Muscle Fibre

8/13/2019 Muscle Fibre

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Human Muscle Fiber Types

Classification

andProperties


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Muscle Fiber Classification

Contractile

Properties

slow

fast

Metabolic

Properties

oxidative

glycolytic

Color

Properties

red

white

Staining

Properties

enzymes

pH

Classification

Basis


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Contractile Properties


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Slow twitch (I) fibers innervated by alpha 2

motor neurons, smaller of the two motor

neurons

Fast twitch (II) fibers innervated by alpha 1

motor neurons, larger of the two motor

neurons Fast twitch (II) fibers have higher excitation

threshold and faster conduction velocity


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Motor Unit Recruitment

Motor neurons recruited

in order of size: Size

Principle

Smallest alpha motorneurons, 2, which

belong to slow twitch

recruited first

Largest alpha motor

neurons, 1, which

belong to fast twitch

recruited last


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Motor Unit Recruitment

Slow twitch with lowthreshold nerves (2)recruited during

light-to-moderateeffort.

Sustainedsubmaximal jogging,

cycling, lifting alight weight at aslow speed.

Fast twitch withhigher threshold

nerves (

1) activatedas forcerequirementsincrease.

Sprint running orswimming or cyclistascends hill.


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Metabolic Properties

All muscle fibers can produce energy both

aerobically and anaerobically

One or the other may predominate or productionmay be balanced

SO (I) fibers rely mainly on oxidative pathways

FG (IIb) fibers rely mainly on glycolytic pathways

FOG (IIa) possess characteristics like SO and FG


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Metabolic Properties

FOG (IIa) and FG

(IIb) have higher

glycogen storesthan SO (I).

SO (I) has more

triglyceride than

either FG or FOG.


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Color Properties

Myoglobin contains heme (Fe)which carries O2 in musclemakes it RED.

Myoglobin cooked turns tan

color. Dark turkey meat (thigh) was

once oxidative.

WHITEturkey meat

(pectoralis) had no myoglobin,was not oxidative.

So turkeys walk all day andflap occasionally but rarelyfly.


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Color Properties

Pheasants fly far to survive.

Pheasant pectoralis muscle has

myoglobinwhich is red or dark.


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Staining Properties

Slow twitch (type I) have myosin isoforms

with low ATPase activity.

Fast twitch (type II) have myosin isoformswith high ATPase activity that promotes

rapid breakdown of ATP for energy of high-

speed muscle shortening.


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Muscle Fiber Types

Type I

Red

Type IIa

Red

Type IIb

White

Diameter

(neuron & fiber)

Medium Large Large

Force Capacity Low (5g) Moderate (20 g) High (50 g)

Myosin ATPase

Activity

Low High High

Myoglobin

Content

High High Low

Metabolic

Properties

Oxidative Oxidative/

Glycolytic

Glycolytic

Fatigue

Resistance

High Intermediate Low


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Distribution of Fiber Types

All muscle composed

of ST & FT fibers

Distribution variesfrom muscle to muscle

within an individual

Most individuals

possess between 45

and 55% ST


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Vastus lateralis on

average 52% SO, 33%

FOG, 14% FG. Same

for deltoid, bicepsbrachii.

Soleus may have as

much as 85% SO. Triceps brachii may

have as few as 30%

SO.


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Great variation betweenindividuals

Vastus lateralis of elite

distance runners had 79%

ST, untrained had 58%

Available evidence

indicates that the

distribution of slow andfast twitch fibers is

genetically determined

and not altered by

training


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Illustration References

McArdle, William D., Frank I. Katch, and

Victor L. Katch. 2000. Essentials of

Exercise Physiology2nded. ImageCollection. Lippincott Williams & Wilkins.

Plowman, Sharon A. and Denise L. Smith.

1998.Digital Image Archive for ExercisePhysiology. Allyn & Bacon.

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