Biomechanics

Question 1

Types of Motion

Answer 1

Linear, translatory, curvilinear

Question 2

Planes of Motion

Answer 2

Transverse, frontal, sagittal plane

Question 3

External Forces

Answer 3

Gravity, wind, water, objects

Question 4

Internal Forces

Answer 4

Muscles, ligaments, bones

Question 5

Scalar

Answer 5

Magnitude
Examples:
Mass
Temperature
Work
Energy
Speed

Question 6

Vector

Answer 6

Magnitude & Direction
Examples:
Weight
Force
Velocity
Acceleration

Question 7

Force Vector

Answer 7

Three Components:
Point of application
Action line with direction indicating push or pull
Magnitude (length of vector)

Question 8

Naming Forces

Answer 8

“Object-on-object”
First object is source
Second object is object being acted on
Contact required for force to act (exception = gravity)

Question 9

Mass vs. Force

Answer 9

Kilogram:
Used to measure mass (a quantity of matter)
Scalar - not a vector quantity
Don't confuse with "weight"
Magnitude, no direction

Question 10

Newton

Answer 10

Measure of force
A vector quantity
A measure of weight
US equivalent: pounds (lbs)

Question 11

Force of Gravity

Answer 11

Attraction of Earth’s mass for the mass of other objects
Causes acceleration: 9.8m/s^2
PTs are always dealing with gravity (Gait, resistance)

Question 12

Weight

Answer 12

Mass (kg) x 9.8m/s^2
Mass is a scalar unit (no direction)
Weight is a vector (force unit): has magnitude and direction

Question 13

Center of Gravity

Answer 13

Gravity acts at all points on an object
“Point of application” is COG or center of mass
Hypothetical point at which all mass would appear to be concentrated (center of mass)
For symmetircal objects, COG is located in geometric center

Question 14

Line of Gravity

Answer 14

Action line and direction of the force of gravity
Always vertically downward toward the center of the Earth
Visualized as a string with a weight on the end (plumb line)

Question 15

Segmental COG

Answer 15

Each segment in the body has its own COG
Adjacent segments grouped together if moving as a unit
Gravity acting on combined segments represented by a single COG
Combined COG is on a line between segmental COGs

Question 16

COG: Human Body

Answer 16

Anterior to S2 in anatomic position
Really depends on a person’s proportions
Magnitude equal to person’s weight

Question 17

COG Displacement

Answer 17

Depends on rearrangement of segments and loads

COG can relocate

Question 18

Stability & the COG

Answer 18

If LOG falls outside of the base of support (BOS) = instability
The larger the BOS, the greater the stability
The closer the COG is to the BOS, the greater the stability

Question 19

PTs: Using COG and base of support

Answer 19

Lifting mechanics - Lower COG, wide base of support = Stable
Ankle rehab - Narrow base of support, challenge stability and muscular control = Instability
Walker/Assistive Devices - Increase base of support, greater stability

Question 20

Relocation of the COG

Answer 20

COG location dependant on arrangement of segments, distribution of mass
Most common redistribution of mass: addition of external mass
Adjustments bring LOG closer to center of base support

Question 21

Newton’s Laws

Answer 21

Law of Reaction (3rd)
Law of Inertia (1st)
Law of Acceleration (2nd)

Question 22

Newton’s Law of Reaction

Answer 22

Forces always come in pairs
“For every action there is an equal and opposite reaction.”
Most forces arise from “contact” - Push-Pull force; Gravity - can ignore opposite force
If book contacts table, table also contacts book

Question 23

Analysis of Forces

Answer 23

Summary: accounting for forces on an object
Forces result from contact
Gravity exerts a force on all objects - also has a reaction force (typically ignored)
All forces come in pairs (whenever 2 objects contact)

Question 24

Equilibrium

Answer 24

Forces on an object determine if object in translatory, rotary, or curvilinear motion
Forces can be applied to an object without causing movement
Statics: the study of conditions under which objects remain in equilibrium (at rest) as a result of forces acting on them

Question 25

Newton's Law of Inertia

Answer 25

Deals with objects in equilibrium An object will remain at rest or in uniform motion unless acted on by an unbalanced force (outside force) For an object to be in static equilibrium: sum of forces - 0 Constant velocity - infrequent Inertia: property that makes an object resist initiation and change in motion

Question 26

Determining Equilibrium of an Object

Answer 26

Identity and magnitude of all forces Gravity acts on book at books COG; magnitude = wt. of book Book on table, therefore, table on book (3rd Law)

Question 27

Determining Equilibrium of a Table

Answer 27

Equilibrium - the sum of all forces acting on the book must equal zero Other forces are irrelevant

Question 28

Linear Force System

Answer 28

``` Two or more forces act on the same object and in the same line Translation Magnitude and Direction Forces to the right or up are + Forces tot eh left or down are - ```

Question 29

Newton's Law of Acceleration

Answer 29

Acceleration (a): change in speed and/or change in direction a = F/m or F = ma Inertia is proportional to the mass of the body or object

Question 30

Law of Acceleration

Answer 30

Acceleration of a body is proportional to and takes place in the direction in which the force acts and is inversely proportional to the body's mass F = ma and a = F/m

Question 31

Joint Distraction in a Linear Force System

Answer 31

Skeletal traction produces joint distraction Parts in equilibrium Known force is gravity-on-weight

Question 32

Refinement of Newton's 1st Law (Inertia)

Answer 32

Sum of all horizontal forces must be zero | Sum of all vertical forces must be zero

Question 33

Force of Friction

Answer 33

Potentially exists whenever two objects contact Has magnitude only when contacting objects moved or attempt to move Friction is a vector force Friction forces also come in pairs: equal in magnitude, opposite in direction Shear Force: force that moves or attempts to move one object on another (parallel to contacting surfaces in the direction of the attempted movement)

Question 34

Force of Friction

Answer 34

Action lines of friction forces always lie parallel to contacting surfaces Magnitude of Friction: Magnitude of contact between objects Slipperiness or roughness of surfaces (m = coefficient of friction) Magnitude of shear force In traction example, leg needs lifting off bed to minimize friction

Question 35

Force of Friction

Answer 35

``` Fx = μ*Fc Fx = Friction force μ = Coefficient of friction Fc = Contact force or Normal force To change Fx μ: Soft-Rough > Hard-Smooth Fc : Heavier object > Fc ```

Question 36

Examples of μs

Answer 36

Cobalt chrome on polyethylene (typical materials used in joint replacement), μs = 0.01 to 0.05. Normal cartilage, μs = 0.005 to 0.02. Dry cartilage μs = 0.27 Wet cartilage μs = 0.002 ­ 0.020