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Muscle Structure,
Function &
Biomechanics
E. Chao
Honorary Chair Professor of BME
10-14-10, NCKU
Outline
• Structure
• Mechanics & function
• Injury, repair, & remodeling
• EMG & force measurement
•Modeling & muscle stress
estimate
Introduction
• Largest tissue mass, 40-45% BW
• Contain cells, nerves, vessels, etc.
• Protect underlying structure
• Connect between bone & CNS
• Provide movement & function
• Offer high frequency loading &
damping effects to bone
Hierarchical
Structure of
Skeletal Muscle
Muscle Architecture
A. Parallel
B. Unipennate
C. Bipennate
D. Fusiform
Advantage of Pennation
• Muscle shortening without
volume change
• Large number of fiber in smaller
cross-section
• Effective PCSA (Physical CrossSectional Area)
Most Powerful Muscle
With largest PCSA (Physiological
Cross-Sectional Area)
Muscle Fiber Cytology
• Plasma membrance - sacrolemma
•Basement membrance, reticular
layer, ECM - Endomysium with
capillaries
• Endomysium
• Muscle fiber - many cells fused
• Satellite cells (stem cells)
• Myofibrils & sarcomeres
Muscle
Fiber
Structure
Fiber Ultra-structure
• Long cell with multiple nuclei
• Surrounded by plasma membranes
• Satellite cells
• Motor end plates
• Structural proteins, “myofibrils”
• Sarcomeres (functional units)
Basic functional unit
Structural protein, “Sarcomeres”
Interconnecting filaments
Muscle Action
• Isometric
• Isokinetic
• Isotonic
• Concentric &
eccentric action
Increase Contractive
force
• Change stimulation frequency
• Recruit more motor units
“Motor unit” - single
motoneuron axon
α
Skeletal Muscle Types
• Type I - Dark appearance, low
strength, slow twitching, fatigue
resistance, aerobic
• Type IIA - White, fast twitching
• Type IIB - White, fast twitching,
anaerobic
Muscle Fiber Types
Fiber Type
• Most muscles have mixed type
• Type distribution is genetically
determined
• Changing fiber type by exercise
is difficult
• Some endurance training may
produce type interconversion
Energetics of Muscle
• Phosphate hydrolysis - AMP,
ADP, CP, ATP
• Aerobic metabolism - O2
dependence
• Anaerobic - hydrolysis of
glucose
• Fat & protein serve as accessory
source of energy
Muscle PCSA
• Maximal force α to PCSA
• Speed of shortening α to
PCSA
PCSA = Muscle volume/Fiber length
In tendon transfer, muscle fiber #, size, PCSA
& architecture arrangement are critical
Muscle Mechanics
Length, shortening speed, time, and the
state of activation
Hills
3-element
model
No damping
effect!
Isometric Length-Tension
Relationship
The Blix
Curve
Force-Velocity Relationship
(F + a) (v + b) = (Po+ a)b
A. V. Hill
Concentric & Eccentric
Contraction
• Agonist vs antagonist
• Eccentric - Strain
hardening effect
• Eccentric - source of
muscle strain
Muscle Training Effect
• Strength - motor unit
recruitment
• Endurance (aerobic) - O2(+)
cardiovascular enhancement
• Power (anaerobic) - hydrolysis
of glucose
Electromyography EMG
• Muscle & tendon - interface
between nerve & skeleton
• EMG - electrical signal from
neuromuscular activation
• Surface, needle, wire eletrodes
• Motor unit involved
• Data processing
• Prediction of muscle force
Electromyography EMG
Only able to
provide
qualitative
muscle activity
Muscle Stress Prediction
Biomechanical Modeling & Analysis
• Inverse dynamics - measure motion
• Direct dynamics - know force
• Synthesis - prescribe motion
Determine the passive stresses in
tendon, ligament, bone & implants
Type of Model used in
Biomechanical Analysis
• Finite element model Forward dynamic problems
• System model - Inverse &
Forward dynamic problems
4 Types of Problems in
Mechanics
• Static equilibrium
• Dynamic equilibrium
• Continnum
• Contact
4 Key Problems
• Forward - know load find motion
• Inverse - know motion find load
• Synthesis - define performance
find load & motion path
• Indeterminate - more unknown
than equations
How to solve problems?
• Develop model
• Identify unknowns
• Bring principles & laws
• Measure or assume initial
& boundary conditions
• Sort out the governing &
constraining equations
Key Laws & Principles
• Equilibrium
• Gravitation
• Conservation
• Continuity
• Constitutive
• Thermodynamics
Why do we need
equations ?
FEM model of muscle
Muscle passive
elements
Tendon
elements
Contraction Unit
Sarcomas
CP = elastic element
CA = active element
δ = muscle length
A0 = Activation level
Virtual Interactive
Musculoskeletal System
“VIMS - Net”
VIMS - Models
VIMS - Tools
VIMS - Labs
The ATP Project:
A 3D Graphic-Based
MS System for
Biomechanical
Analysis
$ 2 million, 3 years!
Contribution of Muscle on
Shoulder Function & Stability
Resultant
force must
be within
the joint
contact
surface
Static Equilibrium Analysis
Analytical
& graphic
methods
High load in shoulder function
Phases of Baseball Pitching
Wind-up
Cocking
FP
Acceleration
TOP
MER
Follow-through
BR
Shoulder Muscle Model
 Eleven
muscles are included:
Deltoid anterior, posterior, and lateral
part, Teres minor, Latissimus dorsi,
subscapularis, supraspinatus,
infraspinatus, pectoralis major, biceps
long head, triceps.
 Muscle Attachment
Visible Human database
Muscle Line of Action
Muscle wrapping around
humeral head
Intermediate
Origin
Point is tangent
to the humeral
head
Reorientation of
the muscle
Minimize the
Straight muscle path
Insertion
Line
Muscle Lines of Action in
Baseball Pitching
Wind up
Early
cocking
Late
cocking
Follow
through
Acceleration
Max External
Rotation
Foot contact
Ball releasing
Scapular Movement
Deltoid Posterior
part
Supraspinatus
Biceps Long Head
Triceps Long Head
Teres Minor
Infraspinatus
Latisimus Dorsi
VIMS applications in
Sports Biomechanics
& Rehabilitation
Inverse Dynamics
.
..
 F = m ( rA + w x rC + w x ( w x rC ))
.
=
+ x
M
IAw
w
IAw
(Chao, 1971)
Conditions
• Measure motion & determine joint load
• Obtain displacement & calculate velocity
& acceleration
Find Joint/Muscle Loads
Redundant Problem
Unknowns > # of Equations
To be solved using the
Optimization Technique
Forward Dynamic Problem
Will occur in forward Dynamic Problem
Sequence of solving
Inverse Problem
• Free-body diagram
• Find resultant force/moment
• Distribute force in joint &
muscle
• Surface contact & internal
stress
Muscle Force Calculation
Optimization Analysis
• Sequential Quadratic
Programming search method
• Using in nonlinear equation
• Varying cost function
Motion Capturing System
• Qualisys system at 500Hz
• 37 reflective markers
Dynamic Analysis of
Baseball Pitching
Muscle Force and
Joint Stability
• Joint contact surface size &
geometry
• Joint capsule & ligaments
• Muscle balance
Musculoskeletal Modeling
Microsensor Pressure (mm Hg)
Intramuscular Pressure
Transducer
• < 2% F.S. repeatability
• < 1.5% F.S. accuracy
• < 4% Hysteresis
250
200
150
100
50
0
0
50
100
150
200
250
Reference Pressure (mm Hg)
Kaufman, Wavering, Morrow, Davis, Lieber, J Biomech, 03’
Skeletal Muscle
Research Opportunity
• Modeling & visualization
• Force estimate joint force muscle force
• Direct measurement
• Neuromuscular stimulation (FES)
• Artificial muscle (TE)
Research ≠ funding ≠ problem solving
But you must know
Mechanics first
Learn how to walk
before you run!!!