MUSCULAR SYSTEM

OBJECTIVE:

•Identify the basic behavioral properties of the musculotendinous unit

•Structure of skeletal muscle

•Change in muscle length with tension development

•Factors affecting muscular force generation

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BEHAVIORAL PROPERTIES OF THE MUSCULOTENDINOUS UNIT

Four behavioral properties of muscle tissue: Extensibility Elasticity Irritability The ability to develop tension

These properties are common to all muscle, including the cardiac, smooth, & skeletal muscle of human beings.

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Extensibility & Elasticity

The properties of extensibility & elasticity are common to many biological tissues.

Extensibility – the ability to be stretched or to increase in length.

Elasticity – the ability to return to normal length after a stretch.

Muscle’s elasticity returns it to normal resting length following a stretch & provides for the smooth transmission of tension from

muscle to bone.

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Two major components of the elastic behavior of muscle:

Parallel elastic component (PEC) Passive elastic property of muscle derived from

the muscle membranes.

Series elastic component (SEC) Passive elastic property of muscle derived from

the tendons. Act as a spring to store elastic energy (EE) when a

tensed muscle is stretched.

Contractile component Muscle property enabling tension development by stimulated muscle fibers.

Membranes & tendons are respectively parallel to & in series (or in line) with the muscle fibers.

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Parallel Elastic Component

Contractile Component

Series Elastic

Component

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The elasticity of human skeletal muscle is believed to be due primarily to the SEC.

When a tensed muscle is stretched, the SEC causes an elastic recoil effect

The stretch promotes subsequent forceful shortening of the muscle

This pattern of eccentric contraction followed immediately by concentric contraction is known as the stretch-shortening cycle.

This phenomenon contributes to effective development of concentric muscular

force in many sport activities.

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The stretch-shortening cycle also promotes storage & use of elastic energy (EE) during running, particularly with the alternating

eccentric & concentric tension present in the gastrocnemius.

Both SEC & PEC have a viscous property that enable muscle to stretch & recoil in a time-

dependent fashion. When static stretch of a muscle group is maintained

over time, the muscle progressively lengthens, increasing joint range of motion

After a group has been stretched, it does not recoil to resting length immediately, but shortens gradually over

time

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ExtensibilityElasticityIrritabilityThe ability to develop tension

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Irritability & the Ability to Develop Tension

Irritability- The ability to respond to a stimulus.

Stimuli affecting muscles are either: Electrochemical – action potential from the

attaching nerve. Mechanical – an external blow to a portion of a

muscle.

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• The ability to develop tension is the one behavioral characteristic unique to muscle tissue.

• Development of tension = contraction (eccentric or concentric)

Muscle stimulus

Develop tension

STRUCTURAL ORGANIZATION OF SKELETAL MUSCLE

Approximately 434 muscles in the human body (40-45% of the body weight of most adult).

About 75 muscle pairs are responsible for body movements & posture, with the remainder involved in activities such as eye control &

swallowing.

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Bone

Perimysium

Endomysium(between individualmuscle fibers)

Muscle fiber (single muscle cell)

Fascicle(wrapped by perimysium)

Epimysium

Tendon

Epimysium

Muscle fiberin middle ofa fascicle

Blood vessel

Perimysium

Endomysium

Fascicle(a)

(b)

Structure of Skeletal Muscle (muscle fiber)

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Epimysium The outermost layer that surround the entire

muscle.Perimysium Connective tissue surround individual bundles

of muscle fibers (inward from the epimysium).Fascicle Individual bundle of muscle fibers.Endomysium Connective tissue surrounded for each muscle

fiber within the fasciculus.

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Sarcolemma◦ The cell membrane surrounding the muscle fiber

cell.

Myofibrils ◦ Numerous threadlike structure that contain the

contractile proteins (protein filaments) Myosin – thick filaments composed of the

protein. Actin – thin filaments composed primarily of

the protein.

Sarcoplasmic reticulum◦ The storage sites for calcium, which plays an

important role in muscular contraction.

Sarcomeres◦ Myofibrils further subdivided into individual

segments.

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Small part of one myofibril enlarged to show the myofilamentsresponsible for the banding pattern. Each sarcomere extends fromone Z disc to the next (basic structural unit of muscle fiber).

Enlargement of one sarcomere (sectioned lengthwise). Notice the myosin heads on the thick filaments.

M line Bisect each sarcomere (middle)

A band Contain thick, rough myosin filament, each of which is surrounded by thin, smooth actin filaments

I band Contain only thin actin filaments

Z lines (disc)

Attachment of thin actin filaments

H zones Center of A bands, contain only thick myosin filaments

Figure 9.5

Myofibril

Myofibrils

Triad:

Tubules ofthe SR

Sarcolemma

Sarcolemma

Mitochondria

I band I bandA band

H zone Z discZ disc

Part of a skeletalmuscle fiber (cell)

• T tubule• Terminal

cisternaeof the SR (2)

M line

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Motor Units

Composed of a single motor neuron & all fibers innervated by it.

Typically, there is only 1 end plate per fiber.

A single mammalian motor unit may contain from less than 100 to nearly 2000 fibers, depending on the type of movements the

muscle executes. Movements that are precisely controlled (eyes, fingers)

produced by motor units with small numbers of fibers Gross, forceful movements (gastrocnemius) result of the

activity of large motor units

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Figure 9.13a

Spinal cord

Motor neuroncell body

Muscle

Nerve

Motorunit 1

Motorunit 2

Musclefibers

Motorneuronaxon

Axon terminals atneuromuscular junctions

Axons of motor neurons extend from the spinal cord to the muscle. There each axon divides into a number of axon terminals that form neuromuscular junctions with muscle fibers scattered throughout the muscle.

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Motor end plate

Fiber Types

Slow twitch fiber (ST)◦ A fiber that reaches peak tension relatively

slowly.

Fast twitch fiber (FT)◦ A fiber that reaches peak tension relatively

quickly.◦ Fast-twitch Oxidative Glycolytic◦ Fast-twitch Glycolytic

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SKELETAL MUSCLE FIBER CHARACTERISTICS

CHARACTERISTIC TYPE 1 SLOW-TWITCH

OXIDATIVE (SO)

TYPE IIA FAST-TWITCH OXIDATIVE GLYCOLYTIC

(FOG)

TYPE IIB FAST-TWITCH GLYCOLYTIC

(FG)

Contraction speed Slow Fast Fast

Fatigue rate Slow Intermediate fast

Diameter Small Intermediate Large

ATPase concentration Low High High

Mitochondrial concentration High High Low

Glycolytic enzyme concentration

Low Intermediate High

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Time

FT

ST

Twitch tensio

n

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Fiber Architecture

Two categories of muscle fiber arrangement

◦ Parallel fiber arrangement Pattern of fibers within a

muscle in which the fibers are roughly parallel to the longitudinal axis of the muscle.

E.g. sartorius, rectus abdominis, biceps brachii.

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Pennate fiber arrangement Pattern of fibers within a

muscle with short fibers attaching to one or more tendons (lie at an angle).

E.g. tibialis posterior, rectus femoris, deltoids

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SKELETAL MUSCLE FUNCTION

When an activated muscle develops tension, the amount of tension present is constant throughout the length of the muscle, & at the sites of the musculotendinous attachments to bone.

The tensile force (stretching force) developed by the muscle pulls on the attached bones & create torque at the joints crossed by the muscle.

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Torque (Tm ) produced by a muscle at the joint center of rotation is the product of muscle force ( Fm ) & muscle moment arm ( d⊥ ).

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The torque exerted by the biceps brachii (Fb) must counteract the torques created by the force developed in the triceps brachii (Ft), the weight of the forearm & hand (wtf), & the weight of the shot held in the hand (wts).

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Recruitment of motor units

The CNS exerts an elaborate system of control that enables:◦ Matching of the speed & magnitude of

muscle contraction to the requirements of the movement so that: Smooth, delicate, & precise movements can

be executed.

Slow twitch (ST) motor units generally have low thresholds & are relatively easy to activate.Fast twitch (FT) motor units are supplied by nerves more difficult to activate.

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Change in Muscle Length with Tension Development

When muscular tension produces a torque larger than the resistive torque at a joint, the muscle shortens, causing a change in the angle at the joint.

Type of contraction;◦ Concentric◦ Eccentric ◦ Isometric

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Concentric Eccentric Isometric

Contraction involving shortening of muscleResulting joint movement is in the same

direction as the net torque generated by the muscle.

A single muscle fiber is capable of shortening to approximately one-half of its normal resting length.

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Concentric Eccentric Isometric

When opposing joint torque exceeds that produced by tension in a muscle, the muscle lengthens.

When a muscle lengthens as it is being stimulated to develop tension.

The direction of joint motion is opposite that of the net muscle torque.

The eccentric tension acts as a braking mechanism to control movement speed.

E.g. without the presence of eccentric tension in muscles, the forearms, hand, & weight would drop uncontrolled because of the force of gravity.

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Concentric Eccentric Isometric

Muscular tension is developed but no change in muscle length.

Opposing torque at the joint crossed by the muscle is equal to the torque produced by the muscle (with zero net torque present),◦ Muscle length remains unchanged & no

movement occurs at the joint.

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Recruitment of motor unitsChange in muscle length with

tension developmentRoles assumed by muscles

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SKELETAL MUSCLE FUNCTION

Roles Assumed by Muscles

Agonist

Antagonist

Stabilizers

Neutralizer

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Agonist

Prime mover.When a muscle contracts & causes movement

of a body segment at a joint.E.g.

◦ During the elbow flexion phase of a forearm curl, the brachialis & the biceps brachii act as the primary agonist, with the brachioradialis, extensor carpi radialis longus, & pronator teres serving as assistant agonist.

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Antagonist

Muscle with actions opposite those of the agonist act.

Opposers by developing eccentric tension at the same time that the agonists are causing movement.

Agonists & antagonists are typically positioned on opposite sides of a joint.

E.g.◦ During elbow flexion when the brachialis &

the biceps brachii are primary agonists, the triceps could act as antagonists by developing resistive tension.

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Stabilizers

Stabilizing a portion of the body against a particular force.◦ The force may be internal, from tension in

other muscles, or external, such as the weight of an object being lifted.

E.g.◦ The rhomboids act as stabilizers by

developing tension to stabilize the scapulae against the pull of the tow rope during water skiing, or on tug-of-war event.

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Neutralizer

Neutralizers muscle prevent unwanted accessory actions that normally occur when agonists develop concentric tension.

E.g.◦ When the biceps brachii develops concentric

tension, it produces both flexion at the elbow & supination of the forearm. If only elbow flexion is desired, the pronator teres act as a neutralizer to counteract the supination of the forearm.

Performance of human movements typically involves the cooperative actions of many muscle groups acting sequentially & in concert.

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Factors Affecting Muscular Force Generation

The magnitude of the force generated by muscle is also related to:

Velocity of muscle shorteningLength of the muscle when it is stimulatedPeriod of time since the muscle received a

stimulus

Factors affecting:Force-Velocity relationshipLength-Tension RelationshipElectromechanical Delay (EMD)

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Force-Velocity Relationship for muscle tissue

When the resistance (force) is negligible, muscle contracts with maximal velocity. As the load progressively increases, concentric

contraction velocity slows to zero at isometric maximum. As the load increases further, the muscle lengthens

eccentrically.

FVR does NOT imply that it is impossible to move a heavy resistance at a fast speed.

The stronger a muscle, the greater the magnitude of maximum isometric tension

FVR also does NOT imply that it is impossible to move a light load at a slow speed.

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Maximally

activated

muscle

Length-Tension Relationship

The total tension present in a stretched muscle is the sum of the active tension provided by the muscle fibers & the passive tension provided by the tendons & muscle membranes.

Within the human body, force generation capability increases when the muscle is lightly stretched.Parallel-fibered muscles produce maximum tensions at

just over resting length.Pennate-fibered muscles generate maximum tension at

between 120% & 130% of resting length.This phenomenon is due to the contribution of the elastic

components of muscle (primarily the SEC), which add to the tension present in the muscle when the muscle is stretched.

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Total tension = active tension (muscle fibers) + passive tension (tendons &muscle membranes)

Electromechanical Delay (EMD)

When a muscle is stimulated, a brief period of time elapse before the muscle begins to develop tension.

ED- time between the arrival of neural stimulus and tension development by the muscle 41

EMD where the period of time is believed to be needed for the contractile component of the muscle to stretch the SEC.During this time, muscle laxity is eliminated.Once the SEC is sufficiently stretched, tension

development proceeds.

Researchers have found shorter EMDs produced by muscles with high percentages of FT fibers as compared to muscles with high percentages of ST fibers.

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Muscular Strength, Power & Endurance

Muscular StrengthThe maximum amount of force a muscle

can produce in a single effort

Muscular PowerThe product of muscular force and the

velocity of muscle shortening

Muscular EnduranceThe ability of a muscle to exert a sub-

maximal force repeatedly over time

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What is the effect of muscle temperature (warm up) ?

The speeds of nerve and muscle functions increase.

Normal body temperature

Elevated body temperature

With warm-up, there is a shift to the right in

the force-velocity curve, with higher

maximum isometric tension and higher

maximum velocity of shortening possible at

a given load.

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velo

city

force

Common Muscle Injuries

Delayed-Onset Muscle Soreness (DOMS)◦ occurs after some period of time following

unaccustomed exercise.◦ arises 24 – 72 hours after participation in a long

or strenuous bout of exercise.

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Strains - overstretching of muscle tissue

Contusions - compressive forces sustained during impacts

Cramps - electrolytes imbalance, deficiencies in calcium & magnesium, dehydration