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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

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

3-2

Center of Gravity

- The point around which a bodys 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 individuals 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 ?

kg/m3 - Specific weight ? 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

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 bodys 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

- 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

Vector Resolution

Graphic Solution of Vector Problems

- Graphic vector manipulation may yield approximate

result

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

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