Chapter 2
The Structure and
Function of the
Musculoskeletal
System
Introduction
• OccBio ==> performance & injury
• requires knowledge of the basic structure
and function of the musculoskeletal system.
• Main mechanical functions of MS system:
• 1) support
• 2) protection
• 3) allow for motion.
The six major substructures of the
musculoskeletal system are:
1. Tendons
2. Ligaments
3. Fascia
4. Cartilage
5. Bone
6. Muscle
The six major substructures of the
musculoskeletal system are:
1. Tendons
2. Ligaments
3. Fascia
4. Cartilage
5. Bone
6. Muscle
Soft Tissues
The six major substructures of the
musculoskeletal system are:
1. Tendons
2. Ligaments
3. Fascia
4. Cartilage
5. Bone
6. Muscle
Connective Tissues
The six major substructures of the
musculoskeletal system are:
1. Tendons
2. Ligaments
3. Fascia
4. Cartilage
5. Bone
6. Muscle
Force generating unit
Connective Tissue
1. Tendons
2. Ligaments
3. Fascia
4. Cartilage
5. Bone
6. Muscle
• provide support
• transmit forces
• maintain
structural
integrity
Connective Tissue consists of
• cells: produce extracellular matrix
• extracellular matrix: consistency
determines CT physical properties
• ground substance (viscous fluid)
• fibers
• collagen
• elastic
Specialized Connective Tissue
cells
• Fibroblasts : cells produce matrix of
loose connective tissue (skin, tendons
& ligaments)
• Chondroblasts: cells produce matrix of
cartilage (transform to chondrocytes)
• Osteoblasts: cells produce matrix of
bone (transform to osteocytes)
Fibers in matrix affect
mechanical characteristics of CT
• Elastin fibers
• branched and wavy
• contain protein elastin
• low tensile strength (stretch
a lot) but return to original
length
• Collagen fibers:
most numerous
• long, slightly wavy and
unbranched
• bundle of protein collagen
• provides high tensile
strength (resists stretch)
Fibers in matrix affect
mechanical characteristics of CT
Thick: collagen
Thin: elastin
Present in different proportions in different structures: Why?
Fibers in matrix affect
mechanical characteristics of CT
Present in different orientations in different structures: Why?
Tendons: connect muscle to bone
• transmit muscle force
Tendons: connect muscle to bone
• transmit muscle force
• Parallel collagen arrangement, minimal elastin
• Surrounded by fibrous tissue sheaths
• reduce friction (bony prominence, tunnels) BIC
• sheath inner lining: synovium => synovial fluid
Tendons: connect muscle to bone
• transmit muscle force
• Parallel collagen arrangement, minimal elastin
• Surrounded by fibrous tissue sheaths
• reduce friction (bony prominence, tunnels)
• sheath inner lining: synovium => synovial fluid
• Damage:
• torn
• tendonitis
• tensynovitis
Trigger Finger
Ligament: connects bone to bone
• provides stability to joints
Ligament: connects bone to bone
• provides stability to joints
• non-parallel collagen arrangement, aligned
in direction of imposed stress
Ligament: connects bone to bone
• provides stability to joints
• non-parallel collagen arrangement, aligned
in direction of imposed stress
• Special functions:
• Transverse ligament (retinaculum)
• pulley for tendon
• change direction of force
Ligament: connects bone to bone
• provides stability to joints
• non-parallel collagen arrangement, aligned
in direction of imposed stress
• Special functions:
• Annular ligament
Ligament: connects bone to bone
• provides stability to joints
• non-parallel collagen arrangement, aligned
in direction of imposed stress
• Special functions:
• Damage: sprain
• First, second, third degree sprain
Fascia: separates muscles and organs
• contains more elastin fibers
Fascia: separates muscles and organs
• contains more elastin fibers
• Damage: irritation & swelling
• plantar fasciitis
Fascia: separates muscles and organs
• contains more elastin fibers
• Damage: irritation & swelling
• plantar fasciitis
• shin splints
• iliotibial band syndrome
Fascia: separates muscles and organs
• contains more elastic fibers
• Damage: irritation & swelling
• plantar fasciitis
• shin splints
• iliotibial band syndrome
Cartilage: covers bone at joints
• hyaline cartilage
• bony surfaces at movable articulation
• reduces friction within the joint
Cartilage: covers bone at joints
• fibrocartilage: dense collagen fibers
• intervertebral disks, menisci
Cartilage: covers bone at joints
• fibrocartilage:
• distributes load at the joint
• absorbs shock (?x?)
Cartilage
• Unique aspect: devoid of nerves and
blood vessels.
• nourished by diffusion
• limits thickness
• Influences healing
Cartilage
• Unique aspect: devoid of nerves and
blood vessels.
• nourished by diffusion
• limits thickness
• influences healing
• Damage
• osteoarthritis
• “torn” cartilage
Cartilage
• Unique aspect: devoid of nerves and
blood vessels.
• nourished by diffusion
• limits thickness
• influences healing
• Damage
• osteoarthritis
• “torn” cartilage
Cartilage
• Unique aspect: devoid of nerves and
blood vessels.
• nourished by diffusion
• limits thickness
• influences healing
• Damage
• osteoarthritis
• “torn” cartilage
Stress-strain relationships
•
•
•
•
Stress: applied load
Strain: deformation
Plastic region: permanent disruption
Ultimate strength: complete tear
Bone
• Axial bones
• skull, vertebrae,
sternum, ribs,
pelvis
• Appendicular
bones
• limbs
Functions of bone
• Provide support
• Allow movement
• Protection
• Mineral storage
• 99% of body Ca in bone
• Blood cell formation
Bones
• Long bones
• shaft (diaphysis)
• two expanded ends (epiphyses)
• cortical (compact)
• provide cortex or lining of bone
• cancellous (spongy, trabecular)
Fractures
• Bone loaded to failure
• tensile & compressive load
• stronger compression than tension
Fractures
• Bone loaded to failure
• tensile & compressive load
• stronger compression than tension
• Traumatic fracture: single load
• Stress fracture: repetitive loading
Traumatic fracture: single load
Fracture: effect of loading type
Stress fracture: “hot spot”
Remodelling
• Throughout life
• youth: length and circumference
Remodelling
• Throughout life
• adult: circumference
Bone Remodelling
• Osteocytes: hard bone cell
• Osteoblasts form bone ==> osteocytes
• Osteoclast resorbs bone
Wolff’s Law (1892)
• bone is deposited where needed and
resorbed where not needed
• bone remodels in response to applied stress
• Bone hypertrophy occurs in areas
where stress and strain are increased.
• Bone atrophy occurs in areas where
stress and strain are decreased.
Remodelling
Factors affecting remodeling
Remodeling: lifestyle
•success ??
• available nutrient
• hormonal levels (estrogen)
• mechanical stress
• activity vs inactivity
Osteoporosis
Characteristics
• decrease in bone mineral content.
• reduction in cortical bone thickness
• reduction in trabecular integrity
• Significantly reduces bone strength
• Very prevalent in elderly females
• Spontaneous compression fractures
Bone strength and aging
Spontaneous
compressive
fractures
Skeletal Muscle
• about 400 muscles in the
body
• ~ 50% of TBW
• ~ 50% of body’s
metabolism
• Controlled by voluntary
nervous system
• somatic nervous system
Skeletal Muscle
Functions
• generate force (tension)
across joints
• moment of force or torque
Triceps
torque
Biceps
torque
Skeletal Muscle
Functions:
• generate force (tension)
across joints
• moment of force or torque
• muscle pump
• aid in venous return
Muscle Structure
• muscle cells
• muscle fibers
• tension producer
• connective tissue
• energy storage
• transfer of energy
• nerve
• motor & sensory
• communication
Gross Muscle Anatomy
Skeletal Muscle Fiber
• Myofibrils
• longitudinal subunits of fiber
• true contractile elements (tension)
• contain myofilaments
• protein filaments actin and myosin
• overlapping filaments give the muscle its
striated appearance.
Actin & Myosin
Electron Microscope image of Sarcomere
1 sarcomere
3D Structure & Cross Bridges
3D Structure & Cross Bridges
Ethier & Simmons (2007)
Introductory Biomechanics
From cells to Organisms
Skeletal Muscle Innervation
Nerve cell carries electrical signal
• Neuron transmits impulse
• CNS to muscle
• Motor nerves
• efferent
• Sensory receptors to CNS
• Sensory nerves
• afferent
Motor neuron (efferent)
• One nerve and all the
muscle fibers it
innervates
• Collaterals
• Neuromuscular
junction
• motor endplate
• synapse
• Single muscle ==>
many MUs
Motor
Unit
In vivo motor neuron
Skeletal Muscle Innervation
Nerve cell carries electrical signal
• Neuron transmits impulse
• CNS to muscle
• Motor nerves
• efferent
• Sensory receptors to CNS
• Sensory nerves
• afferent
Sensory nerve (afferent)
• Golgi tendon organs
• sensitive to amount of tension produced
• inhibits tension production
Sensory nerve (afferent)
• Golgi tendon organs
• sensitive to amount of tension produced
• inhibits tension production
In vivo
Sensory nerve (afferent)
• Muscle Spindles
• sensitive to length of muscle (rate of lengthening)
• enhances tension production
Sensory nerve (afferent)
• Muscle Spindles
• sensitive to length of muscle (rate of lengthening)
• enhances tension production
Skeletal
Muscle
Innervation
Principles of Activation
• All-or-none principle
• if & when a motor unit is
activated, all the fibers in
the motor unit produce
tension
Excitation - Contraction Coupling
Process of converting an electrical signal (action
potential) into the mechanical process of sarcomere
contraction and force production
Sliding Filament Theory
Describes the physiological - biomechanical process of
sarcomere shortening and force production
Proposed in 1953
by Hugh E. Huxley
and Jean Hanson,
published in Nature,
1954. Focus of
extensive research
ever since.
Molecular basis of muscle contraction
•
•
•
•
•
•
Acetylcholine released at NMJ
Muscle membrane is depolarized.
Calcium is released.
Troponin combines with Ca exposing actin active sites
Cross-bridge attachment.
Myosin crossbridges swivel and rotates
•
•
Pulls actin over myosin  Sarcomere shortens
Crossbridge breaks, myosin returns to original shape
•
Cross Bridge Cycling
Cross Bridges from Myosin
Molecule
Cross Bridges Produce Power Stroke
Energy metabolism of muscle
• Contraction consumes chemical energy
• ATP (adenosine triphosphate)
• Source of ATP: CHO, fat, (protein)
• aerobic metabolism (with oxygen)
• byproducts CO2 and H20
• anaerobic metabolism (without oxygen)
• byproduct lactate (lactic acid)
Energy metabolism of muscle
• Contraction consumes chemical energy
• Source of ATP: CHO, fat, (protein)
• Fatigue
• lactic acid, depletion of ATP, neural,
psychological
Skeletal Muscle
• Muscle fibers terminate at the
tendon
• the myotendinous junction
• ***mysiums ==> tendon
• weakest part of the muscle?
Connective tissue of muscle
• Provides pathway (blood, nerve)
• Affects mechanical characteristics
• Three components
• Epimysium - outside covering (fascia).
• Perimysium - septa separating bundles of
fibers (fasciculi) internally.
• Endomysium - surround individual fibers.
Muscle x-section
Gross Muscle Anatomy
Muscle Actions (contractions)
• Muscle only pulls
• Concentric - shortening of the
muscle (causes movement)
• Eccentric - lengthening of the muscle
(slows or controls movement)
• Isometric - muscle tension without
movement (prevents movement) 10.13
Comparing Types of Contractions
Concentric
Eccentric
Isometric
Positive acceleration
Negative acceleration
No. accel.
Increase velocity
Decrease velocity
Maintain v.
Jumping up
Landing from jump
Standing
Throwing (start to
ball release)
Throwing (after ball
release – stop arm)
Holding
Weakest
Strongest
Middle
Important roles of muscle
• Concentric muscle activity distributes
and reduces the stress within bone by
generating mechanical energy
• Eccentric muscle activity distributes
and reduces the stress within bone by
absorbing mechanical energy
• Continuous strenuous activity can
cause abnormally high stress on
bone/muscle as muscle fatigues
Three Component Muscle Model
(explains mechanical behavior of muscle)
Schematic diagram
of muscle
“Muscle”
represented with
SEE, CE, & PEE
(SEC, CC, PEC)
Tendon is another
component of
SEE
(Zajac, 1992)
Three Component Muscle Model
Muscles with fibers in
series shorten more and
have faster shortening
velocities
Muscles with fibers in
parallel produce more
force
Shorten Sarcomeres to Stretch Springs
Muscle model with
sarcomere and SEE
(tendon)
(McMahon,
1987)
Sarcomere activated and
shortened to keep (SEE)
tendon stretched
(Roberts,
1997)
Mechanical Muscle-Tendon Model
Mechanical model explains:
Contractile and elastic components – Contractile
component exerts force on SEE and SEE exerts
force on bone
Electromechanical Delay – delay between contractile
activation and bone moving
Actin & myosin, slack in SEE, overcome inertia
Eccentric force greater than concentric force
Definitions of concentric and eccentric contractions
Electromechanical Delay
Explain the term, wrt 3 component model of muscle.
Functional Properties of Muscle
Muscles contract and produce force.
Force production comes from contractile and elastic
elements which have entirely different force producing
characteristics.
Total muscle force production is a complex interaction
between the force producing components and is
affected by several functional characteristics.
Basically, the quantity and quality of x-bridges.
Factors affecting force produced:
• Length-tension relationship
• Total Tension reflects
• amount of overlap between actin &
myosin
• utilization of elastic component
• Stretch-Shorten Cycle (SSC)
Length – Tension Relationship
Sarcomere Length – Tension
Relationship
Force development in the
sarcomeres depends on
the length (overlap) of
the sarcomere.
Sarcomere produces
maximum force at midlength (resting length)
Length Tension in Sarcomeres
Mid or resting length
optimizes the arrangement
of actin and myosin
filaments.
More crossbridge sites
available – more cross
bridges formed – more
force.
Length Tension in Sarcomeres
Shorter sarcomere lengths reduce available crossbridge
sites – actin filaments overlaps and cover some sites
Length Tension in Sarcomeres
Longer sarcomere lengths
reduce available crossbridge
sites – actin filaments
stretched past binding sites
on myosin filaments.
Few or no crossbridges formed.
Length Tension in Elastic Elements
Tension (idealized units)
Force development in the elastic elements depends on the
length of the element according to Hooke’s Law.
(F=-kx)
or
Short
Resting
Long
Elastic Element Length
Elastic element
maximum force at
longest lengths and
no force at resting
shorter lengths.
Length Tension in Elastic Elements
Elastic element
length-tension
for different
muscles
Depends on tissue
stiffness which
depends on
size and
molecular
composition
Length Tension in Elastic Elements
Force development in the elastic elements also depends on
the velocity of the stretch – modified Hooke’s Law to
include rate of stretch: F= kx + Vel.
Elastic element has
more force with
longer and faster
stretch.
Length – Tension Relationship
Combined
Length–Tension Relationship
Total muscle force depends
on individual
component
contributions.
At mid lengths – sarcomere
dominates
At long lengths – elastic
elements dominate
Length - Tension & Stretch - Shorten
Stretch – shortening cycle employs the length tension
relationship of muscle.
This effect includes maximizing the amount and rate of
muscle stretch.
How is stretch-shortening cycle used in
throwing, batting, running, and darts?
How about lifting, hammering, pushing, sweeping?
Factors affecting force produced:
• Force-velocity relationship
• concentric: decrease force with increase
velocity
• inefficient coupling of actin & myosin
• eccentric: increase (?) force with increase
velocity
• viscosity of muscle fluids
• effective coupling of actin & myosin
Functional Properties of Muscle
Muscles contract and produce force.
Muscle forces are produced in many different situations
some of which involve slower contractions and some
of which involve faster contractions. Contraction
velocity (rate of shortening or lengthening of muscle
fibers and elastic tissue) affects force production.
Contraction velocity interacts with type of contraction –
velocity effect is different for concentric and eccentric
contractions.
Force – Velocity Relationship
Contraction Type
Eccentric
Concentric
Concentric:
Force production decreases
as contraction velocity
increases.
Higher forces can be
produced during slower
contractions.
Why?(sliding filaments and cross bridge
formation, elastic components: quality)
Force – Velocity Relationship
Contraction Type
Eccentric
Concentric
Eccentric:
Force production increases
as contraction velocity
increases.
Higher forces can be
produced during faster
contractions.
Force – Velocity Relationship
Contraction Type
Eccentric
Concentric
Eccentric:
Increased force production
due to faster stretch of
elastic tissues:
F= -kx + Vel.
Stretch-shortening cycle –
faster pre-stretch
enhances muscle force
production.
Force – Velocity Relationship
Contraction Type
Eccentric
Concentric
Isometric:
(No velocity, constant
length)
Force Hierarchy:
Eccentric strongest.
Isometric in between.
Concentric weakest.
Force – Velocity Relationship
Contraction Type
Eccentric Concentric
Force Hierarchy:
Eccentric – stretch utilizes elastic
component and contractile works at
optimum length
Isometric – both elastic and contractile
components can contribute but one or
both will not be at optimum length
Concentric – contractile and elastic
components shortening & moving
away from optimum length
Force – Velocity
Relationship
Concentric:
Knee joint velocity and muscle
torque during the stance phase
of running. Torque (and muscle
force) are highest in midstance
when the joint stops moving
(zero velocity). Muscle
shortening velocity is low at this
time.
Force–Velocity Relationship
Weight lifting ability
limited by concentric
strength.
Eccentric capability not
trained as effectively.
Length – Tension Relationship
Sticking Region in Lifting
Sticking region or sticking
point is the weak point
in weightlifting, the
phase when the lift may
fail.
The region is defined as the
period during the early
lift when less force than
the weight of the bar is
being applied to the bar.
Concentric Force–Velocity
Relationship
F – V has identical form
in males & females
Training raises F – V but
maintains basic form
F –V has basic form in
all muscles but force
output at each
velocity varies across
muscles
Concentric Force–Velocity
Relationship
F – V has identical form
in slow and fast
twitch fibers but fast
twitch have higher
force at each velocity
Concentric Force–Velocity
Relationship
F – V has identical
form in different
muscles, some
muscles have
larger force at
each velocity.
Functional Properties of Muscle
Muscles contract and produce force and this force is applied
to a bone in the skeletal system. Sometimes the muscle
force causes the bone to move
the force does work
Work – result of a force applied with movement
Work = Force * displacement or Work = Torque * angular
displ.
Functional Properties of Muscle
Power represents the rate at which work is being done.
P = Work / time
P= Work / time
= Force * displ. / time
= Force * velocity
= Torque * ang. displ./ time
= Torque * angular velocity
measured in Watts (W)
Powerful people are Strong and Fast
Functional Properties of Muscle
Powerful people are Strong and Fast
Power During Lifting
0.20
m
40 N
Work = Force * displacement
= 0.20 m (40N)
= 8 Nm = 8 J
Lift in 0.5 s: P = Work/time = 16 W
Lift in 1.0 s: P = Work / time = 8 W
Lift in 2.0 s: P = Work / time = 4 W
Work was performed from a
concentric contraction.
Power – Velocity Relationship
Concentric:
Power is maximized at
about 1/3 maximum
shortening velocity.
Muscle force is still
moderately high and
velocity is
moderately fast.
Power – Velocity Relationship
Concentric:
Power is higher in fast
vs. slow twitch
muscle fibers at all
velocities.
Power – Velocity Relationship
Concentric:
Power – velocity form
identical for different
muscles but some
muscles are more
powerful than other
muscles.
Joint Power Produced By Joint
Torques
Knee power, torque, and angular
velocity during stance phase of
running.
Peak torque at zero velocity – at
maximum knee flexion, maximum
quadriceps stretch – muscle force
maximized early in movement.
Peak power at mid levels of torque
and velocity – both torque and
velocity contribute to power – muscle
work maximized in middle of
movements.
Functional Properties of Muscle
Combined understanding of L-T, F-V, and P-V relations
1) Stretch-shortening cycle provides pre-stretch
(eccentric contraction) prior to the desired concentric
contraction
2) Stretched muscle provides high force at the start of
concentric contraction due to favorable L-T and F-V
characteristics
3) Muscle power and work maximized about 1/3 into
movement – muscle contribution to movement (force
& displacement) maximized at this time
What can strength training do?
Muscle force
affected by
active
generation of
tension,
enhancement
from muscle
spindle, and
inhibition from
Golgi Tendon
Organ
Explains the effectiveness of plyometric training
Joints
• Fibrous joints
• fibrous tissue bridges the joint (ie skull).
• Cartilaginous joints
• cartilage bridges the joint (e.g.,
intervertebral joints, sacrum).
• Synovial joints
• no tissue between the articular surfaces
Synovial joints
• Structure
•
•
•
•
bone ends
articular cartilage (hyaline cartilage)
joint capsule (ligaments)
synovial membrane
• synovial fluid
• Lubrication: fluid & hyaline
Osteoarthritis
• aka Degenerative joint disease.
• Cartilage does not repair because .....
• Characteristics
• wearing away of cartilage
• bony deformation (osteophytes)
• pain with use
• Cause
• genetics, mechanical wear (injury)
Intervertebral discs
• The Jelly Donut of our back
• Donut: annulus fibrosus.
• Jelly: nucleus pulposus
Intervertebral discs
• The Jelly Donut of our back
• Donut: annulus fibrosus.
• Jelly: nucleus pulposus
• Relationship to spinal nerves
Back pain 'starts in school
By Roger Highfield
Around half of all children are at risk of suffering a lifetime of back problems
because of awkward postures during lessons and using computers, furniture and other
equipment designed for adults.
Forty per cent of schoolchildren suffer health problems considered in adults to be
"work related” that could affect them for the rest of their lives, said Prof Peter Buckle, of the
University of Surrey's Robens Centre for Health Ergonomics in Guildford.
He said a Danish study showed that 51 per cent of children aged 13 to 16 reported
low back pain in the previous year, and 24 per cent of 11- to14-year-olds in the north-west of
England reported having back pain in the month prior to completing a questionnaire.
"Under European laws the health of workers is protected," he said. "But when we
start to look at young adults and children the picture is far less clear.
"Worryingly, evidence is starting to show that, for some health problems, we may
be leaving it too late before we start helping."
A study found that those reporting low back pain in school were more likely to report low
back pain as adults.
(Filed: 10/09/2002) © Copyright of Telegraph Group Limited 2002.
Intervertebral
discs
• The Jelly Donut of our back
• Donut: annulus fibrosus.
• Jelly: nucleus pulposus
• Relationship to spinal nerves
• Herniation (low back pain)
• nucleus causes annulus to bulge at weak
area
Back loads
Affected by:
Load lifted
HAT inertia & posture
Muscle Activity
McGill, 2001
Back muscle modeling