Chapter 3: Kinetic Concepts for Analyzing Human Motion

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Chapter 3Kinetic Concepts for Analyzing Human
Motion

• Basic Biomechanics, 4th edition
• Susan J. Hall
• Presentation Created by
• TK Koesterer, Ph.D., ATC
• Humboldt State University

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Objectives

• Define and identify common units of measurement
  for mass, force, weight, pressure, volume,
  density, specific weight, torque, and impulse
• Identify and describe the different types of
  mechanical loads that act on the human body
• Identify and describe the uses of available
  instrumentation for measuring kinetic quantities
• Distinguish between vector and scalar quantities
• Solve quantitative problem involving vector
  quantities using both graphic and trigonometric
  procedures

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Basic Concepts Related to Kinetics

• Inertia
• Mass
• Force
• Free body diagram
• Center of Gravity
• Weight

• Pressure
• Volume
• Density
• Specific weight
• Torque
• Impulse

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Inertia

• Tendency for a body to resist a change in its
  state of motion

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Mass

• Quantity of matter contained in an object

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Force

• Push or pull
• the product of mass and acceleration

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Free body diagram

• a sketch that shows a defined system in isolation
  with all of the force vectors acting on the system

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Center of Gravity

• The point around which a bodys weight is
  equally balanced, no matter how the body is
  positioned

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Weight

• Attractive force that the earth exerts on a body

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Sample Problem 1

• If a scale shows that an individual has a mass
  of 68 kg, what is that individuals weight?
• Known m 68 kg
• Solution Answer
• Wanted weight wt 667 N
• Formulas wt mag wt 150 lbs
• 1 kg 2.2 lbs

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Sample Problem 2

• What is the mass of an object weighting 1200
  N?
• Known wt 1200 N
• Solution Answer
• Wanted mass m 122.32 kg
• Formulas wt mag

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Pressure

• Force per unit of area over which the force acts

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Sample Problem 3

• Is it better to be stepped on by a women wearing
  a spike or by a court shoe?
• Known wt 556 N As 4 cm2 Ac 175 cm2
• Solution
• Wanted Answer
• Pressure exerted by the spike heel p
  139N/cm2
• Pressure exerted by the court shoe p 3.8
  N/Cm2
• Formulas p F/A 43.75 times
• more pressure

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Volume

• Space occupied by a body

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Density

• Mass per units of volume

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Specific Weight

• Weight per unit of volume

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Torque

• Rotary effect of a force

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Impulse

• Product of force and the time over which the
  force acts

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Common Units for Kinetic Quantities

• Quantity Symbol Metric Unit English Unit
• Mass m kg
  slug
• Force F
  N lb
• Pressure P Pa
  psi
• Volume (solids) V m3
  ft3
• (liquids)
  liter gallon
• Density ?
  kg/m3
• Specific weight ? N/m3
  lb/ft3
• Torque T
  N-m ft-lb
• Impulse N
  s lb s

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Mechanical Loads on the Human Body

• Compression pressing or squeezing force
  directed axially through a body
• Tension pulling or stretching force directed
  axially through a body
• Shear force directed parallel to a surface

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Mechanical Loads on the Human Body

• Stress distribution of force within a body,
  quantified as force divided by the area over
  which the force acts
• Torsion load producing twisting of a body
  around its longitudinal axis
• Bending asymmetric loading that produces
  tension on one side of a bodys longitudinal axis
  and compression on the other side
• Combined loading simultaneous action of more
  than one of the pure forms of loading

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Sample Problem 4

• How much compressive stress is present on the L1,
  L2 vertebral disk of a 625 N woman, given that
  approximately 45 of body weight is supported by
  the disk

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Sample Problem 4

• When she stands in anatomical positions?
• Given F (625 N) (0.45) A 20 cm2
• Formula Stress F/A
• Stress (625 N) (0.45) / 20 cm2
• Stress 14 N / cm2

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Sample Problem 4

• b) When she stands erect holding a 222 N
  suitcase?
• Given F (625 N) (0.45) 222 N A 20 cm2
• Formula Stress F/A
• Stress (625 N) (0.45) 222 N / 20 cm2
• Stress 25.2 N / cm2

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The Effects of Loading

• Deformation
• When an external force is applied to the human
  body, several factors influence whether an injury
  occurs
• Magnitude and direction of force
• Area over which force is distributed
• Load-deformation curve
• Yield point (elastic limit)
• Failure

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Repetitive vs. Acute Loads

• Repetitive loading repeated application of
  subacute load that is usually of relatively low
  magnitude
• Acute loading application of a single force of
  sufficient magnitude to cause injury
• Macrotrauma a single force large enough to
  cause injury acting on biological tissue
• Microtrauma when repeated or chronic loading
  over a period of time produces an injury

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Tools for Measuring Kinetic Quantities

• Electromyography (EMG)
• To study neuromuscular function
• Dynamography
• Primarily employed in gait research
• Starts, takeoffs, landings, baseball golf
  swings, and balance

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Vector Algebra

• Vector any physical quantity that posses both
  magnitude and direction
• Kinetic vector quantities
• force weight, pressure, specific weight torque
• Kinematic vector quantities
• Displacement, velocity acceleration
• Scalar quantities magnitude only
• Mass, volume, length speed

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Vector Composition

• Resultant vector
• Tip-to-tail vector composition

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Vector Resolution
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Graphic Solution of Vector Problems

• Graphic vector manipulation may yield approximate
  result

30 N 3 cm
45 N 4.5 cm
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Trigonometric Solution of Vector Problems

• A more accurate procedure for quantitatively
  dealing with vector problems

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Summary

• This chapter introduced basic concepts related to
  kinetics
• Several types of mechanical loads act on the
  human body.
• The distribution of force within a body structure
  is termed mechanical stress.
• Vectors quantities have magnitude direction
• Vector problems may be solved by a graphic or a
  trigonometric approach.

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The End