Center of Gravity (COG)

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Center of Gravity (COG)

• the point around which a bodys weight is equally
  distributed in all directions

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Center of Gravity (COG)

• COG COM
• Geometric center?
• Fixed?
• Located outside of the body?
• Moves in the direction of added mass

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Center of Gravity (COG)

• COG COM
• Geometric center?
• Fixed?
• Located outside of the body?
• Moves in the direction of added mass

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Applications

• Muscle Function Rotational Effects
• Segmental Alignments
• Whole Body Stability

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Basic Kinetic Concepts
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Course Content

1. Introduction to the Course
2. Biomechanical Concepts Related to Human Movement
3. Anatomical Concepts Related to Human Movement
4. Applications in Human Movement

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Biomechanical Concepts

• Basic Kinematic Concepts
• Vector Algebra
• Basic Kinetic Concepts

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What is kinetics?

• The study of forces tending to cause, causing, or
  resulting from motion.
• Can identify and alter forces to change motion
  for desirable results.

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Basic Kinetic Concepts

1. Force
2. Torque
3. Free Body Diagrams

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Basic Kinetic Concepts

1. Force
2. Torque
3. Free Body Diagrams

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Force

• A push or pull
• Vector quantity with 4 characteristics
• Magnitude
• Direction
• Point of application
• Line of force
• The interaction of an object with its surroundings

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For a force to cause acceleration

• Must overcome opposing forces (net force)
• Must overcome inertia (mass)
• Even if acceleration does not occur, deformation
  of the object will occur.

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Common Forces

• Weight
• Reaction forces
• GRF
• Friction
• JRF
• Muscle force
• Elastic force
• Intraabdominal pressure
• Inertial force
• Fluid force

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Basic Kinetic Concepts

1. Force
2. Torque
3. Free Body Diagrams

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Torque

• AKA moment of force, or moment
• The ability of a force to create rotation

• moment arm - perpendicular distance from the line
  of force to the axis of rotation
• Must have a moment arm (be an eccentric force)

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For a torque to cause angular acceleration

• Must overcome opposing torques (net torque)
• Must overcome moment of inertia (mass, length of
  the rotating body)

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Which way will the door turn?
FC 200 N
FB 100 N
dC .05 m
dB 1 m
TC FCdC TB FBdB TC 200 N .05 m TB
100 N 1 m TC 10 Nm TB 100 Nm TN 100 Nm
(-10 Nm) TN 90 Nm
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What must be done for the class to win?
FC 200 N
FB 100 N
dC .05 m
dB 1 m
? TC or decrease ? TB sufficiently. How? TC ? ?
FC ? ? dC
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Increase the magnitude of FC.
FC 300 N
TC FCdC TB FBdB TC 300 N .05 m TB
100 N 1 m TC 15 Nm TB 100 Nm TN 100 Nm
(-15 Nm) TN 85 Nm
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It would take 2000 N of force to get 100 Nm of
torque.
TC FCdC TB FBdB TC 2000 N .05 m TB
100 N 1 m TC 100 Nm TB 100 Nm TN 100 Nm
(-100 Nm) TN 0 Nm
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Increase the magnitude of dC by changing the
point of force application.
FC 200 N
dC .15 m
TC FCdC TB FBdB TC 200 N .15 m TB
100 N 1 m TC 30 Nm TB 100 Nm TN 100 Nm
(-30 Nm) TN 70 Nm
FB 100 N
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It would take a moment arm of .5 m to get 100 Nm
of torque. We can not apply the force far enough
away to get a moment arm this large. We run out
of door!
TC FCdC TB FBdB TC 200 N .5 m TB
100 N 1 m TC 100 Nm TB 100 Nm TN 100 Nm
(-100 Nm) TN 0 Nm
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TC FCdC TB FBdB TC 200 N .20 m TB
100 N 1 m TC 40 Nm TB 100 Nm TN 100 Nm
(-40 Nm) TN 60 Nm
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We would not be able to change the angle enough
to get 100 Nm of torque.
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It will take a combination of all three, or else
TB would have to be reduced in a similar (but
opposite) manner.
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How would the door rotate if a force were applied
to the door in this manner?
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Summary

• Net torque determines rotation
• Can change net torque by changing one or more
  individual torques
• Can change individual torques by changing
• Magnitude of force
• Direction of force (moment arm)
• Point of application of force (moment arm)
• Often takes a combination of all of these

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Applications of Torque in Human Movement

• Muscle Function
• Movement Analysis

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What kind of torque does the biceps brachii
create?
What kind of torque does gravity create?
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Which way will the arm rotate?
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What conditions have to be true for the arm to
flex? For the arm to extend?
What role does the triceps brachii play?
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What is the torque output of this muscle in the
frontal plane?
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What is the torque output of this muscle in the
frontal plane?
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Can this muscle cause frontal plane rotation of
the scapula?
What are the angular movements of the scapula in
the frontal plane called?
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Yes, if it has a moment arm for that axis of
rotation!
Will this muscle cause downward rotation?
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Yes, if its torque is larger than the opposing
torques.
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What happens to the length of the moment arm of
the muscle throughout the ROM?
What about muscle torque?
Does muscle force stay constant through the ROM?
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What happens to the length of the moment arm of
the muscle throughout the ROM?
What about muscle torque?
Does muscle force stay constant through the ROM?
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Applications of Torque in Human Movement

• Muscle Function
• Movement Analysis

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What happens to the resistive force throughout
the ROM?
What about the resistive torque?
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What happens to the resistive force and
resistive torque as she goes through the ROM?
d
W
d
W
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What would happen to resistive torque if she put
her hands behind her head?
d
W
d
W
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Basic Kinetic Concepts

1. Force
2. Torque
3. Free Body Diagrams

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Summary

• Net force determines magnitude and direction of
  acceleration for linear motion.
• Net torque determines magnitude and direction of
  acceleration for angular motion.
• FBDs are a necessary first step in analyzing
  human motion.

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For the next lecture unit

• Lecture Topic 3
• Subtopic A The Skeletal System