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Chapter 3: Kinetic Concepts for Analyzing Human Motion

Chapter 3: Kinetic Concepts for Analyzing Human Motion. Basic Biomechanics, 4 th edition Susan J. Hall Presentation Created by TK Koesterer, Ph.D., ATC Humboldt State University. Objectives.

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Chapter 3: Kinetic Concepts for Analyzing Human Motion

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  1. Chapter 3:Kinetic Concepts for Analyzing Human Motion Basic Biomechanics, 4th edition Susan J. Hall Presentation Created by TK Koesterer, Ph.D., ATC Humboldt State University

  2. 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

  3. Basic Concepts Related to Kinetics • Inertia • Mass • Force • Free body diagram • Center of Gravity • Weight • Pressure • Volume • Density • Specific weight • Torque • Impulse

  4. Sample Problem 1 1. If a scale shows that an individual has a mass of 68 kg, what is that individual’s weight? Known: m = 68 kg Solution Answer Wanted: weight wt = 667 N Formulas: wt = mag wt = 150 lbs 1 kg = 2.2 lbs

  5. Sample Problem 1 2. What is the mass of an object weighting 1200 N? Known: wt = 1200 N SolutionAnswer Wanted: mass m = 122.32 kg Formulas: wt = mag

  6. Sample Problem 2 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

  7. 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 lb/ft3 Specific weight  N/m3 lb/ft3 Torque T N-m ft-lb Impulse N • s lb • s

  8. Mechanical Loads on the Human Body • Compression • Tension • Shear • Stress • Torsion • Bending • Combined loading

  9. Sample Problem 3 • 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

  10. Sample Problem 3 • 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

  11. Sample Problem 3 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

  12. 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

  13. Repetitive vs. Acute Loads • Repetitive loading • Acute loading • Macrotrauma • Microtrauma

  14. 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

  15. Vector Algebra • Vector • Kinetic vector quantities • force weight, pressure, specific weight & torque • Kinematic vector quantities • Displacement, velocity & acceleration • Scalar quantities • Mass, volume, length & speed

  16. Resultant vector Vector #2 Vector #1 Vector Composition • Resultant vector • “Tip-to-tail” vector composition

  17. Example: A ball is thrown into the air Vertical Horizontal Vector Resolution

  18. 1 cm = 10 N Graphic Solution of Vector Problems • Graphic vector manipulation may yield approximate result 30 N = 3 cm 35 N = 4.5 cm

  19. Trigonometric Solution of Vector Problems • A more accurate procedure for quantitatively dealing with vector problems

  20. 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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