Chapter 6
The Biomechanics
of Human Skeletal
Muscle
Behavioral Properties of the
Musculotendinous Unit
1)
extensibility: ability to be
stretched or to increase in
length
2)
elasticity: ability to return to
normal resting length following a
stretch
6-2
Behavioral Properties of the
Musculotendinous Unit
Components of elasticity:
•
parallel elastic component passive elasticity derived
from muscle membranes
•
series elastic component passive elasticity derived from
tendons when a tensed
muscle is stretched
6-3
Behavioral Properties of the
Musculotendinous Unit
Parallel Elastic
Component
Contractile
Component
Series Elastic
Component
From a mechanical perspective, the musculotendinous unit behaves as a contractile component
(muscle fibers) in parallel with one elastic component (muscle membranes) and in series with another
elastic component (tendons).
6-4
Behavioral Properties of the
Musculotendinous Unit
What is the stretch-shortening cycle?
•
eccentric contraction (in which
the muscle is actively
stretched) followed
immediately by concentric
contraction
• Can you think of examples?
6-5
Behavioral Properties of the
Musculotendinous Unit
3)
irritability: ability to respond
to a stimulus
4)
ability to develop tension: the
contractile component of muscle
function
6-6
Properties of Muscle Tissue
• Irritability
ability to respond to a stimulus
• Extensibility
ability to be stretched or to increase in length
• Elasticity
ability to return to normal resting length following a stretch
• Contractility
ability to develop tension: the contractile component of muscle function
6-7
Structural Organization of Skeletal
Muscle
What is a muscle fiber?
(single muscle cell surrounded by a
membrane called the sarcolemma
and containing specialized
cytoplasm called sarcoplasm)
6-8
Structural Organization of Skeletal
Muscle
Sarcolemma
6-9
R E V I E W: O R G A N I Z AT I O N A L
OF
MUSCLE
S K E LE TA L
LEVELS
SCL
FA S C I C L E
USCLE C E L L
USCLE FIBER
YOFIBRIL
MYOFILAMENTS
6-10
R E V I E W : O R G A N I Z AT I O N A L
O F S K E L E TA L MLJSCLE
LEVELS
MUSCLE
FA S C I C L E
I
MUSCLE CELL
(M U S C L E F I B E R )
MYOFIBRIL
MYOFILAMENT$
6-11
Structural Organization of Skeletal
Muscle
What do we know about 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
6-12
Example 1: Muscle can generate approximately 90 N of force per square
centimeter of cross-sectional area. If a biceps brachii has a cross-sectional
area of 10 cm2, how much force can it exert?
Solution: 90 N (10 cm2) = 900 N
6-13
A
M
lin/. linel
Z
B
II band t - 4 -
A band
II band
A band
I
Myosin (thick)
filaments
c
Actin (thin)
filaments
6-14
Structural Organization of Skeletal
Muscle
Sarcomere
The sarcomere is the basic
structural unit of the muscle
fiber. The alternating dark and
light bands give muscle its
striated appearance. The A
bands contain thick, rough
myosin filaments surrounded
by six thin, smooth actin
filaments. The I bands contain
only thin actin filaments.
6-15
Skeletal Muscle Fiber Model - Myofibrils
6-16
Structural Organization of Skeletal
Muscle
What is a motor unit?
• single motor neuron and all fibers
it innervates
• considered the functional unit of
the neuromuscular system
6-17
Physiology : neuromuscular junction - motor unit
6-18
Structural Organization of Skeletal
Muscle
FT
ST
Twitch Tension
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.)
Time
6-19
Skeletal Muscle Fiber Characteristics
TYPE IIA
Fast-Twitch
Oxidative
Glycolytic
(FOG)
fast
CHARACTERISTIC
Contraction Speed
Type I
Slow-Twitch
Oxidative
(SO)
slow
Fatigue rate
slow
intermediate
fast
Diameter
small
intermediate
large
ATPase concentration low
Mitochondrial
high
concentration
high
high
high
low
Glycolytic enzyme
concentration
intermediate
high
low
Type IIB
Fast-Twitch
Glycolytic
(FG)
fast
6-20
Structural Organization of Skeletal
Muscle
How are muscle fibers organized?
•
parallel fiber arrangement: fibers
are roughly parallel to the
longitudinal axis of the muscle;
examples are?
•
pennate fiber arrangement: short
fibers attach to one or more tendons
within the muscle; examples?
6-21
Structural Organization of Skeletal
Muscle
Parallel fiber arrangements
Pennate fiber arrangements
6-22
Structural Organization of Skeletal
Muscle
The angle of
pennation increases
as tension
progressively
increases in the
muscle fibers.
Relaxed
With tension
development
6-23
Skeletal Muscle Function
How are motor units (MUs) recruited?
•
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
6-24
Skeletal Muscle Function
What terms are used to describe muscle
contractions based on change in muscle
length?
• concentric: involving shortening
• eccentric: involving lengthening
• isometric: involving no change
6-25
Skeletal Muscle Function
What roles are assumed by muscles?
•
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
•Synegist: Assist prime mover and refine
motion
6-26
Tnceps
Steeps
Brachn
Anconeus
Brachtahs
Extensor Carp1
.....,._
Brachtoradtalts
Radlalts Lonaus
6-27
Tnceps
Steeps
Brachn
Anconeus
Extensor Carpt
Radtalts Lonaus
Adductor muscles of the hip
-----+- Brachtoradtalts
6-28
The difference between antagonist and agonist muscles is that they work in the
opposite direction to complete an action. Agonist muscles react in response to
voluntary or involuntary stimulus and create the movement necessary to complete a
task. Antagonist muscles act against the agonist muscle and help to move the body
part back in place after the action is completed.
Muscles always work in pairs. This is because they can only pull on bones. As one
muscle pulls, the other relaxes
6-29
bleeps
(contracted)
radius
triceps
(relaxed)
., _ o r i g i n
bleeps
(relaxed)
triceps
(contracted)
ulna
Figure 6
6-30
pectorals
(p cs)
rectus
bdominus
(abs)
- - - quadriceps
(quads)
gastrocnem us ,-
gluteals
(glut )
- "'"
- - - - sol us
6-31
Hamstring : Antagonist
Quadriceps: Agonist
EXTENSION
Hamstring : Agonist
FLEXION
Quadriceps: Antagonist
6-32
Anterior
Deltoid
Posterior
Deltoid
6-33
The tibialis anterior muscle is the most medial
muscle of the anterior compartment of the leg.
The tibialis anterior is responsible for dorsiflexing
and inverting the foot.
Antagonists are plantar-flexors of the posterior
compartment such as soleus and gastrocnemius.
In humans, the gastrocnemius muscle is a very powerful superficial bipennate muscle
that is in the back part of the lower leg. It runs from its two heads just above the knee to
the heel, and is involved in standing, walking, running and jumping. Along with the
soleus muscle it forms the calf muscle. Its function is plantar flexing the foot at the ankle
joint and flexing the leg at the knee joint.
6-34
Wrist extensors
Lateral epicondyle
t
I
Wrist flexors
Medial epicondyle
IM G 1000
6-35
Skeletal Muscle Function
What are disadvantages associated with
muscles that cross more than one joint?
•
active insufficiency: failure to
produce force when slack
• passive insufficiency: restriction of joint
range of motion when fully stretched
6-36
Skeletal Muscle Function
active insufficiency: failure to
produce force when muscles are
slack (decreased ability to form a
fist with the wrist in flexion)
6-37
Skeletal Muscle Function
passive insufficiency: restriction of
joint range of motion when muscles
are fully stretched (decreased ROM
for wrist extension with the fingers
extended)
6-38
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
6-39
Factors Affecting Muscular Force
Generation
isometric maximum
Force
The force-velocity
relationship for
muscle tissue: As
the load increases,
concentric
contraction velocity
slows to zero at
isometric maximum.
Velocity
6-40
Factors Affecting Muscular Force
Generation
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)
6-41
Factors Affecting Muscular Force
Generation
What is electromechanical delay?
Myoelectric activity
Force
Stimulus
Electromechanical delay
(time between arrival of a neural stimulus
and tension development by the muscle)
6-42
Muscular Strength, Power and
Endurance
How do we measure muscular strength?
(the amount of torque a muscle group
can generate at a joint)
6-43
Muscular Strength, Power and
Endurance
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.
6-44
Muscular Strength, Power and
Endurance
What 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
6-45
Muscular Strength, Power and
Endurance
What 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
6-46
Skeletal Muscle Function
Torque produced
by a muscle (Tm) at
the joint center of
rotation is the
product of muscle
force (Fm) and
muscle moment
arm (d ).
6-47
Muscular Strength, Power and
Endurance
A
B
C
The mechanical advantage of the biceps brachii 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.
6-48
Muscular Strength, Power and
Endurance
What is muscular 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
6-49
Muscular Strength, Power and
Endurance
Power
Force
Force-Velocity
Power-Velocity
Velocity
The general shapes of the force-velocity and
power-velocity curves for skeletal muscle.
6-50
Muscular Strength, Power and
Endurance
What is muscular endurance?
•
the ability of muscle to exert
tension over a period of time
• the opposite of muscle fatigability
6-51
Muscular Strength, Power and
Endurance
What is the effect of muscle temperature
on (warm up) ?
(the speeds of nerve and muscle functions
increase)
6-52
Muscular Strength, Power and
Endurance
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
6-53