BIOMECHANICS of HUMAN SKELETAL

MUSCLE

Mohd Yusof BaharuddinBBiomedEng(UM)

MBiomedEng(Melbourne)

myusof@unimap.edu.my

ENT 214 Biomechanics

Picture from MisterBisson Flickr pages

Muscle Tissue Behavioral Properties

ExtensibilityElasticityIrritabilityAbility to develop tension

Extensibility & Elasticity

What is extensibility? Ability to stretch or to increase in length

How about elasticity? Ability to return to normal length after stretch

Figure from http://www.flashplayer.com/forum/showthread.php?t=20485

Elastic Behaviour

1. Parallel elastic component (PEC) Muscle membrane Supply resistance when muscle is passively stretched

2. Series elastic component (SEC) Tendons Act as spring to store elastic energy

3. Contractile component (CC) Muscle property enabling tension by stimulated

muscle fibers

Muscle Elastic Behavior Model

Muscle Tissue Behavioral Properties

Irritabilityability to respond to a stimulus

Ability to develop tension the contractile component of muscle

function

Structural Organization of Skeletal Muscle

What is a muscle fiber?Because of threadlike shapesingle muscle cell surrounded by a

membrane called the sarcolemma and containing specialized cytoplasm called sarcoplasm

Skeletal Muscle

Silverthorn, Human Physiology,Benjamin-Cummings, ISBN 0-321-22687-9, Fig. 12-3, p. 392

Skeletal muscle

Silverthorn, Human Physiology,Benjamin-Cummings, ISBN 0-321-22687-9, Fig. 12-1a, p. 390

Image taken from http://courses.cm.utexas.edu/jrobertus/ch339k/overheads-1.htm

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

Muscle Fiber (cont’d)

Muscle Fiber (cont’d)

Muscle Fiber (cont’d)

Myosin & Actin

Motor unit

single motor neuron and all fibers it innervates

considered the functional unit of the neuromuscular system

There are 2 types Fast twitch (FT) Slow twitch (ST)

Structural Organization of Skeletal Muscle

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

Skeletal Muscle Fiber Characteristics

TYPE IIAType I Fast-Twitch Type IIB

Slow-Twitch Oxidative Fast-Twitch Oxidative 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

Fiber architecture

There are 2 types of fiber architecture parallel fiber arrangement

fibers are roughly parallel to the longitudinal axis of the muscle

example: sartorius, rectus abdominis, biseps brachii

pennate fiber arrangement short fibers attach to one or more tendons within the

muscle example: tibialis posterior, rectus femoris, deltoid

Fiber architecture (cont’d)

parallel fiber arrangement Muscle become shorten due to fiber shortening

pennate fiber arrangement When muscle shorten, they rotate about their

tendon attachment This will increase the angle of pennation Greater angle of pennation will induced smaller

amount of effective force

Sample Problem 1

Ft

Ffα

How much force is exerted by tendon of pennate muscle when tension in fiber is 100 N, given that the angle of pennation is 60 degree.

Solution

Ft

Ff

α

Ft = 100 cos 60

= 100 (0.5)

= 50

Recruitment of Motor Units (MU)

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

Terms used to describe muscle contractions

concentric: involving shortening eccentric: involving lengthening isometric: involving no change

Muscles Roles

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

Two Joint and Multijoint Muscles

active insufficiency: failure to produce force when slack

passive insufficiency: restriction of joint range

of motion when fully stretched

Active Insufficiency

Failure to produce force when muscles are slack (decreased ability to form a fist with the wrist in flexion)

Passive insufficiency

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

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)

Force – Velocity Relationship

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

Length – Tension Relationship

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.

Te

nsio

n

Length (% of resting length)50 100 150

Active Tension

Passive Tension

Total Tension

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.

Te

nsio

n

Length (% of resting length)50 100 150

Active Tension

Passive Tension

Total Tension

Force – Time Relationship

What is electromechanical delay (EMD)?

EMD is the time between arrival of a neural stimulus and tension development by the muscle

Time needed for muscles reached maximum isometric is 0.2 - 1 second after EMD

Myoelectric activity

Force

Electromechanical delayStimulus

Muscular Strength

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

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

Sample Problem 2

α

How much tork is produced at the elbow by biceps brachii inserting at an angle 60 degree on the radius when tension in the muscle is 400 N?

Solution

1. Find Fp which is perpendicular to bone

2. Calculate tork

Tm = Fp x d

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

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

Effects for differences angle

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

Muscle 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

Muscular Power

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

For

ce

Velocity

Pow

er

Power-VelocityForce-Velocity

Muscular Endurance

the ability of muscle to exert tension over a

period of time

the opposite of muscle fatigability

Effect of muscle temperature(Warm up)

the speeds of nerve and muscle functions increase

Effect of muscle temperature(Warm up) cont’d

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

Summary

1. Basic behavioral properties of the muscle unit

2. Relationships of fiber types and architecture to muscle function

3. Skeletal muscles function to produce coordinated movement of the human body

4. Effects of the force-velocity and length-tension relationships and EMD on muscle function

5. Muscular strength, power, and endurance