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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
