ENT 214
Biomechanics
BIOMECHANICS of
HUMAN SKELETAL
MUSCLE
Mohd Yusof Baharuddin
BBiomedEng(UM)
MBiomedEng(Melbourne)
myusof@unimap.edu.my
Picture from MisterBisson Flickr pages
Muscle Tissue
Behavioral Properties
Extensibility
Elasticity
Irritability
Ability
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)


2.
Series elastic component (SEC)


3.
Muscle membrane
Supply resistance when muscle is passively stretched
Tendons
Act as spring to store elastic energy
Contractile component (CC)

Muscle property enabling tension by stimulated
muscle fibers
Muscle Elastic
Behavior Model
Muscle Tissue
Behavioral Properties
Irritability
 ability
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 shape
 single muscle cell surrounded by a
membrane called the sarcolemma
and containing specialized
cytoplasm called sarcoplasm
Skeletal Muscle
Silverthorn, Human Physiology,
Benjamin-Cummings, ISBN 0321-22687-9, Fig. 12-3, p. 392
Skeletal muscle
Silverthorn, Human Physiology,
Benjamin-Cummings, ISBN 0321-22687-9, 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
Peak tension is
typically greater for
FT than for ST fibers.
FT
ST
Twitch Tension
Fast twitch (FT) fibers
both reach peak
tension and relax
more quickly than
slow twitch (ST)
fibers.
Time
Skeletal Muscle Fiber
Characteristics
CHARACTERISTIC
Contraction Speed
Type I
Slow-Twitch
Oxidative
(SO)
slow
TYPE IIA
Fast-Twitch
Oxidative
Glycolytic
(FOG)
fast
Fatigue rate
slow
intermediate
fast
Diameter
small
intermediate
large
ATPase concentration low
high
high
Mitochondrial
concentration
Glycolytic enzyme
concentration
high
high
low
low
intermediate
high
Type IIB
Fast-Twitch
Glycolytic
(FG)
fast
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
Ff
α
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
α
Ff
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)
The force-velocity
relationship for
muscle tissue:
When resistance
(force) is negligible,
muscle contracts
with maximal
velocity.
Force
Factors Affecting Muscular Force
Generation
(Low resistance,
high contraction
velocity)
Velocity
Force – Velocity Relationship
isometric
maximum
Force
The force-velocity
relationship for
muscle tissue: As
the load increases,
concentric
contraction velocity
slows to zero at
isometric
maximum.
Velocity
Length – Tension Relationship
Total
Tension
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.
Active
Tension
Passive
Tension
50
100
150
Length (% of resting length)
Force – Time Relationship
What is electromechanical delay (EMD)?
Myoelectric activity
Force
Stimulus
Electromechanical delay
 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
Muscular Strength
the amount of torque a muscle group can
generate at a joint
How do we measure muscular
strength?
Ft
Ft
The component of muscle force that
produces torque (Ft) at the joint is directed
perpendicular to the attached bone.
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
A
B
C
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 of
the muscle is lessened because more and more of the force is pulling
the radius toward or away from the elbow rather than contributing
to forearm rotation.
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
Power
Force
Force-Velocity
Power-Velocity
Velocity
The general shapes of the force-velocity and
power-velocity curves for skeletal muscle.
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
Normal body temperature
Elevated body temperature
Force
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
Summary
1.
2.
3.
4.
5.
Basic behavioral properties of the muscle unit
Relationships of fiber types and architecture to
muscle function
Skeletal muscles function to produce coordinated
movement of the human body
Effects of the force-velocity and length-tension
relationships and EMD on muscle function
Muscular strength, power, and endurance