– Peripheral (muscular) mechanisms
– Central (nervous) mechanisms
• When motor unit fatigues:
– Change in frequency content
– Change in amplitude of EMG signal
• Sufficient rest restores initial signal content and
amplitude
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Strengthening Muscle
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Strengthening Muscle (cont.)
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Principles of Training
• Genetic predisposition
• Training specificity
• Intensity
• Rest
• Volume
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Strength Training and the Nonathlete
• ACSM
– 2 days per week
– 8–12 exercises per day
• Counteracts atrophy of muscle and bone
• Elderly
• Children
– High intensity not recommended
– Epiphyseal plates susceptible to injury under high loads
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Training Modalities
• Isometric
–
No visible movement
–
Rehabilitation
• Isotonic
–
Same weight throughout range of motion (ROM)
• Isokinetic
–
Same velocity, varied resistance
• Close-linked
–
Isotonic, in which one segment is fixed in place
• Variable resistive
–
Supposedly overloads muscle throughout ROM
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Injury to Skeletal Muscle
• At risk
–
Two-jointed muscles at greatest risk of strain
–
Eccentrically contracted to slow limb movement
• Hamstrings, rotator cuffs
–
Fatigued or weak muscles
–
When performing unique task for first time
–
Already injured
• Prevention
–
Warm-up
–
Build up when starting new program
–
Recognize signs of fatigue
–
Give body adequate rest
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Summary
• Characteristics of muscle tissue
– Irritability, contractility, extensibility, elasticity
• Often act in compartmentalized groups
• Fiber organization
– Fusiform, penniform
• Fiber types
– Type I, IIa, IIb
• Functions of muscles
– Produce movement, maintain postures, stabilize
joints, and others
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