Biomechanics III- link segment module ..

biomechanical analysis of the human body

1. LINK SEGMENT MODELS
•Bodysegment:individual
partsofthehumanbodythat
moverelativetoeachother.

2. Mechanical properties of Body Segment
•Eachbodysegmenthasuniquemechanicalproperties.
E.g.
•Mass
•Length
•Centerofgravity

3. Link Segment Models
•Agroupofbodysegmentsthat
areofinterestandarelinked
togetherbyanatomicaljoints
definesLinkBodySegment.
•LBSarebiomechanicalmodels
usedtorepresentthevarious
framesoftheskeletalsystem
duringanalysis.

4. Assumptions applied to biomechanical
models.
1.Thebodysegmentsarerigid.
2.Thejointsbetweentwobodysegmentsisfrictionless.
3.Thepositionofthecenterofmassremainsfixedrelative
tothesegment.
4.Themomentofinertiaofeachsegmentremainsconstant.
5.Thelengthofeachsegmentremainsconstant.

5. Lecture 3
Topic: Direct and Inverse Dynamics
Objectives:
1.Explain Dynamics as relates to mechanics.
2.Differentiate between Direct and Inverse
Dynamics.

6. Dynamics
•Dynamicsundermechanicsreferstothestudy
offorceandtorqueandtheireffectonthe
motionofabody.
•DynamicsissynonymoustoKinetics

7. Direct Dynamics
•Whenalltheforcesactingonabodysegmentare
known,thereisapossibilitytopredicttheresulting
motionofthatbodysegment.Thepredictingtoolto
determinethemotionisKINETICS.
•Directdynamicsinvolvesusingtheprinciplesof
kineticstodeducetheresultingmotionthatanobject
wouldexperiencegiventheforceappliedtoit.

8. Inverse Dynamics
•This involves calculating the acceleration
of body segment in motion and using the
mass of that body segment to quantify or
measure the applied force.

9. Lecture 4
Topic: Analysis of forces acting on body segments
Objectives:
•Explain Internal, External and Inertial forces
•Sources of these forces.

10. Forces acting on Body Segments
•Three fundamental forces can act on a body segment in a
link segment model, namely:
•External force
•Internal force
•Inertial force
Review Questions
•What is a force?
•What is the formula and unit of force?
•Differentiate between weight and force.
•What is gravity?

11. External Force
•Theseareforcesapplieddirectlytoa
bodysegmentandthosedueto
gravity.
•Sourcesofexternalforce,namely:
•Thoseapplieddirectlytothebody
segment.
•Gravity.
•Teaser
Howmanyexternalforcesactonthe
bodyduringstanding?

12. Conti…external force
•Externalforcesapplieddirectlytothebodyare
measuredusingTRANSDUCERS.
•Thoseduetogravityarecalculated.
•Inordertocalculatejointforces,externalforcesmust
bemeasured.

13. How many external forces are acting
on the girl?

14. Conti…external force
•Inlinksegmentmodelexternalforcesactoneach
bodysegment.Thisisdueto:
i.Interactionofbodysegmentwithobjectsexternaltothe
linksegmentmodel.
ii.Theweightofthebodysegment.
iii.Interactionwithadjacentbodysegments.

15. INTERNAL FORCE
•Theseareforcesthatareinternaltothebody.
•Internalforcesaregeneratedby:
i.ContractionofMuscle
ii.Ligament
iii.Frictionwithinjoints.However,jointforcesareverysmalland
negligible.Besides,jointforcesduetofrictionwon’tbe
consideredbecauseoftheassumptionthat,thejointare
frictionlessjoints

16. INERTIAL FORCE:
•Theyareforcesactingduetotheinertiaofabody
segmentasitaccelerates.
•Theinertialforceofasegmentisequaltotheproduct
ofthemassofthesegmentanditsacceleration.
•However,thisisrelativelyverysmall,thuswouldbe
considerednegligibleforthepurposesofthisclass.

17. EQUILIBRIUM
•Equilibriumdescribesthestateofabodywhereforcesor
influencesactingonthebodyarebalancedorcounteract
eachother.
•StatesofEquilibrium
1.Static
2.Dynamic
3.Quasi-static
•Teaser
StateNewton’slawofEquilibrium.

18. Static Equilibrium
•Abodythatisinstaticequilibriumhasazero-resultant
force.Suchabodyisseentobeatrest.
•Teaser
•FindanexpressionfortheSUMof
alltheforcesactingonthesegmentinthe
figureonyourright.
•AssumethatF
1=3.5KN,W=5.2KN
andF
2=8.7KN.Calculatetheresultant
forceonthesegment.

19. Dynamic Equilibrium
•Anobjectisindynamicequilibrium,whenitis
movingwithaconstantvelocity.
Teaser
•Findtheexpressionforthe
resultantforceforthesegment.
•Ifthemass,F
1,F
2,ofthebodyare
6kg,7Nand10Nrespectively,
calculatetheaccelerationofthe
body.[g=10ms
-2
]

20. Quasi-Static
•Forbiomechanicalbodies,whenthebodysegment
undergoessmallacceleration,theresultantforcewhich
isequivalenttotheinertialforceisnegligible,
comparingitwithotherforcesactingonthesegment.
•Theaccelerationofthatsegmentistherefore
neglected.
•Thismakesthe
resultantforceequal
tozero.