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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
Groups of Muscles
Muscles typically act in unison – not individually Fascia
Sheet of fibrous tissue Compartmentalizes groups of muscles
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
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
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
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
Fiber Organization
Penniform 3 Types
Unipennate Off one side of tendon Semimembranosus
Bipennate Off both sides of tendon Gastrocnemius
Multipennate Both varieties Deltoid
PCS > ACS
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
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
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
Functions of Muscle
Produce movement Maintain postures and positions Stabilize joints Other functions
Support and protect visceral organsAlter and control cavity pressureMaintain body temperatureControl entrances/exits to the body
Role of Muscle
Prime mover Muscle(s) primarily responsible for a given movement
Assistant mover Other muscles contributing to movement
Agonist Muscles creating same joint movement
Antagonist Muscles 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
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
Stretch-Shortening Cycle
Pre-stretch Quick lengthening of a muscle before contraction Generates greater force than contraction alone Utilizes elastic component of muscle
Prestretch & Fiber Type Type I
Slower pre-stretch best because of slow cross-bridging
Type IIFaster pre-stretch best because of fast cross-
bridging
Plyometrics
Conditioning protocol that utilizes pre-stretching Single-leg bounds, depth jumps, stair hopping
One- and Two-Jointed Muscles
Muscles can cross one or two joints One-Jointed Muscles
Brachialis, pectoralis major Two-Jointed Muscles (biarticulate)
Save energyGastrocnemius, hamstrings, biceps brachii
Motor Unit
Group of muscles innervated by the same motor neuron
From 4 to 2000 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
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
Muscle Contraction (cont.)
Depolarization Loss of polarity
Repolarization Movement to the initial resting (polarized) state
Hyperpolarization State before repolarization
Sliding Filament Theory
A.F. Huxley Seeks to explain production of
tension in muscle Myosin & actin
Create cross-bridges Slide past one another Cause the sarcomere to
contract
Mechanical Model of Muscle
A.V. Hill 3 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
Factors Influencing Muscle Force
Angle of attachment Force-time characteristics
Force increases non-linearly due to elastic components
Length-tension relationship Force-velocity relationship
Electromyography
Correct placement of electrodes is critical Electrodes oriented parallel to muscle fiber Prepare skin by…
Shaving Abrading Cleaning with alcohol
Signal must be amplified Sample rate ≥ 1000 Hz required
Electromyography (cont.)
Rectification Taking the absolute value of raw signal
Linear envelope Time domain Frequency domain Electromechanical delay (EMD)
Temporal delay between onset of EMG signal & development of muscle tension
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
Clinical Gait Analysis
EMG used to investigate which muscle group is used in a certain phase of gait
Determine activation order Raw or rectified EMG signal
is used
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
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
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