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
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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
3-2
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
Mass
m
Force
F
Pressure
P
Volume (solids)
V
(liquids)
Density
ρ
Specific weight
γ
Torque
T
Impulse
Metric Unit English Unit
kg
slug
N
lb
Pa
psi
m3
ft3
liter
gallon
kg/m3
N/m3
lb/ft3
N-m
ft-lb
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
3-4
3-5
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
3-6
3-7
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
3-10
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
Resultant vector
Vector #2
Vector #1
Vector Resolution
Example: A ball is thrown into the air
Vertical
Horizontal
Graphic Solution of
Vector Problems
•  Graphic vector manipulation may yield
approximate result
1 cm = 10 N
30 N = 3 cm
45 N = 4.5 cm
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