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

Two-Jointed Muscles

RF: Better leg extensor, H: better hip extensor

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

Ergonomics

Ergonomics EMG used to investigate effects of…

Sitting posture Carrying loads

Principles of Training

Genetic predisposition Training specificity Intensity Rest Volume

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

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 muscle

Produce movement, maintain postures, stabilize joints, and others