Introduction to Biomechanics V: The Accommodation of Forces Flashcards Preview

DPT 726: Orthopaedic Foundations > Introduction to Biomechanics V: The Accommodation of Forces > Flashcards

Flashcards in Introduction to Biomechanics V: The Accommodation of Forces Deck (28)
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The Momentum-Impulse Relationship

-gives another perspective to understand human movement, mechanism of injury, and protective devices
-may be calculated for either linear or rotary motion



-describes quantity of motion by a body
-derived from Newton's second law
-linear momentum F=m v
-generally is represented by letter p, measured in units of kgm/s
-smaller guy moving faster makes the bigger hit
-person moving faster has more momentum, and is less likely to get hurt



-measures what is required to change momentum of a body
-also derived from Newton's 2nd law
-linear impulse= force x time
-rotary impulse= torque x time
-momentum of an object can be changed by a large force delivered for a brief instant or a small force delivered over a longer time


Application of Momentum-Impulse Relationship

-often applied to design of tools and equipment
-ex:padding in bike helmets, outsoles in shoes, dashboard padding


Momentum-Impulse Relationship: Application to Clinical Practice

-every day our bodies absorb forces necessary to control momentum
-some are more mechanically efficient or "skilled" at this
-those with lesser motor skills more likely to break down over time
-if we don't address ability to withstand these forces a PT has not fully rehabbed a patient
-PTs must have conceptual understanding in order to address stress and recovery in musculoskeletal system
-interaction of organism, task, and environment (dynamic action theory) influences most tasks in work, play, and ADL


Stress-Strain Diagram

-aka load-deformation curve
-plot that quantifies relationship between force applied to a structure and deformation produced
-tells us structural properties of a tissue or material
-used to study response of structure to physical stress, fracture behavior and repair
-useful information in training and rehab
-stress=load per unit area of a sample (a measure of force within a tissue)
-strain=a measure of effect of stress, defined as difference between beginning state and ending state



-seen in a material subjected to a force
-force per unit area in a structure
-specific in a structure to a point of application and a direction of application
-units of measure are pascals, N/m^2, or psi
-normal stress is perpendicular to cross-sectional plane of structure
-shear stress is parallel to cross-sectional plane of structure



-seen in material specimen subjected to a force
-ratio of deformation ot original length
-normal strain is perpendicular to cross-sectional plane of structure
-shear strain is parallel to cross-sectional plane of structure


Stress-Strain Diagram: Linear and Nonlinear Behavior

-linear behavior exists when deformation is directly proportional to applied load and ratio of one variable quantity to the other variable quantity is constant
-nonlinear behavior exists when deformation demonstrates any deviation from linearity
-non-linear behavior is common in biologic tissues


Zones in Stress-Strain Diagram

-a way of characterizing the non-linearity of a load-deformation curve
-neutral zone: region of laxity
-elastic zone: region of resistance
-plastic zone: region of microfailure
-the typical load-deformation curve may be divided clinically into physiologic and traumatic regions


The Neutral Zone in the Stress-Strain Diagram

-aka toe region
-exists in most biological tissues and structures
-region where wavy collagen fibers straighten out
-a region of very low stiffness
-located immediately at start of loading, that is, before linear segment of elastic range


The Elastic Region in Stress-Strain Diagram

-material/tissue returns to original length and shape on removal of load
-look at graph on page 7
-linear part of elastic region: stress is proportional to strain
-nonlinear part of elastic region: stress not proportional to strain
-no permanent deformation occurs during this phase
-ex: spring, pole in pole vault


The Plastic Region in Stress-Strain Diagram

-material/tissue does not return to original length and shape on removal of load: residual deformation, permanent deformation
-Look at graph on page 8
-ex: taffy pull, bending nail, sprained ankle or knee


The Failure Region of Stress-Strain Diagram

-material/tissue fails
-exists a sudden decrease in stress without any additional strain
-look at graph on page 8
-ex: cracking an egg, ACL rupture


Stiffness and Strength of Material/Tissue

-slope of diagram is known as stiffness
-stiffness represented by slope of curve in elastic region
-stiffness obtained by dividing stress by strain at given point in elastic region
-strength determined by following criteria before failure: ultimate load, ultimate deformation, energy storage


Stress-Strain Curve for Muscle

-muscle is viscoelastic
-deforms easily under low load
-then responds stiffly
-graph on page 9


Stress-Strain Curve for Tendon

-tendon is capable of handling high loads
-end of elastic limit is also ultimate strength of tendon
-secondary to having no plastic phase
-graph on page 9


Stress-Strain Curve for Bone

-bone is brittle
-responds stiffly initially
-then undergoes minimal deformation before failure
-graph on page 10


Anisotropy of Materials

-anisotropic if mechanical properties are different in different directions
-isotropic if mechanical properties are consistent in different directions
-ex of anisotropic materials: wood, bone, IVD
-ex of isotropic materials: many metals, ice


Deformation Energy

-amount of work done on material or tissue by deforming load
-unit of measure is joule or newton meter
-load-deformation curve is excellent indicator of deformation energy
-elastic vs. plastic deformation energy



-process in which a lag occurs between application and removal of a force and its subsequent effect
-energy is lost during this process
-magnitude of energy loss determined by area between curves


Ductility and Brittleness

-material toughness defined by amount of energy required to failure
-tougher material is generally ductile
-absorbs large amounts of plastic energy before failure
-indicated by area under stress-strain curve



-mechanical property of high capacity for plastic deformation without fractures
-undergo a relatively large deformation before failure
-quantified by percentage elongation in length at failure
-ex: gum, beef jerky, most metals



-measure of tendency to deform or strain before fracture
-can absorb relatively little energy
-undergo little deformation before failure
-ex: chips, ceramics, cortical bone
-quantified by percentage elongation in length at failure: less than 6% and up are called brittle, more than 6% and dow are called ductile


Creep Phenomenon

-describes tendency of material to move or deform permanently to avoid stress
-results from long-term exposure to levels of stress that are below the ultimate strength of a material
-app: height in morning vs night
-deformation may become so large the component can no longer function properly
-app: poor posture can lead to change in muscle mechanics


Material Fatigue

-process of birth and growth of cracks in structures
-due to repetitive load cycles
-the fatigue clock starts once structure is subjected to a repetitive load
-speed of clock depends upon frequency of load and magnitude of load


Material or Tissue Fatigue

-magnitude of cyclic load that eventually produces failure: is generally in what was once the elastic load range; is far below the original failure load of structure


Take Home Messages

-momentum-impulse relationship describes the motion of a body and the force necessary to change it
-the momentum-impulse relationship is often integrated into clinical application and design of tools and equipment
-understanding the behavior of materials under load helps us know and modify the response of tissues under physical stress