Biomechanics Basics
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Biomechanics Basics
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Biomechanics
Bio Mechanics
PhysicalTherapy
Biological SystemsOsseousJoints & LigamentsMuscles & FasciaeCardiovascularCNSPNSOrgans of sensesIntegumentaryRespiratoryDigestiveUrogenital LymphaticDuctless glands
Health professionApplication of Scientific PrinciplesMovement DysfunctionClinical practice, research, educationPathologyPrevention, evaluation, treatment
FluidsIdeal FluidsViscous FluidsCompressible Fluids
Solids
DeformableBodies
Material strengthElasticityPlasticity
Rigid Bodies
StaticsDynamics
Kinematics Kinetics
From Smidt GL. Biomechanics and Physical Therapy.Physical Therapy. 64(12): 1807-08, 1984.
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Biomechanics
Study of mechanics in the human body
Mechanics statics – bodies @ rest or moving w/
constant velocity dynamics – bodies in motion
undergoing acceleration
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Biomechanics
Bio Mechanics
PhysicalTherapy
Biological SystemsOsseousJoints & LigamentsMuscles & FasciaeCardiovascularCNSPNSOrgans of sensesIntegumentaryRespiratoryDigestiveUrogenital LymphaticDuctless glands
Health professionApplication of Scientific PrinciplesMovement DysfunctionClinical practice, research, educationPathologyPrevention, evaluation, treatment
FluidsIdeal FluidsViscous FluidsCompressible Fluids
Solids
DeformableBodies
Material strengthElasticityPlasticity
Rigid Bodies
StaticsDynamics
Kinematics Kinetics
From Smidt GL. Biomechanics and Physical Therapy.Physical Therapy. 64(12): 1807-08, 1984.
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Definition Kinematics
Kinetics
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Kinematic Variables Temporal characteristics Position or location Displacement Velocity Acceleration
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Linear versus Angular Kinematics Position or location Displacement (d vs. ) Velocity (v vs. ) Acceleration (a vs. )
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Kinetics Forces
Mechanical action or effect applied to a body that tends to produce acceleration
Push or pull
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Kinetics - Forces
Mutual interaction between 2 bodies
- produces deformation of bodies
and/or - affects motion of bodies
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Force (vector) Point of application
Direction
Magnitude
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Mass
Quantity of matter (kg)
Center of Mass
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Force Systems
Linear
Parallel
F1 F2
F1F2
F3
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Force Systems
Concurrent
General
F1
F2
F1
F3
F3
F2 F4
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Force Systems
Force Couple
F1
F2
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Center of Mass/Gravity
Point at which body’s mass is equally distributed
Balance point
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Pressure
Force / Area
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Moment or Force / Torque (T)
Degree to which a force tends to rotate an object
Torque twist
Moment bend
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Moment or Force / Torque (T)
T = f * ma
ma = moment arm, lever arm, torque arm
Shortest distance () from AOR to line of force
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Moment
• T = F * ma
• T = 20 lbs. * 12 in.
• T = 240 in-lbs.
12”
20 lbs.
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Moments
Coxa Varum
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Newton’s Laws of Motion
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Law of Inertia (1)
Body at rest or in uniform motion will tend to remain at rest or in uniform motion unless acted upon by an external force
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Law of Acceleration (2)
a f causing it
Acceleration acts in same direction as f
f = m * a
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Law of Reaction (3)
Every action = & opposite reaction
Biomechanics Book
- w = mg
+ w = mg
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Law of Reaction Ground Reaction Forces
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Equilibrium At rest (static) or Constant linear/angular velocities
(dynamic) Sum of forces = 0 (3d) Sum of moments = 0 (3d)
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Work and Power Work = Force * distance
Power = Work / time
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Momentum “quantity of motion”
p = m * v (linear)
Bigger & faster they are, the harder they hit
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First Class Lever
EA RA
FEFR
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First Class Lever
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First Class Lever few in body
Triceps on olecranon
Splenius Capitis on OA joint
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First Class Lever
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Mechanical Advantage M. Adv. = FR / FE
M. Adv. = EA / RA (forces levers)
M. Adv. > 1 advantage M. Adv. < 1 disadvantage
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Second Class Lever
EA
RA
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Second Class Lever
FR
FE
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Second Class Advantage
M. Adv. always > 1
FR
FE
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Second Class Lever
Very few in body
Heel raise (fixed distal segment)
Eccentric: G is FE
muscle is FR
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Second Class Lever
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Third Class Lever
EA
RAFR
FE
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Third Class Lever
FR
FE
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Third Class Disadvantage
M. Adv. always < 1
FR
FE
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Third Class Lever Most common
Concentric contractions
Exchange between 2nd and 3rd class levers
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Third Class Lever
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Inefficient Human Body? 3rd class:
FE > movement of distal segment (goal)
2nd class:FE (gravity) < movement of
distal segment > control
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Forces Acting on Human
Internal- muscles, ligaments, tendons, bones
External- Gravity, wind, water, another person
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Stress
Internal resistance of a material to an imposed load
= force / area
Pascal = 1 N/m2
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Axial Stress
Axial (Normal) stress ()- compressive- tensile
Shear stress ()- forces acting parallel or tangential
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Strain
Change in shape or deformation as a result of an imposed external load/stress
shape / original shape L / L0
Compressive,tensile, shear(angulation)
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Strain
TT
C
S
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Linear Stress-Strain Curves
Str
ess
()
Strain ()
A
B
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Stress and Strain Slope = /
as slope stiffness
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Stress and Strain
Elastic Region
Yield Point or Elastic Limit
Ultimate Failure or Fracture Point
Strain or Deformation()
Str
ess
or L
oad
()
Plastic Region
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Stress and Strain Elastic Region stiffness
Young’s Modulus (E) = slope in elastic region
E = /
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Mechanical Stress and Strain
Wet Bone
Stress
Strain
Dry Bone
Glass
Aluminum
Steel
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Poisson’s Effect/Ratio
C
TT
Applied compressive load tensile stress & strain
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Poisson’s Effect/Ratio
Applied tensile load compressive stress & strain
T
T
C C
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Poisson’s Ratio = - (transverse strain / axial
strain)
= - (t / a)
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Viscoelasticity
Viscosity resistance to flow ability to lessen shear forceElasticity ability to return to original
shape after deforming load is removed
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Viscoelasticity
Purely elastic – returns to original shape w/ no energy loss
Load (defo
rm)
Unload (return)
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Viscoelastic Delayed return response and loss
of heat energy (hysteresis)
Load (d
eform
)
Unload (return)
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Viscoelastic Elastic effects - rate of elastic
return dependent on material properties
Viscous effects (time-dependent properties)
- Creep- Stress-Relaxation
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Creep Test Material/tissue is subjected to a sudden,
constant load ()
Constant is maintained
Deformation () is recorded over time
Measure of viscoelastic nature of material
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Creep Tissue deforms rapidly 20 sudden load
(elastic)
Continues to deform or creep beyond initial deformation (viscous)
Definition – material deforms as a function of time under the action of a constant load
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Creep – FSU
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Creep – FSU
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Stress Relaxation Constant strain () level
Develops an initial resistance or stress at that held deformation
At that held deformation the stress () or relaxes
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Stress Relaxation
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Stress Relaxation
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Stress Relaxation
tt0
tt0
Viscoelastic “Solid”Viscoelastic “Fluid”
tt0
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Creep Effect of temp.
temp rate of creep