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