Kinesiology, NTUPT

The Musculature

Structure of Skeletal Muscles
Skeletal Muscle Contraciton
Muscle Coordination

Objectives： After studying this topic, the students will be able to

To understand the classification of the muscles and their characteristics
To describe functions of a skeletal muscle
To explain the mechanism of muscle contraction and coordination
To distinguish the differences among agonists, antagonists, and synergists
To distinguish the action of multi-joint muscles from that of single-joint motion

Brown DA (2002). Muscle: The ultimate forc generator in the body. In Neumann DA: Kinesiology of the Musculoskeletal System: Foundations for Physical Rehabilitation. Philadelphia: Mosby. Chapter 3, pp. 41-55
Smith LK et al., 1996. Chapter 4
Lorenz T et al. (2001). Biomechanics of skeletal muscles. In Nordin M & Frankel VH: Basic Biomechanics of the Musculoskeletal System. Philadelphia: Lippincott Williams & Wilkins. Chapter 6, pp.148-176

Structure of Skeletal Muscles

Types of Muscles

	striated feature	voluntary contraction	examples
skeletal muscles	yes	yes	extremity muscles
cardiac muscles	yes	no	heart
smooth muscles	no	no	internal organs

Some statistics about skeletal muscles

the most abundant tissue in the body, accounting for 40-45% of total BW

> 430 skeletal muscles

Most movements are completed by < 80 pairs of skeletal muscles

Properties of skeletal muscles

muscle fiber

extensibility

elasticity

contractility

tendon or aponeurosis

viscoelasticity

non-contractility

Organization of skeletal muscles

epimyosium muscle： contractile elements that are embedded within a network of connective tissues perimyosium fasciculus endomyosium sacrolemma muscle fiber myofibril myosin actin

Muscle fibers

a long cylindrical cell with hundreds of nuclei

10-100 mm in diameter

1-30 cm in length

contractile component: myofabril

non-contractile component: endomyosium

types

slow twitch fiber (type I)

red in color because of abundant blood supply

slower to the peak when contracted

fatigue resistant

fast twitch fiber (type IIA)

pale in color because of less blood supply

rapidly to the peak when contracted

easy fatigue

intermediate fiber (type IIB)

Skeletal muscle 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 fibers of the deltoid

Note： Lieber RL(1992) divided skeletal muscle architecture into 3 general types
longitudinal architecture： biceps brachii
unipennate architecture： vastus lateralis
multipennate architecture： gluteus medius

effect of the angle of pennation

the greater the angle of pennation, the smaller the amount of effective force transmitted to the tendon

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

The pennate arrangement will allow the packing of more fibers given the same space.

Skeletal Muscle Contraction

Functions of skeletal muscles

To move the body limb by creating motion

To provide strength by generating active force

To protect joints by absorbing shock

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

Definition

The act that muscle fibers generate tension which leads to the muscle become shortening, remain the same length, or lengthening.

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

Types based on changes in length

concentric contraction (shortening contraction)

definition： muscle contraction with the length of the entire muscle shortened

In daily activities, if the gravity is the only external force acting on the body, the agonist muscle contracts concentrically during gravity-resisted motions

examples：

the abdominal muscles contract concentrically when the body sits up during curl-up (仰臥起坐)

the triceps brachii muscle contracts concentrically when the body rises up during press up (伏地挺身)

the quadriceps femoris contracts concentrically during upstairs

Have you arm raise to the shoulder level and try to flex the elbow to 90 degrees. Which of the muscles in the arm responsible for this motion? What type of the contraction? If you extend the elbow, what muscle contracts and which type of contraction is?
Similar to #1, if the elbow is flexed from 90 degrees to 130 degrees. Which of the muscles in the arm responsible for this motion? What type of the contraction? If you extend the elbow from 130 degrees to 90 degrees, what muscle contracts and which type of contraction is?
How to have your triceps brachii muscle contract eccentrically?
If you lie down from sitting without using the abdominal muscles, what would happens?

isometric contraction (static contraction)

isos = equal； metron = measure

definition： muscle contraction with muscle length kept no change

The joint angle remains the same when an isometric strength is developed.

There is no motion existed during isometric contraction

eccentric contraction (lengthening contraction)

definition： muscle contraction with the length of the entire muscle lengthened

In daily activities, if the gravity is the only external force acting on the body, the antagonist muscle contracts eccentrically during gravity-assisted motions

examples：

the abdominal muscles contract eccentrically when the body lies down during curl-up (仰臥起坐)

the quadriceps contracts eccentrically during downstairs

Types of muscle contraction, based on development of tension

isotonic

iso = equal； tonus = tension

Muscle physiologists defined a kind of muscle contraction that develops constant tension throughout the whole muscle excursion as isotonic contraction; however, it is seldom seen in the living body

Clinicians use isotonic contraction commonly and 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

the moment arm to the joint axis is changing throughout the motion
the resistance with respect to the gravity is changing throughout the motion

isometric

equal muscle length and same joint angle

zero motion speed with varying resistance

isokinetic

iso = equal； kinetos = move

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)

first introduced by Hislop and Perrine in 1967

equal motion speed with accommodating resistance

comparison of different types of muscle contraction

	Tension	Length	Speed
isotonic	varying	varying	varying
isometric	varying	equal	zero
isokinetic	accomodating resistance (varying)	varying	constant

Length-Tension Relationship

mechanical model of muscle fiber contractile component： actin and myosin crossbridges structures parallel elastic component： muscle connective tissue e.g. epimyosium, perimyosium, or endomyosium series elastic component： connective tissues within the tendon
	tension generated by active contraction resting length of a sacromere： the length that allows the greatest number of cross-bridge attachments and the greatest potential active force active length-tension curve： an inverted U-shape with its peak at the resting length tension generated by passive stretch developed when series and parallel elastic components are stretched passive length-tension curve： the tissue is slack before stretched and then the tension builds as an exponential function
total length tension curve of muscle at shortened lengths： active contraction dominates force generation just beyond its resting length： passive tension begins to contribute and active tension is compromised at more elongated lengths： passive tension accounts for most of the total force

Force-Velocity Relationship

force decreased as velocity increased during concentric contraction

force increased as velocity decreased during eccentric contraction

force = 0 during isometric contraction

Muscle Coordination

Muscle activities during motion

focal muscle

agonist or prime mover

agon = contest

the principal muscle that produces a joint motion or maintains a static posture

can be concentric, isometric, or eccentric

antagonist

anti = against; agon = contest

the muscle that contracts in the opposite direction of the agonist

passively elongates or shortens to allow motion acted by agonist

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

postural muscle

anticipatory postural adjustment (APA)

Co-contraction

agonists and antagonists contract simultaneously

leading to joint approximation

Actions of multi-joint muscles

single-joint muscle 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.

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

the contractile tension of the agonist is markedly weak when a multi-joint muscle is attempt to contract at a shortened position, i.e. the muscle contracts at the lower portion of its length-tension curve

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

NOTE： The totally insufficient grip strength produced with the wrist fully flexed is due to the combination of active insufficiency of the long finger flexors and passive insufficiency of the long finger extensors

Have your friend keep his/her arm by the side and try to flex the elbow to 90 degrees. Feel the maximum isometric strength he/she can generate. Then, have his/her upper raise to the shoulder level and try to flex the elbow to 90 degrees. Feel the maximum isometric strength again. See which one is stronger. Is this an example of active insufficiency or passive insufficiency? Which one of the elbow flexor is responsible for this phenomenon?
Consider the gastrocnemius that is a two-joint muscle at lower leg. Please give an example that the gastronemius demonstrates active insufficiency and another example that it presents passive insufficiency.
Think the muscles you have already learned from the Anatomy class. Which one is the single joint muscle? Which one is the two-joint muscle? Multi-joint muscle?

Established on 09/30/2002 and Last Updated 03/31/2005