Chapter 6 The Biomechanics of Human Skeletal Muscle Basic Biomechanics, 6 th edition By Susan J. Hall, Ph.D. © 2012 The McGraw-Hill Companies, Inc. All

Chapter 6

The Biomechanics of Human Skeletal

MuscleBasic Biomechanics, 6th edition

By Susan J. Hall, Ph.D.

© 2012 The McGraw-Hill Companies, Inc. All rights reserved.McGraw-Hill/Irwin

6-2

Behavioral Properties of the Musculotendinous Unit

1) extensibility: ability to be stretched or to increase in length

2) elasticity: ability to return to normal resting length following a stretch

Basic Biomechanics, 6th editionBy Susan J. Hall, Ph.D.

6-3


Components of elasticity:

• 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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Parallel Elastic Component

Series Elastic ComponentContractile

Component

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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What is the stretch-shortening cycle?

• eccentric contraction (in which the muscle is actively stretched)

followed immediately by concentric contraction• Can you think of examples?



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3) irritability: ability to respond to a stimulus

4) ability to develop tension: the contractile component of muscle function


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Structural Organization of Skeletal Muscle

What is a muscle fiber?

(single muscle cell surrounded by a membrane called the sarcolemma and containing specialized cytoplasm called sarcoplasm)


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Sarcolemma

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What do we know about muscle fibers?• some fibers run the entire length of a

muscle; others are shorter• skeletal muscle fibers grow in both

length and diameter from birth through adulthood

• fiber diameter can be increased through resistance training


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Sarcomere

The sarcomere is the basic structural unit of the muscle

fiber. The alternating dark and light bands give muscle

its striated appearance. The A bands contain thick, rough

myosin filaments surrounded by six thin, smooth actin

filaments. The I bands contain only thin actin filaments.

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What is a motor unit?

• single motor neuron and all fibers it innervates

• considered the functional unit of the neuromuscular system


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Fast twitch (FT) fibers both reach peak tension and

relax more quickly than slow twitch

(ST) fibers. (Peak tension is typically greater for FT than

for ST fibers.)

Tw

itch

Ten

sion

Time

FT ST


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Skeletal Muscle Fiber Characteristics TYPE IIA

Type I Fast-Twitch Type IIB Slow-Twitch Oxidative Fast-Twitch

Oxidative Glycolytic GlycolyticCHARACTERISTIC (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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How are muscle fibers organized?

• parallel fiber arrangement: fibers are roughly parallel to the longitudinal axis of the muscle; examples are?

• pennate fiber arrangement: short fibers attach to one or more tendons within the muscle; examples?


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Parallel fiber arrangements Pennate fiber arrangements

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Relaxed With tension development

The angle of pennation increases as tension progressively increases in the muscle fibers.

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Skeletal Muscle Function

How are motor units (MUs) recruited?

• 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 MUs with higher and higher activation thresholds


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What terms are used to describe muscle contractions based on change in muscle length?

• concentric: involving shortening

• eccentric: involving lengthening

• isometric: involving no change


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What roles are assumed by muscles?

• agonist: acts to cause a movement• antagonist: acts to slow or stop a movement• stabilizer: acts to stabilize a body part against some other force• neutralizer: acts to eliminate an unwanted action produced by an agonist


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What are disadvantages associated with muscles that cross more than one joint?

• active insufficiency: failure to produce force when slack • passive insufficiency: restriction of joint

range of motion when fully stretched


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active insufficiency: failure to produce force when muscles are slack (decreased ability to form a fist with the wrist in flexion)


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passive insufficiency: restriction of joint range of motion when muscles are fully stretched (decreased ROM for wrist extension with the fingers extended)


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Factors Affecting Muscular Force Generation

The force-velocity relationship for muscle tissue:

When resistance (force) is

negligible, muscle contracts with

maximal velocity.

Velocity

For

ce (Low resistance, high contraction velocity)


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The force-velocity relationship for

muscle tissue: As the load increases,

concentric contraction velocity

slows to zero at isometric maximum.

Velocity

For

ce

isometric maximum


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The length-tension relationship: Tension

present in a stretched muscle is the sum of

the active tension provided by the muscle fibers and the passive

tension provided by the tendons and membranes.

Ten

sion

Length (% of resting length)50 100 150

Active Tension

Passive Tension

Total Tension


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What is electromechanical delay?

(time between arrival of a neural stimulus and tension development by the muscle)

Myoelectric activity

Force

Electromechanical delayStimulus


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Muscular Strength, Power and Endurance

How do we measure muscular strength?

(the amount of torque a muscle group can generate at a joint)


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How do we measure muscular strength?

The component of muscle force that produces torque (Ft) at the joint is

directed perpendicular to the attached bone.

Ft Ft


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What factors affect muscular strength?

• tension-generating capability of the muscle tissue, which is in turn affected by:

• muscle cross-sectional area

• training state of muscle


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What factors affect muscular strength?• moment arms of the muscles crossing the joint (mechanical advantage), in turn affected by:• distance between muscle attachment to

bone and joint center• angle of the muscle’s attachment to bone


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Torque produced by a muscle (Tm) at the joint center of

rotation is the product of muscle

force (Fm) and muscle moment

arm (d ).

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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 ofthe muscle is lessened because more and more of the force is pullingthe radius toward or away from the elbow rather than contributing to forearm rotation.

CA B


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What is muscular power?

• the product of muscular force and the velocity of muscle shortening • the rate of torque production at a joint• the product of net torque and angular velocity at a joint


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The general shapes of the force-velocity and power-velocity curves for skeletal muscle.

For

ce

Velocity

Pow

er

Power-VelocityForce-Velocity


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What is muscular endurance?

• the ability of muscle to exert tension over a period of time

• the opposite of muscle fatigability


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

(the speeds of nerve and muscle functions increase)


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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.Velocity

For

ce

Normal body temperature

Elevated body temperature


Documents

Chapter 6 The Biomechanics of Human Skeletal Muscle Basic Biomechanics, 6 th edition By Susan J. Hall, Ph.D. © 2012 The McGraw-Hill Companies, Inc. All