1

Skeletal Muscles and Functions

Huei-Ming Chai, PT, Ph.D.

School of Physical Therapy

National Taiwan University

Classification of Muscles

• striated muscles – skeletal muscles: voluntary contraction

– cardiac muscles

• non-striated (smooth) muscles

2

Skeletal Muscle Statistics

• the most abundant tissue in the body,

accounting for 40-45% BW

• > 430 skeletal muscles

• Most movements are completed by < 80 pairs

of skeletal muscles

Skeletal Muscle and Function

Structure of skeletal muscle

• Muscle contraction

• Muscle coordination

3

Properties of Skeletal Muscle

• Muscle fiber – extensibility

– elasticity

– contractility

• tendon or aponeurosis – viscoelasticity

– non-contractility

Organization of Skeletal Muscle

muscle fasciculus

muscle epimysium

perimysium

muscle fiber endomysium

muscle fibril

4

Muscle Fiber

• a long cylindrical cell with hundreds of nuclei – 10-100 m in diameter

– 1-30 cm in length

• Contractile component: myofabril

• Non-contractile component: endomyosium

Organization of Muscle Fiber

muscle fibril

endomysium muscle fiber

myosin filament actin filament

sarcomere

cross section

5

Contractile Component

-- Sarcomere

actin myosin crossbridge

Z line A band

I band H band

Types of Muscle Fibers

• slow twitch fiber (Type I) – red in color

– slow to peak when contracted

– fatigue resistant

• fast twitch fiber (Type IIA) – white in color

– fast to peak when contracted

– easy fatigue

• intermediate (Type IIB)

Smith, p.88

6

Fiber Architecture

• parallel fiber arrangement： parallel to the longitudinal axis of the muscle – longitudinal： sartorius – quadrate or quadralateral： rhomboid – triangular or fan-shaped： pectoralis major – fusiform or spindle-shaped： biceps brachii

• pennate fiber arrangement： at an angle to the longitudinal axis of the muscle, – unipenniform： extnesor digitorum longous – bipenniform： flexor hallucis longus – multipenniform： middle deltoid

Effect of Pennation

• pennation angle effective force

transmitted to the tendon

• tension in the muscle fibers pennation

angle

• pennate arrangement:

to allow packing of

more fibers

given the same space.

7

Skeletal Muscle and Function

• Structure of skeletal muscle

Muscle contraction

• Muscle coordination

Functions of Skeletal Muscle

• To move the body limb by creating motion

• To provide strength by generating active force

• To protect joints by absorbing shock

• specific functions of connective tissues within muscle – To provide gross structure to muscle

– To generate passive tension against stretch

– To transmit force to the bone and across the joint

8

Sliding Filament Mechanism

• AF Huxley & HE Huxley, 1964

• active shortening of sacromere, resulting from

the relative movement of actin and myosin

filaments with retaining its original length

• force of contraction is developed by the

crossbridges of myosin

Movement of Cross Bridges

shortening Lengths of myosin and actin

keep the same

9

Types Based on Changes in Length

concentric

(shortening)

resting

eccentric

(lengthening)

isometric

(static)

Abdominal Muscle Contraction

• concentric contraction – to create trunk flexion (resisting gravity)

• eccentric contraction – to control trunk extension (checking gravity)

10

Triceps Brachii Action

• downward motion

– elbow flexion

– elbow extensors (antagonist) eccentric contraction

• upward motion

– elbow extension

– elbow extensors (agonist)

concentric contraction

Abdominal Muscle Actions

concentric

eccentric

direction of motion muscle length

concentric gravity-resisted shortening

eccentric gravity-assisted lengthening

11

Elbow Flexion at 90 of Shoulder ABD

elbow flexor, concentric elbow extensor, eccentric

elbow flexor, eccentric elbow extensor, concentric

open

kinematic

chain

closed

kinematic

chain

Quadriceps Actions

motion? gravity? muscle contraction? muscle length?

12

What if ….

shoulder extensor

concentric contraction

to create force

Shoulder Extension

13

Types Based on

Tension Development

• isotonic contraction

• isometric contraction

• isokinetic contraction

Isotonic Contraction

• iso = equal； tonus = tension

• defined by muscle physiologists as a kind of muscle

contraction that develops constant tension throughout

the whole muscle excursion as isotonic contraction – seldom seen in the living body

– clinically refer to a muscle contraction that causes a joint to

move through some range of motion

• Even though the resistance remains the same, the

tension generated by the muscle is not equal tension

because – moment arm to the joint axis changing throughout the motion

– resistance with respect to the gravity changing throughout the

motion

14

Slight Squatting

• quadriceps action

• calf action

• shoulder flexor

• elbow flexor

Isokinetic Contraction

• iso = equal； kinetos = move

• first introduced by Hislop and Perrine in 1967

• definition: one kind of muscle contraction that

occurs when the rate of movement is constant

• not occur in the living body without using

special machine (isokinetic dynamometer)

• equal motion speed with

accommodating resistance

15

Isokinetic Testing

Isokinetic Testing System

• Cybex: torque

• Kin-Com: force

Kin-Com:

load cell

angle

am

pli

tud

e ext.

flex.

Cybex:

dynamometer

16

Isokinetic Contraction

joint angle

joint moment

Isokinetic

F

mg

M

Isotonic

dynamometer

Comparison of Muscle Contraction

• isotonic contraction – varying tension

– varying length

– varying speed

• Isometric contraction – varying tension

– equal length

– zero speed

• isokinetic contraction – accommodating resistance (various tension)

– varying length

– equal speed

17

Tension Developed by

A Single Muscle Fiber Contraction

tensio

n

length

active

tension

resting

length

Mechanical Model of

Musculotendinous Unit

• Keele, Neil, Joels, 1982

parallel elastic component

contractile component

series

elastic

component

18

Length-Tension Curve

-- maximum isometric contraction

tensio

n

length

total tension

active

tension

passive

tension

resting

length

Force-Velocity Curve

Hill’s model

0

contraction velocity

forc

e

eccentric concentric

isometric

19

Skeletal Muscle and Function

• Structure of skeletal muscle

• Muscle contraction

Muscle coordination

Muscle Activities During Motion

• focal muscle – agonist (prime mover)

– antagonist

– synergist

• stabilizer

• neutralizer

• postural muscle – anticipatory postural adjustment (APA)

20

Agonist

• the principal muscle that produces a joint

motion or maintains a static posture

• can be concentric, isometric, or eccentric

Antagonist

• 拮抗肌

• the muscle that contracts in the opposite

direction of the agonist

• passively elongates or shortens to allow

motion acted by agonist

21

Synergist

• Syn = together; ergon = work

• the muscle that contracts together with the

agonist – stabilizer: to stabilize the proximal component of

the joint involved

– neutralizer: to rule out unwanted motions

Stabilizer

teres minor

• scapular muscles stabilize the scapula

deltoid can elevate the arm

teres minor can rotate the arm externally

22

Neutralizer

scapula

adduction

upper trapezius

lower trapezius neutralize

Cocontraction

co-contraction joint approximation

co-contraction: agonist and antagonist

contract simultaneously

23

Single- vs. Multi-Joint Muscle

• single-joint muscle: a muscle that cross one

joint only, e.g. the brachialis, the short head

of the biceps brachii

• two-joint muscle: a muscle that cross two

joints, e.g. the long-head of the biceps brachii,

the grastrocnemius, etc.

• multi-joint muscle: a muscle that cross more

than one joint e.g. the long finger flexors, the

long finger extensors, etc.

Action of Two-Joint Muscle

• active insufficiency – unable to reach the contraction force because of

the limit of muscle length

– e.g. make a fist as wrist extended vs. that as wrist flexed

• passive insufficiency – unable to reach full range of motion because of

the limit of muscle length

– e.g. open the hand as wrist extended vs. that wrist flexed

24

Active Insufficiency

• unable to reach the contraction force because

of the limit of muscle length

• examples：

– Making a fist with the wrist extended is stronger

than that with the wrist flexed

– the strength of the elbow flexor decreases as the

shoulder joint is more flexed

Mechanism of Active Insufficiency

• the contractile tension of the agonist is

markedly weak when a multi-joint muscle is

attempt to contract at a shortened position

25

Passive Insufficiency

• unable to reach full range of motion because

of the limit of muscle length

• examples：

– automatically open the hand as wrist flexed

– difficult to reach the toes with the knee extended

as compared to that with the knee flexed

• Even though the agonist may contract

strongly, motion may be limited because of

the lack of excursion of the antagonist

Practice of Two Joint Muscles

• biceps brachii

• hamstring

• gastrocnemius