biomechanics of muscles

7/29/2019 biomechanics of muscles

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Chapter 6

The Biomechanics of

Human Skeletal

Muscle

PRESENTED BY: MUHAMMAD ASAD

CHAUDHARY

ROLL NO: 33 DPT 2nd Year


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MUSCLE

Contractile tissue that produces motion forfunctions, including body movements,digestion, focusing, circulation, and body

warmth. It can be classified as: Striated

Cardiac

Smooth


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Behavioral Properties of the

Musculotendinous Unit

The characteristic behavioral

properties of muscle are:

1) Extensibility: ability to be stretched

or to increase in length

2) Elasticity: ability to return to normalresting length following a stretch


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Components ofElasticity:

Parallel elastic component - passive

elasticity derived from muscle

membranes

Series elastic component - passive

elasticity derived from tendons

when a tensed muscle is

stretched


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Both the SEC and the PEC have a viscous

property that enables muscle to stretch and

recoil in a time-dependent fashion.

This visco-elastic response is independent

of gender.


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Parallel ElasticComponent

Series Elastic

ComponentContractileComponent

From a mechanical perspective, the musculotendinous unit behaves as a

contractile component (muscle fibers) in parallel with one elastic component

(muscle membranes) and in series with another elastic component (tendons).


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Series and parallel elastic elements in muscle.

A.Resting muscle contains elastic elements in series with the contractileelements (sarcomeres) and in parallel with them.

B.During an isometric contraction, the muscle does not change length, but

sarcomeres shorten, stretching the series elastic elements.

C.During isotonic contraction, the contractile elements shorten, stretching the

series elastic elements, before they develop tension to lift the load.

D.Muscle begins to shorten when contractile elements shorten further.



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3) Irritability: the ability to respond to a

stimulus

Stimuli affecting muscles are eitherelectrochemical, such as an action potential

from the attaching nerve, or mechanical,

such as an external blow to a portion of amuscle.

If the stimulus is of sufficient magnitude,

muscle responds by developing tension.


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Behavioral Properties of theMusculotendinous Unit

4) Contractility - the ability of a muscle

to shorten in length.

When a muscle contracts, it pulls with equalforce on each attachment.

A muscles line of pull refers to the direction of

the resultant force produced at an attachment.


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Structural Organization of SkeletalMuscle

There are approximately 434 muscles in thehuman body, making up

4045% of the body weight of most adults.Muscles are distributed in pairs on the right andleft sides of the body.

About 75 muscle pairs are responsible for bodymovements and posture, with the remainderinvolved in activities such as eye control and

swallowing.


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Structural Organization of Skeletal

Muscle

What is a muscle fiber?

(single muscle cell surrounded by amembrane called the sarcolemma

and containing specialized

cytoplasm called sarcoplasm)

The number of muscle fibers present is

genetically determined and varies from

person to person.


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Muscle

Sarcolemma


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Muscle

some fibers run the entire length of a

muscle; others are shorter

skeletal muscle fibers grow in both

length and diameterfrom birth

through adulthood

fiberdiametercan be increasedthrough resistance training


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Structural Organization of Skeletal Muscle The sarcoplasm of each fiber contains a number

of nuclei and mitochondria, as well as numerousthreadlike myofibrils that are aligned parallel to

one another. The myofibrils contain two types ofprotein filaments whose arrangement producesthe characteristic striated pattern after which

skeletal,or striated, muscle is named.


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MuscleSarcomere

The sarcomere,compartmentalized between twoZ lines, is the basic structural unit

of the muscle fiber. The A bandscontain thick, rough myosinfilaments surrounded by six thin,smooth actin filaments. The I

bands contain only thin actinfilaments. In the center of the Abands are the H zones, whichcontain only

the thick myosin filaments.


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Muscle

What is a motor

unit?single motor neuron

and all fibers itinnervates

considered the

functional unit of theneuromuscular system


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Muscle

The fibers of a motor unit may be spread over a several-centimeter area and be interspersed with the fibers of othermotor units. Motor units are typically confined to a singlemuscle and are localized within that muscle. A single

mammalian motor unit may contain from less than 100 tonearly 2000 fibers, depending on the type of movements themuscle executes

Most skeletal motor units in mammals are composed oftwitch-type cells that respond to a single stimulus by developingtension in a twitch like fashion. The tension in a twitch fiberfollowing the stimulus of a single nerve impulse rises to a peak

value in less than 100msec and then immediately declines.


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Muscle

Fast twitch (FT)

fibers both reach

peak tension and

relax more quicklythan slow twitch

(ST) fibers. (Peak

tension is typically

greater for FT thanfor ST fibers.)

Tw

itchTension

Time

FT ST


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Skeletal Muscle Fiber CharacteristicsTYPE IIA

TypeI Fast-Twitch TypeIIB

Slow-Twitch Oxidative Fast-TwitchOxidative Glycolytic Glycolytic

CHARACTERISTIC (SO) (FOG) (FG)

Contraction Speed slow fast fast

Fatigue rate slow intermediate fast

Diameter small intermediate large

ATPase concentration low high high

Mitochondrial high high lowconcentrationGlycolytic enzyme low intermediate highconcentration


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Structural Organization of SkeletalMuscle

The role of genetic expression on fiber type and suggeststhat skeletal muscle adapts to altered functional demandswith changes in the genetic phenotype of individual fibers.

Myogenic stem cells called satellite cells are normally inactivebut can be stimulated by a change in habitual muscle activityto proliferate and form new muscle fibers. It has beenhypothesized that muscle regeneration following exercise

may provide a stimulus for satellite cell involvement inremodeling muscle by altering genetic expression in termsof muscle fiber appearance and function within the muscle.


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Structural Organization of Skeletal Muscle

Fiber ArchitectureParallel fiber arrangement: Fibers are roughly parallel tothe longitudinal axis of the muscle; e.g.Sartorius,rectusabdominis, and biceps brachii etc.

In most parallel-fibered muscles, there are fibers that donot extend the entire length of the muscle, but terminatesomewhere in the muscle belly. Such fibers have structuralspecializations that provide interconnections withneighboring fibers at many points along the fibers surface to

enable delivery of tension when the fiber is stimulated.When tension is developed, any shortening of the muscle is

primarily the result of the shortening of its fibers


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Structural Organization of Skeletal Muscle Pennate fiber arrangement: Short fibers attach to

one or more tendons within the muscle; e.g. Tibialisposterior, rectus femoris, and deltoid etc. A pennate fiber arrangement is one in which the fibers lie at

an angle to the muscles longitudinal axis.

The fibers of a muscle may exhibit more than one angle ofpennation (angle of attachment) to a tendon.

When the fibers of a pennate muscle shorten, they rotate

about their tendon attachment or attachments, progressivelyincreasing the angle of pennation

Pennate fiber arrangement promotes muscle force production, and

parallel fiber arrangement facilitates muscle shortening.


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Muscle

Parallel fiber arrangements Pennate fiber arrangements


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Muscle

Relaxed With tension

development

The angle of pennation increasesas tension progressively increasesin the muscle fibers. The greaterthe angle of pennation, the smallerthe amount of effective forcetransmitted to the tendon ortendons to move the attached

bones. Once the angle of exceeds60, the amount of effective forcetransferred to the tendon is lessthan one-half of the force actually

produced by the muscle fibers


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Skeletal Muscle Function

When an activated muscle develops tension, the amount oftension present is constant throughout the length of themuscle, as well as in the tendons, and at the sites of themusculotendinous attachments to bone.

The magnitude of the torque generated is the product of theforce developed by the muscle and the perpendiculardistance of the line of action of that force from the center ofrotation at the joint.

The weight of the attached body segment, external forcesacting on the body, and tension in any muscle crossing a jointcan all generate torques at that joint.


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Tm = Fm d

d

Fm

Fb

Ft

Wtf Wts


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How are motor units (MUs) recruited?The central nervous system exerts an elaborate system ofcontrol that enables matching of the speed and magnitude ofmuscle contraction to the requirements of the movement so that

smooth, delicate, and precise movements can be executed.Slow Twitch (ST) fibers are easier to activate than fast twitch(FT) fibers ST fibers are always recruited first. Increasing speed, force, or

duration of movement involves progressive recruitment of MUswith higher and higher activation thresholdsSlow-twitch motor units have low threshold and always produce

tension first, whether the resulting movement is slow or fast.


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Muscle ContractionWhen muscular tension produces a torque larger than theresistive torque at a joint, the muscle shortens, causing a

change in the angle at the joint.Concentric muscle action - when a muscle shortensunder tension.A single muscle fiber is capable of

shortening to approximatelyone-half of its normal resting length. The resultingjoint movement is in the same direction as the net torquegenerated by the muscles.


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Eccentric muscle action - when a muscle lengthens under

tension.The direction of joint motion is opposite that of the netmuscle torque. The eccentric tension acts as a braking mechanismto control movement speed. Otherwise body parts will drop inan uncontrolled way.

Isometric muscle action - when a muscle produces tension,but there is not movement. As the opposing torque at the jointcrossed by the muscle is equal to the torque produced by the

muscle, muscle length remains unchanged, and no movementoccurs at the joint.

The development of tension increases the diameter of themuscle, that is why body builders develop isometric tension to

display their muscles when competing.



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Roles are Assumed By Muscles

Agonist- a muscle that causes movement. The primemover.

Antagonist - a muscle that resists movement.Synergist- a muscle that assists the agonist inperforming a movement.

Stabilizer, Neutralizer, Fixator- role played by amuscle acting to stabilize a body part against some otherforce or eliminate an unwanted action produced by an

agonist.


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The muscles which cross two or more then two joints are called

two-joint and multi-joint Muscles

The effectiveness of a two-joint or multijoint muscle in causingmovement at any joint crossed depends on the location andorientation of the muscles attachment relative to the joint, the

tightness or laxity present in the musculotendinous unit, and theactions of other muscles that cross the joint.

one-joint muscles produce force directed primarily in line with abody segment, two-joint muscles can produce force with asignificant transverse component

Two-Joint and Multijoint MusclesSkeletal Muscle Function


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Disadvantages associated with musclesthat cross more than one joint Active Insufficiency: Failure to produce force when

slack. For example, the finger flexors cannot produce astight a fist when the wrist is in flexion as when it is in aneutral position.

Passive Insufficiency: Restriction of joint range ofmotion when fully stretched. For example, a larger range ofhyperextension is possible at the wrist when the fingers are

not fully extended.


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active insufficiency: failure toproduce force when muscles are

slack (decreased ability to form afist with the wrist in flexion)


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passive insufficiency: restriction ofjoint range of motion when muscles

are fully stretched (decreased ROMfor wrist extension with the fingersextended)


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Factors Affecting Muscular ForceGeneration

The magnitude of the force generated by muscleis also related to the velocity of muscle

shortening, the length of the muscle when it isstimulated, and the period of time since themuscle received a stimulus.


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Factors Affecting Muscular Force

Generation

The force-velocity relationship for muscle tissue:Thisforcevelocity relationship was fi rst described for concentrictension development in muscle by Hill in 1938.Accordingly, the forcevelocity relationship does not imply that it

is impossible to move a heavy resistance at a fast speed. Thestronger the muscle is, the greater the magnitude of maximumisometric tension. This is the maximum amount of force that amuscle can generate before actually lengthening as the resistance

is increased. However, the general shape of the forcevelocitycurve remains the same, regardless of the magnitude ofmaximum isometric tension.


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The force-velocityrelationship for muscle

tissue

When resistance (force) isnegligible, muscle contractswith maximal velocity.

The general pattern holds true

for all types of muscle, even thetiny muscles responsible for therapid fluttering of insect wings Velocity

Force (Low resistance,

high contraction

velocity)


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Generation

The force-velocity

relationship for

muscle tissue:As

the load increases,concentric

contraction velocity

slows to zero at

isometric maximum.

Velocity

Force

isometric maximum



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GenerationLength-Tension Relationship:

Tension present in a stretched muscleis the sum of the active tensionprovided by the muscle fibers and thepassive tension provided by the

tendons and membranes.The amountof maximum isometric tension amuscle is capable of producing ispartly dependent on the muscles

length. In single muscle fibers, forcegeneration is at its peak when themuscle is slightly stretched.

Tension

Length (% of resting length)50 100 150

Active

Tension

Passive

Tension

TotalTension

Parallel-fiber muscles produce maximumtensions at just over resting length, and pennate

fiber muscles generate maximum tensions atbetween 120% and 130% of restin len th.


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Stretch-Shortening Cycle

When an actively tensed muscle is stretched just prior tocontraction, the resulting contraction is more forceful than in theabsence of the prestretch. This pattern of eccentric contractionfollowed immediately by concentric contraction is known as thestretch-shortening cycle (SSC).

A muscle can perform substantially more work when it is activelystretched prior to shortening than when it simply contracts.

The metabolic cost of performing a given amount of mechanicalwork is also less when the SSC is invoked than the cost without it.

The SSC also promotes storage and use of elastic energy duringrunning, particularly with the alternating eccentric and concentric

tension present in the gastrocnemius.


F t Aff ti M l F


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Generation

Electromechanical delay

Myoelectric activity

Force

Electromechanical delayStimulus

(time between arrival of aneural stimulus and tension

development by themuscle)The length of EMD varies

considerably among humanmuscles, with values of 20100 msec reportedFast twitch fibers haveshort EMD.


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Development of higher contraction forces is associated with shorterEMDs.

Factors such as muscle length, contraction type, contraction velocity.

EMD is longer under the following conditions:

Immediately following passive stretching,

Several days after eccentric exercise resulting in muscle damage,

After a period of endurance training, When contraction is initiated from a resting state as compared to an

activated state

EMD in children is also significantly longer than in adults.

Electromechanical delay


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Muscular Strength, Power and

EnduranceMuscular StrengthMuscular strength is most commonly measured as the amountof torque a muscle group can generate at a joint. Thecomponent of muscle force directed perpendicular to the

attached bone produces torque, or a rotary effect, thiscomponent is termed the rotary component of muscle force. Thesize of the rotary component is maximum when the muscle isoriented at 90 to the bone

Ft Ft


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Endurance

Factors Affecting Muscular Strength

Tension-generating capability of the

muscle tissue, which is in turnaffected by:

Muscle cross-sectional area

Training state of muscle


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Endurance

Moment arms of the muscles crossingthe joint (mechanical advantage), inturn affected by:

Distance between muscle attachment tobone and joint center

Angle of the muscles attachment tobone

Factors Affecting Muscular Strength


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Torque produced by amuscle (Tm) at the

joint center ofrotation is the

product of muscleforce (Fm) and muscle

moment arm (d ).


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Endurance

The mechanical advantage of the biceps bracchi is maximum when

the elbow is at approximately 90 degrees (A), because 100% of muscle

force is acting to rotate the radius. As the joint angle increases (B)or decreases (C) from 90 degrees, the mechanical advantage of

the muscle is lessened because more and more of the force is pulling

the radius toward or away from the elbow rather than contributing

to forearm rotation.

CA B


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Endurance

Muscular PowerThe product ofmuscular force and the velocity of muscleshorteningThe rate of torque production at a joint The product ofnet

torque and angular velocity at a jointMaximum power occurs at approximately one-third of maximumvelocity and at approximately one-third of maximum concentric

force.Individuals with a predominance of FT fibers generate more powerat a given load than do individuals with a high percentage of STcompositions. Those with primarily FT compositions also develop

their maximum power at faster velocities of muscle shortening


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Endurance

The general shapes of the force-velocity and

power-velocity curves for skeletal muscle.

F

orce

Velocity

Po

wer

Power-VelocityForce-Velocity


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Endurance

Muscular Endurance

The ability of muscle to exert tension over

a period of time

The opposite of muscle fatigability

Effect ofmuscle temperature on (warmup)

The speeds of nerve and muscle functionsincrease


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Endurance

With warm-up, there

is a shift to the right

in the force-velocity

curve, with highermaximum isometric

tension and higher

maximum velocity of

shortening possibleat a given load.

Velocity

Force

Normal body temperature

Elevated body temperature


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Muscle Fatigue Muscle fatigue has been defined as an exercise-induced reduction in

the maximal force capacity of muscle

A complex array of factors affects the rate at which a muscle fatigues,including the type and intensity of exercise, the specific muscle groupsinvolved, and the physical environment in which the activity occurs.Moreover, within a given muscle, fiber type composition and thepattern of motor unit activation play a role in determining the rate atwhich a muscle fatigues.

Characteristics of muscle fatigue include: Reduction in muscle force production capability and shortening

velocity, as well as prolonged relaxation of motor units. High-intensitymuscle activity over time also results in prolonged twitch duration and

a prolonged sarcolemma action potential of reduced amplitude


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COMMON MUSCLE INJURIES Muscle injuries are common, with most being relatively minor. Fortunately,

healthy skeletal muscle has considerable ability to self-repair. Strains

Muscular strains result from overstretching of muscle tissue. Most typically,an active muscle is overloaded, with the magnitude of the injury related to

the size of the overload and the rate of overloading. Strains can be mild,moderate, or severe.

Mild strains characterized by a feeling of tightness or tension in the muscle.

Second-degree strains involve a partial tear in the muscle tissue,

with symptoms of pain, weakness, and some loss of function.

Third degree sprains, there is severe tearing of the muscle, functional loss,and accompanying hemorrhage and swelling.

The hamstrings are the most frequently strained muscles in the human body.


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COMMON MUSCLE INJURIES CONTUSIONS

Contusions, or muscle bruises, are caused by compressive forces

sustained during impacts. They consist of hematomas within themuscle tissue. A serious muscle contusion, or a contusion that isrepeatedly impacted, can lead to the development of a much moreserious condition known as myositis ossificans. Myositis ossificans consists

of the presence of a calcifiedmass within the muscle. After six or sevenweeks, resorption of the calcified mass usually begins, althoughsometimes a bony lesion in the muscle remains.

CRAMPS

The etiology of muscle cramps is not well understood, with possiblecausative factors including electrolyte imbalances, deficiencies incalcium and magnesium, and dehydration. Cramps can also occursecondary to direct impacts. Cramps may involve moderate to severe

muscle spasms, with proportional levels of accompanying pain


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COMMON MUSCLE INJURIES Delayed-Onset Muscle Soreness

Muscle soreness often occurs after some period of time followingunaccustomed exercise. Delayed-onset muscle soreness (DOMS) arises2472 hours after participation in a long or strenuous bout of exerciseand is characterized by pain, swelling. Micro tearing of the muscle tissuis involved, with symptoms of pain, stiffness, and restricted range ofmotion.

Compartment Syndrome

Hemorrhage or edema within a muscle compartment can result from

injury or excessive muscular exertion. Pressure increases within thecompartment, and severe damage to the neural and vascular structureswithin the compartment follows in the absence of pressure release.Swelling, discoloration, diminished distal pulse, loss of sensation, and

loss of motor function are all progressively apparent symptoms.


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biomechanics of muscles