Biomechanics Basics

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.

Biomechanics

Study of mechanics in the human body

Mechanics statics – bodies @ rest or moving w/

constant velocity dynamics – bodies in motion

undergoing acceleration

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.

Definition Kinematics

Kinetics

Kinematic Variables Temporal characteristics Position or location Displacement Velocity Acceleration

Linear versus Angular Kinematics Position or location Displacement (d vs. ) Velocity (v vs. ) Acceleration (a vs. )

Kinetics Forces

Mechanical action or effect applied to a body that tends to produce acceleration

Push or pull

Kinetics - Forces

Mutual interaction between 2 bodies

- produces deformation of bodies

and/or - affects motion of bodies

Force (vector) Point of application

Direction

Magnitude

Mass

Quantity of matter (kg)

Center of Mass

Force Systems

Linear

Parallel

F1 F2

F1F2

F3

Force Systems

Concurrent

General

F1

F2

F1

F3

F3

F2 F4

Force Systems

Force Couple

F1

F2

Center of Mass/Gravity

Point at which body’s mass is equally distributed

Balance point

Pressure

Force / Area

Moment or Force / Torque (T)

Degree to which a force tends to rotate an object

Torque twist

Moment bend

Moment or Force / Torque (T)

T = f * ma

ma = moment arm, lever arm, torque arm

Shortest distance () from AOR to line of force

Moment

• T = F * ma

• T = 20 lbs. * 12 in.

• T = 240 in-lbs.

12”

20 lbs.

Moments

Coxa Varum

Newton’s Laws of Motion

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

Law of Acceleration (2)

a f causing it

Acceleration acts in same direction as f

f = m * a

Law of Reaction (3)

Every action = & opposite reaction

Biomechanics Book

- w = mg

+ w = mg

Law of Reaction Ground Reaction Forces

Equilibrium At rest (static) or Constant linear/angular velocities

(dynamic) Sum of forces = 0 (3d) Sum of moments = 0 (3d)

Work and Power Work = Force * distance

Power = Work / time

Momentum “quantity of motion”

p = m * v (linear)

Bigger & faster they are, the harder they hit

First Class Lever

EA RA

FEFR

First Class Lever

First Class Lever few in body

Triceps on olecranon

Splenius Capitis on OA joint

First Class Lever

Mechanical Advantage M. Adv. = FR / FE

M. Adv. = EA / RA (forces levers)

M. Adv. > 1 advantage M. Adv. < 1 disadvantage

Second Class Lever

EA

RA

Second Class Lever

FR

FE

Second Class Advantage

M. Adv. always > 1

FR

FE

Second Class Lever

Very few in body

Heel raise (fixed distal segment)

Eccentric: G is FE

muscle is FR

Second Class Lever

Third Class Lever

EA

RAFR

FE

Third Class Lever

FR

FE

Third Class Disadvantage

M. Adv. always < 1

FR

FE

Third Class Lever Most common

Concentric contractions

Exchange between 2nd and 3rd class levers

Third Class Lever

Inefficient Human Body? 3rd class:

FE > movement of distal segment (goal)

2nd class:FE (gravity) < movement of

distal segment > control

Forces Acting on Human

Internal- muscles, ligaments, tendons, bones

External- Gravity, wind, water, another person

Stress

Internal resistance of a material to an imposed load

= force / area

Pascal = 1 N/m2

Axial Stress

Axial (Normal) stress ()- compressive- tensile

Shear stress ()- forces acting parallel or tangential

Strain

Change in shape or deformation as a result of an imposed external load/stress

shape / original shape L / L0

Compressive,tensile, shear(angulation)

Strain

TT

C

S

Linear Stress-Strain Curves

Str

ess

()

Strain ()

A

B

Stress and Strain Slope = /

as slope stiffness

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

Stress and Strain Elastic Region stiffness

Young’s Modulus (E) = slope in elastic region

E = /

Mechanical Stress and Strain

Wet Bone

Stress

Strain

Dry Bone

Glass

Aluminum

Steel

Poisson’s Effect/Ratio

C

TT

Applied compressive load tensile stress & strain

Poisson’s Effect/Ratio

Applied tensile load compressive stress & strain

T

T

C C

Poisson’s Ratio = - (transverse strain / axial

strain)

= - (t / a)

Viscoelasticity

Viscosity resistance to flow ability to lessen shear forceElasticity ability to return to original

shape after deforming load is removed

Viscoelasticity

Purely elastic – returns to original shape w/ no energy loss

Load (defo

rm)

Unload (return)

Viscoelastic Delayed return response and loss

of heat energy (hysteresis)

Load (d

eform

)

Unload (return)

Viscoelastic Elastic effects - rate of elastic

return dependent on material properties

Viscous effects (time-dependent properties)

- Creep- Stress-Relaxation

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

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

Creep – FSU

Creep – FSU

Stress Relaxation Constant strain () level

Develops an initial resistance or stress at that held deformation

At that held deformation the stress () or relaxes

Stress Relaxation

Stress Relaxation

Stress Relaxation

tt0

tt0

Viscoelastic “Solid”Viscoelastic “Fluid”

tt0

Creep Effect of temp.

temp rate of creep