Lecture 1Lecture 1 (revised).docx

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McGill University
Kinesiology&Physical Education
EDKP 206
David J Pearsall

2/11/2013 3:56:00 PM zBiomechanics  Interdisciplinary field o Mathematics and physics o Biology, physiology, pathophysiology and clinical medicine  Basis: determination of how forces generated within the body, or forces applied to body, influence the way we move o How we move = kinematics o Causes of body movements = kinetics History: Borelli- 17thcentury  Published the first book of biomechanics o Considered the father of the field th Marey- 19 century  Uses cinematography to understand movement patterns o Quantified information th Muybrides (after 19 century)  Tried to understand the details of captured movement Coordinate systems  Anterior poster  Medial lateral (from side of body to extremeties)  Inferior and superior  Sagittal—cuts through the body forwards and backwards  Frontal—cuts through the body side to side (like a mirror) Levers  Parallel force system that is very common  Center of rotation (COR) o Forces on different sides – 1 class level o Forces on same side, External force is closer to COR than muscle force – 2 ndclass lever o Forces on same side, Muscle force closer to COR than external rd – 3 class lever  Properties that depend on the amount of material are called ―extensive properties‖ o Ex volume and internal energy, strengeth  Properties that do NOT depend on the amount of material are called ―intensive properties‖ o Ex. density  normalized for weight (volume/mass) Stress and strain  Stress: units of force/area o Measure that is independent of the amount of material o Types of force  Compression force= pushing on an object  Diameter increases, length decreases  Tension force= pulling on an object  Diameter decreases, length increases  Shear stress- back and forth motion in materials  Sheer force/area over which it acts  Failure occurs at maximal shear stress  Strain: normalized stretch or displacement of a material o Engineering strain: measuring the change in length  Change in length/original length  Change in length = measure length – original length o note elastic: the material returns to its original length/shape after loading Youngs model- stress vs strain line  Aka modulus of elasticity  It tells you o How much a material stretches or strains when it is subjected to acertain stress o How much stress build up in a material when it is stretch or strained by a certain amount  Also dictates a materials ―stiffness‖ o A material with a high youngs modulus will have less strain under a given load  stiff material  Intensive property & material propery o Material property- no matter how much of the material you have, they should have the same intensive properties Load to failure  Ultimate tensile stress/strength o Intensive definition of strength based on the maximum stress a material can withstand o Fracture toughness like ultimate strength but it is how much stress something can withstand when it already has a crack or break in it  Longer the intial crack, the less forced needed to fully break material  Inelastic/plastic deformation o When we load a material and it stretches/deforms and we then unload it o The stress will return to 0 but the strain DOES NOT  permanent deformation  Does not return to its original shape when load is removed  Yielded o Where matrial begins to deform plastically (permanently)  Yield point/elastic limit o Ben and lastic deformation begin  Yield strength o Stress at which the material begins to deform plastically o Once a material exceeds its yield strength it is permanently deformed and will NOT recover to original shape  Fatigue fracture o Occurs when a material is loaded and unloaded repeatedly and the max loads are below the ultimate tensile strength/yield strength Ways a material can behave before it breaks  No plastic deformation before it breaks  material’s behavior is BRITTLE o Behavior is brittle not the material itself per se  F material deforms plastically before it breaks (i.e. yields first)  matrials behavior is DUCTILE Loading rate  In general: the faster a material is loaded, the more brittle it becomes  Time dependant behaviors are caused by the liquid properties of material o Viscous properties in viscoelastic material  Viscoelastic properties cause for 2 phenominon o Creep: continued deformation of a material over time when material is subjected to a constant load o Stress relaxation: reduction in stress over time as the material is subjected to a constant deformation Bending and torsion  Bending: caused by tosion and compression on opposite sides of material (i.e. opposite sides of the neutral axis  where no stress is felt)  Torsion: generates shear stress Characteristics 2/11/2013 3:56:00 PM Note: the cellular level of our make up is what determines the function of our body components/parts  i.e. micro-architecture is the building blocks and will dictate the strength/stress properties of the tissues Funcitons of bone  Protect internal organs  Support body weight  Provide rigid kinematic links and muscle attachment sights  Facilitate muscle action and body movement  Act as storage area for minerals  Manufactures blood cells  note bone is LIVING TISSUE (i.e. needs blood etc—can be considered an organ) Structure  Components o Inorganic (calcium, phosphate) = ~70%  Found between organic components o Organic (collagen) ~ 30%  measure based on DRY weight o In vivo= water + 30% organic/inorganic components Types  Cortical (aka compact bone) o Hard, dense o Found in the mid section of bones o Resists primary forces o Concentric circle formation of fibers to create ― Osteon‖ (hollow tubes)  Inside the formation, black sections are called ―osteocytes‖  Within these osteocytes are osteocytes lacuna which contain bone cells  Bone cells = osteoclasts o Contains many vascular structures that penetrate through the bone o Contains canaliculi  Routes of communication for living cells that take down/create bone structures o Composed of collagen fibers that have a crimped formation  Cancellous (spongey bone) o Inner aspect of bone  Typically located at distal ends o Composed of thin threads of bone called ―trabeculae‖  Less dense then compact (cortical) bone Comparison b/w cortical and cancellous  % total skeletal mass o Cortical ~80% o Cancellous ~ 20%  Bone surface o Cortical ~ 33% o Cancellous ~67%  Surface – volume ratio o Cortical: 2.5 o Cancellous: 20  Porosity o Cortical: low o Cancellous: high  Turn over o Cortical: slow o Cancellous: fast  Marrow o Cortical: fatty o Cancellous: hematopoietic (def: production of blood cells and platelets occurring in bone marrow)  Development o Cortical: intramembranous ossification o Cancellous: hematopoietic  Function o Cortical: support of external loading, muscle attachement o Cancellous: support of internal loading, mineral homeostasis Mechanics 2/11/2013 3:56:00 PM Stress = Force / Area  Compression stress: arrows point into bone o Length of the bone decreases, width increases  Tensile stress: arrows point out of bones (pulling each end away from each other) o Length of the bone increases, width decreases  Sheer stress: line goes perpendicular to bone  Compressive/tensile and sheer stress o When the arrow is pointed on an angle to the bone  Strain- chan
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