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
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
Basic Concepts Related to Kinetics
• Inertia
• Mass
• Force
– Free body diagram
• Center of Gravity
• Weight
• Pressure
• Volume
• Density
• Specific weight
• Torque
• Impulse
Inertia
• Tendency for a body to resist a change in its state of motion
Mass
• Quantity of matter contained in an object
Force
• Push or pull
• the product of mass and acceleration
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 body’s weight is equally balanced, no matter how the body is positioned
Weight
• Attractive force that the earth exerts on a body
Sample Problem 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
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
Pressure
• Force per unit of area over which the force acts
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
Volume
• Space occupied by a body
Density
• Mass per units of volume
Specific Weight
• Weight per unit of volume
Torque
• Rotary effect of a force
Impulse
• Product of force and the time over which the force acts
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 ρ kg/m3
Specific weight γ N/m3 lb/ft3
Torque T N-m ft-lb
Impulse N • s lb • s
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 body’s 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
Sample Problem 4
a) 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
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
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
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
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
Vector Composition
• Resultant vector
• “Tip-to-tail” vector composition
Vector #1
Vector #2
Resultant vector
Vector Resolution
Vertical
Horizontal
Example: A ball is thrown into the air
Graphic Solution of Vector Problems
• Graphic vector manipulation may yield approximate result
30 N = 3 cm
45 N = 4.5 cm
1 cm = 10 N
Trigonometric Solution of Vector Problems
• A more accurate procedure for quantitatively dealing with vector problems
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.
The End