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

Introduction to Biomechanics V: The Accommodation of Forces Flashcards

1
Q

The Momentum-Impulse Relationship

A
  • gives another perspective to understand human movement, mechanism of injury, and protective devices
  • may be calculated for either linear or rotary motion
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2
Q

Momentum

A
  • 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
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3
Q

Impulse

A
  • 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
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4
Q

Application of Momentum-Impulse Relationship

A
  • often applied to design of tools and equipment

- ex:padding in bike helmets, outsoles in shoes, dashboard padding

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5
Q

Momentum-Impulse Relationship: Application to Clinical Practice

A
  • 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
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6
Q

Stress-Strain Diagram

A
  • 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
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7
Q

Stress

A
  • 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
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8
Q

Strain

A
  • 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
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9
Q

Stress-Strain Diagram: Linear and Nonlinear Behavior

A
  • 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
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10
Q

Zones in Stress-Strain Diagram

A
  • 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
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11
Q

The Neutral Zone in the Stress-Strain Diagram

A
  • 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
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12
Q

The Elastic Region in Stress-Strain Diagram

A
  • 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
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13
Q

The Plastic Region in Stress-Strain Diagram

A
  • 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
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14
Q

The Failure Region of Stress-Strain Diagram

A
  • 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
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15
Q

Stiffness and Strength of Material/Tissue

A
  • 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
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16
Q

Stress-Strain Curve for Muscle

A
  • muscle is viscoelastic
  • deforms easily under low load
  • then responds stiffly
  • graph on page 9
17
Q

Stress-Strain Curve for Tendon

A
  • 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
18
Q

Stress-Strain Curve for Bone

A
  • bone is brittle
  • responds stiffly initially
  • then undergoes minimal deformation before failure
  • graph on page 10
19
Q

Anisotropy of Materials

A
  • 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
20
Q

Deformation Energy

A
  • 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
21
Q

Hysteresis

A
  • 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
22
Q

Ductility and Brittleness

A
  • 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
23
Q

Ductility

A
  • 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
24
Q

Brittleness

A
  • 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
25
Q

Creep Phenomenon

A
  • 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
26
Q

Material Fatigue

A
  • 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
27
Q

Material or Tissue Fatigue

A

-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

28
Q

Take Home Messages

A
  • 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

DPT 726: Orthopaedic Foundations (14 decks)

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