Lecture 1
Objectives:
1.
2.
3.
4.
5.
Discuss class rules
 Attendance is mandatory
 Class meeting times are Wed 8-8:55 and Fridays 8-11:00
 Expected to know: Anatomy and Kinesiology
Review Newton’s Laws of Motion
Review Classifications of Levers
Review the concept of Equilibrium
Introduce Stress/Strain and Load/Deformation
Biomechanics – study of mechanics in the human body
Mechanics – the branch of physics that considers the action of forces on bodies or fluids that are both at
rest (statics) and in motion (dynamics).
Kinematics – describes motion in the body without regard to the forces producing the motion.
(type, location, magnitude, direction)
Kinetics – area concerned with the forces producing the motion or maintaining equilibrium.
KINEMATICS
Types of Motion
1.
Rotary/angular – movement around a fixed axis in a curved path (degrees, radians)
2.
Translatory/linear – movement in a straight line (glide)
3.
Curvilinear – roatary and translatory movement (no fixed axis)
4.
General planar motion – multisegmental motion
What type of motion occurs in the human body most often? Explain?
Planes of Motion



Transverse/horizontal
Frontal/coronal
Sagittal
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Direction

Various motion descriptors – flexion, extension, abduction, adduction

Measured in degrees, radians, inches, cm with rulers, goniometers, inclinometers, etc.
Quantity
KINETICS
Forces – mutual interaction between 2 bodies that produces the deformation of the bodies and/or affects the
motion of the bodies.

Contact forces – direct contact between bodies
Give an example of a Contact Force ? push or pull

Field forces – not from direct contact but applied remotely
Give an example of a Field Force? Gravity, electromagnetic
Forces in the Body
External Forces – arise from sources outside the body
E.g. wind, water, other humans, GRAVITY, electromagnetic
Internal Forces – arise from muscles, ligaments, and bone usually to overcome external forces.
Forces are vectors defined by:
 Point of application
 Direction/Line of action
 Magnitude
Newton’s Laws of Motion
First Law – Law of Inertia
A body continues in its state of rest (equilibrium) or of uniform motion unless an unbalanced force acts on
it.
Equilibrium – all the forces acting on an object are balanced.
 Forces = 0
 Torques = 0
Inertia – the property of an object that causes it to remain at rest or in motion. Because of inertia, force is
needed to change the velocity of an object
Amount of force needed to alter the object’s velocity  to the amount of inertia it has.
Inertia = mass ( > mass the > inertia)
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Second Law – Law of Acceleration
The acceleration of an object is directly proportional to the force causing it, is in the same direction as the
froce, and is inversely proportional to the mass of the object.
a  F/m or F  ma
=>
F = ma
Third Law – Law of Reaction
For every action there is an equal and opposite reaction.
Eg. Book on the table
Is the book in equilibrium?
How?
What force is the book exerting?
If the book is exerting a force then the table must be exerting an = and opposite force =>
EQUILIBIUM
Levers
First Class (see-saw) – fulcrum is located between the force and the resistance.
EA
- relatively few in the human body
- Triceps acting at the olecranon
RA
Lever Arm – distance from the axis to the point at which the effort force is applied to the lever.
Second Class (wheel barrow/nutcracker) – the resistance is between the fulcrum and the force.
-
EA > RA
almost no examples in the human body
Opening the mouth against resistance
muscle eccentrically contracting against an
external moving force
muscle acting on its proximal segment
while the distal is fixed (heel raise)
EA
RA
Third Class – (spring closing a door)
-
EA
EA < RA
Most common lever class in the human body
Biceps on the forearm
Muscle is the effort force (concentric contraction)
RA
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Exchange between second and third class levers
2nd class – muscles acting eccentrically where the muscle is applying the resistance force
3rd class – muscles acting concentrically where the muscle is applying the effort force
Isometric contraction: effort force (internal) = resistance force (external)
Effort force is in the direction of the movement
Torque (moment) = tendency of a force to cause an object to rotate = F d = F x Moment Arm = Fr x Lever
Arm (shortest distance between point of application and joint axis.
Fm
fr
ft
Lever Arm
Most of the force is translatory  inefficiency but what is the purpose?? STABILITY
Pulleys – Change the direction of the force but NOT the magnitude – increase the MOMENT ARM but
force remains the same.
Anatomic – patella
F = ma = mg = w
Moment of inertia
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Mechanical Advantage – measure of efficiency of the lever
MechAdv = EA/RA  when MA > 1 the EF can overcome the R without expending as much
force as R
(EF)(EA) > (R)(RA)
Based on the equation for Mech Adv, what do we know about the Mech Adv for the various levers?
Center of Gravity/Center of Mass
 Point at which the mass is concentrated
 Symmetric objects – geometric center
 Asymmetric objects – toward the heavier end
Line of Gravity
 Vertical line running through the COG
Humans


Segmental centers of gravity
Whole body COG  S2
What is the significance of the COG/LOG on balance when standing, ambulating, squatting, or
lifting?
Balance
Cone of Stability
How does gaining weight affect an individual’s COG?
How does an amputation affect an individual’s COG?
What is the significant of knowing the COM properties of various segments in the body?
Types of Forces
Shear – two externally applied forces that are equal, parallel, and applied in the opposite direction but are
not in line with each other.
Compressive – two externally applied forces that are equal and act in the same line toward each other on
opposite sides of the structure.
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Tensile/Tension (Distraction) – two externally applied forces that are equal and act along the same line and
in opposite directions.
Properties of Connective Tissues
Stress/Strain
Stress = Force/Area (N/cm2, N/m2(pascals), MN/m2 (Mpascals))
Stress = Load
Strain =
Length / Original Length (percentage change)


Linear Strain – change in length/original length (non-dimensional)
Shear Strain – change
Plastic Region
Yield Point or
Elastic Limit
Stress or
Load
Ultimate
Failure or
Fracture Point
Elastic Region
Strain or Deformation
Young’s Modulus = slope in the elastic region = stress/strain
 stiffness of material   Young’s Modulus
Viscoelasticity –
 Elasticity – ability to return to its original shape following deformation after the removal of
the deforming load
 Non-time dependent – returns
 Stores energy (RUBBER BAND)
 Viscosity – ability to dampen/lessen shear forces
 time and rate dependent properties
Viscoelastic – sensitive to the duration of the force application
Creep – occurs when a viscoelastic solid is subjected to a constant load
 Typically responds with a rapid initial deformation followed by a slow, time-dependent,
increasing deformation until equilibrium is reached.
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Deformation
Time
Loading
Load
Unloading
Hysteresis – loss of energy
Deformation
Demonstration
Plastic six-pack holders – Stress and Strain
Glass
Lead
Effect of heat and cold on – Stress / Strain Curve
X-rays of bow radius/ulna
Muscle Testing for Sit-ups (see Kendall)
Group Exercises (8 Groups of 5 students):
Stress/Strain Group - Groups 1-4
Describe the significance of the stress/strain curve when stretching a patient?
What can you do, as a physical therapist that may effect the stress/strain curve for muscle?
Go to the literature and determine the optimal time to stretch muscle / scar tissue (post-surgical)
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