Basic
Biomechanics
Kinematics is not Kinetics
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Body Planes & Axes
Osteokinematics & Degrees of Freedom
Closed & Open Kinematic Chains
Arthrokinematics & Convex/Concave Rules
Examination thru testing AROM, PROM,
flexibility,
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Kinetics = Analysis of forces
Our limited focus on Kinetics (Biomechanics)
– Concepts of COG, LOG, BOS & how they
relate to stability.
– Parallel force systems (levers) in static
situations, typically at 90
degree joint angles.
– Concepts of “Moment Arm” and an
“internal” and “external” moment arms as
well as internal and external torques.
Types of Motion
• Translatory
• Rotary / Angular
• Curvilinear
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Forces
• A force is a physical quantity that can
accelerate and/or deform a body.
– Tensile, Compressive, and Shear forces
• Gravity
– Center of gravity (COG)
– Line of Gravity (LOG) is the vertical vector force
• Muscle
– Produces vector force on the body by contraction,
exerted at its point of attachment
• Friction – created by the
sliding of one surface against
another
Forces
• Force implies both magnitude and
direction, and is a vector quantity.
• Characteristics of a vector
– Base of arrow represents point of application of
force
– Orientation & Tip of arrow represents line of
application & direction of force
– Length of arrow represents magnitude of force
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ID vector components; COG; LOG
Torque
• “The ability of a force to cause rotation of the
lever around an axis”
• Is dependent upon magnitude of force AND
distance from the axis that the force is applied
• Torque = f X lever arm distance
– Common units for torque: N·m or ft·lbs
– lever arm distance = shortest distance between
the line of applied force and the axis of lever;
aka: length of a line drawn perpendicular to line of force to
the axis.
• “Moment”
• “moment arm”
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Example of External Torque (gravity)
80N
0.5m
80N
0.25m
Levers
• A rigid apparatus that rotates (eg. angular
motion) around an axis; the rotation is
caused by Torque
• Components of a Lever
–
–
–
–
–
Axis (A)
Effort Force (EF)
Resistance Force “Weight or Load” (R)
Effort Arm (EA)
aka – internal moment arm
Resistance Arm (RA) aka – external moment arm
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Identify the parts of the Lever
• Static Equilibrium Equation (“Law of
Levers”)
EF x EA = R x RA
aka: Internal Torque = External Torque
• Mechanical Advantage Equation
MA = EA / RA (divide)
As MA increases, the amount of angular
motion will decrease and vice versa.
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1st Class Lever
Be Able to Calculate Equilibrium & MA
R
EA
RA
EF
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2nd Class Lever
2nd Class Lever
EF x EA = R x RA
Solve for any of these 4
variable OR estimate
their relative value
EF
EA
MA = EA / RA
Solve for any of these 3
variable OR estimate
the relative value of
MA
RA
R
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3rd Class Lever
3rd Class Lever
EF x EA = R x RA
Solve for any of
these 4 variable
OR estimate their
relative value
EF
MA = EA / RA
Solve for any of
these 3 variable
OR estimate the
relative value of
MA
EA
R
RA
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What type of lever? MA?
Relative value of R compared to EF?
R
EF
RA
EA
The COG of a region of the body can
move. What is happening to the external
moment arm? Impact on external torque?
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What happens to the external moment arm
throughout the ROM? Where is it
greatest? Impact on external torque?
What happens to the internal
moment arm throughout the ROM?
Where is it greatest?
Impact on internal torque?
EF
EF
EF
EF
EF
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What is illustrated
here?
Understand of the
impact of both
biomechanical
considerations AND
physiologic
considerations on
muscle function
Can you explain the combined
impact of both biomechanical
considerations AND physiologic
considerations on muscle function
on internal torque production?
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Type of lever? Label it
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Why is “R” at that location?
Why is “EF” at that location?
What type of lever is this?
What Type of Lever?
R= 150 lbs
RA= 0.25 ft
EA= 0.5 ft
EF= ? lbs
Calculate the torque
caused by gravity (exerted at
EF
EA
the ball of the foot, in foot-pounds)
Do the equilibrium
equation (solve for EF)
RA
Calculate Mechanical
Advantage. MA=
R
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What Type of Lever?
R= 120 N
RA= ? m
EA= 0.05 m
EF= 960 N
Calculate the torque
caused by biceps
(exerted at the elbow,
in newton-meters)
EF
Equilibrium equation;
solve for RA
Calculate Mechanical
Advantage. MA=
R
EA
RA
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