Biomechanical Concepts Flashcards
How do we define the basic properties of standing balance?
Using
- Forces
- Moments
- Displacement
What are Tissue Mechanics?
- The Foundation to understand tissue injury
What is movement essential for in life?
Life Processes
- Blood Circulations
- Respiration
- Muscle Contraction
Activities
- Walking
- Grasping
- Talking
- Stance
What are the two types of motion in mechanics?
- Linear
- Angular
What is human movement a combination of?
- Linear and Angular Motion
What are the two perspectives human movement can be viewed from?
- Internal Mechanics
- External Mechanics
Describe the Internal Mechanics perspective of human movement. What are some examples?
- Mechanical factors that produce and control movement from inside the body
ex. - Muscle Action
- Ligaments
Describe External Mechanics of the perspective of human movement. What are some examples?
- Mechanical factors affecting the body from without
ex. - Gravity
- Brick falling on your head
- Car Accident
What is an Idealized Force Vector?
- Single force vector representing the NET effect of all other vectors
- SImplifies Computation (Free Body Diagrams)
What is Injury Analysis?
Describes forces acting on the body such as:
- Gravity
- Impact of feet and hands
- Objects impacting body
- Musculotendinous Forces
- Ligament forces
- Compressive forces acting on long bones of lower extremities
What are Net Muscle Forces?
- Clavicular and Sternal Components of the force added vectorially
- Muscle force has antagonist and agonist effect on total force
Why is it useful to use an idealized force vector?
- What is lost in information is gained in creating a simplified model for calculations
What is the Centre of Mass (CoM)?
- Reducing the distributed mass of a body to a single point
- Represents the entire body or point about which a body’s mass is equally distributed
What is the Centre of Mass equivalent to?
- Idealized force vector
Is the Centre of Mass always located on the body?
- NO
What is a Free Body Diagram? What is it useful for?
FBD
- Graphical representation of all forces acting on a system
Useful
- Biomechanical analysis of injury and modeling through idealized force vectors
What is the Moment of Force? when is it used?
Moment of Force
- Equivalent of force for angular motion
Used
- When net force is not applied perpendicular to segment
How is the Moment of Force calculated?
- Using Trigonometric Function
What is the Equation for Moment of Force?
M = dxF (units = Nm)
F = Force
d = Moment arm (torque arm or lever arm)
How can you increase the moment of force?
- Increase force
- Increase moment arm
What happens when you apply force through the axis of rotation?
- No moment of force is produced
What kind of forces can act on tissues with no moment of force created? What happens when that occurs?
Compressive forces on vertebrae
- no rotation induced
- High risk of compressive fractures
How many components of force are involved in producing a moment?
- Usually only one component of the force
What is the system’s response based on?
NET moment
- Mnet = M1 + M2 +….
What is the Joint Reaction Force?
Forces experienced between segments of a free-body diagram
- Difficult to measure experimentally
Where is the Free body diagram usually broken? What must be shown?
Joints
- force acting across joints must be shown
What is Newton’s Third Law? How does it relate to the Free body diagram? ex.
There is an equal and opposite force acting on each joint
- Influenced by any effect included in the free body diagram
ex.
- Ground reaction force
- Muscle-ligament-joint capsule forces
When does Equilibrium exist?
- When forces and moments are balanced
- Exists for the body at rest or moving with constant linear and angular velocity
What happens to the sum of forces and moment of force at rest?
- Equals 0
What is the equation of motion?
- F = ma
- M = la
What is pressure?
The total applied force divided by the total area over which the force is applied
What is the equation for Pressure?
p = F/A (units SI: Pa) (1PA = 1N.m^-2)
What is the Centre of Pressure (CoP)? what units?
- Average location of all forces acting on a surface
Units - m
When does pressure have an effect on injuries?
- When one object impacts another
What is the general rule for pressure resulting in injury?
- Force applied over a larger area decreases the likelihood of injuries
What is standing upright in relation to movement?
- common form of active balance
What is the aim of balancing?
- Keeping the position of the CoM above and within the base of support
How can the base of support be modified?
- changing the stance width
What happens to the Centre of Mass while humans maintain balance?
- Moves horizontally and vertically (sway)
What is the difference between Centre of Mass and Centre of Pressure?
CoM
- Idealized point representing the mass of the whole body (vertical projection on the ground)
CoP
- Weighted average location of all downward forces acting on the ground (e.g. force platform)
What does Centre of Pressure depend on? What does it not depend on?
Depends on:
- Foot placement on the force platform
- Neuromuscular response to maintain balance
Does not:
- Measure of the sway of the Centre of Mass
What is the Centre of Pressure equations?
- CoPx = -My/Fz
- CoPy = Mx/Fz
What are the Torso Movements influenced by?
- changes in stance
What are the 5 degrees of Freedom?
- 2 hip joints
- 2 ankle joints
- motion of torso relative to lower body
What happens as Stance width increases?
- A change in ankle angle leads to a larger change in hip angle
- Proprioceptors at the hip can detect motion
- CoM excursion increases
What happens to the dynamics with a larger stance width?
- Same hip moment produces a greater total moment on the motion of the four-bar linkage (inertia also decreases)
What are some difficulties of measuring Angular Kinematics?
- Finite rotation cannot be treated as vectors in 3D
What is the equation of Angular Kinematics?
Wnet = w1 + w2 + w3 +….
W = angular velocity
What are the Material Mechanic terms?
- Stress, strain
- Compression, tension, shear, bending, torsion
- Viscoelasticity
- Material Fatigue and Failure
What is a Load?
- An externally applied force
What is Deformation?
- Change in shape of a body when a load is applied to it
What is material mechanics?
- Study of Internal Response of material to externally applied loads
Describe Stress in Material Mechanics
- Resistance Developed by tissue when externally loaded
- Internal measure of force divided by cross-sectional area of the tissue
What is the equation for stress?
- σ = F/A (SI Units: Pa)
What is Stress categorized as?
- Compressive
- Tensile
- Shear
Describe Strain relative to Material Mechanics
- Deformation of Tissue subjected to the external load
What kind of strain can we measure?
- Absolute strain
What is the equation for Relative Strain?
- relative stain: e = l - lo / lo
- l = length after deformation
- lo: resting length
What is Strain Categorized as?
- Compressive
- Tensile
- Shear
What does the slope of the load-deformation curve define?
- Stiffness
What is the opposite of stiffness?
- Compliance
What are stiff materials? what is their load-deformation curve profile?
Stiff materials
- Bones
Curve
- Steep slopped
When do stress and strain operate according to Hooke’s Law?
- When they are linearly related
What is Hooke’s Law?
- σ = E * ε ( E: Elastic Modulus or Y: Young’s Modulus)
Why do human tissues not behave linearly?
- Due to their fluid component
When is Stiffness related to the slope of the torque-angle relationship?
- for Standing Balance
What is the load stiffness of the human body during standing balance? Why?
Load Stiffness = mgh
why?
- Small angles during standing balance (less than 6 degrees)
What are the two major types of bone?
- Cortical (compact)
- Trabecular (cancellous bone)
How much weight of the bone comes from cortical bone?
- 80%
Describe Trabecular Bone
- Spongy
- Makes up interior of bones
What happens in the linear region of the bone?
- where elastic modulus is calculated
What leads to large bone deformation?
- Small increases in force
What happens when you release the load from a bone?
- bone does not return to original configuration (remains deformed)
What is the elastic modulus of Cortical bone?
- 13-17GPa
What is the elastic modulus of the Trabecular Bone?
- 10-2000MPa
Describe the composition relationship between the femur and phalanx bones
Femur
- Greater ability to carry load due to larger diaphysis
Phalanx
- Smaller diaphysis
Femur and Phalanx
- Similar composition of Compact Bone
What happens to the difference in maximal stress of the femur and phalanx when normalized for cross-sectional area?
- minimal difference
What is the Stress Response a function of?
- Strain and Strain Rate
What is Strain Rate?
- Speed at which deformation occurs
- Units (SI: s^-1)
What does biological tissue have?
- Fluid component
What happens when you increase strain rate?
- increases slope of σ/ε relationships (and stiffness)
What happens to purely elastic material when a load is removed?
- Returns to original shape (or length)
How do purely elastic materials deform?
- according to their σ/ε relationship
Describe Viscoelastic Tissue.
- Loses energy during deformation
- Return following unloading is retarded
- Return path different from initial path during loading
What are the 3 properties of viscoelastic tissue?
- Hysteresis
- Creep Response
- Stress Relaxation
Describe Hysteresis
- Energy lost during loading-unloading cycle
Describe Creep Response
- Tissue Subjected to constant load continues to deform
Describe Stress Relaxation
- tissue stretch maintained at a given length will result in decreased stress
What happens to materials subjected to repeated loads?
- Experience Fatigue
What is the difference between the mechanical response of the initial load vs subsequent ones?
- They differ
What happens to the continued loading of fatigued material?
- May lead to failure
What is the susceptibility to material failure determined by? Explain
Determined by:
- how the stress is distributed throughout the material
Explanation
- Stress concentrated in one area = greater likelihood of failure
