Biomechanical Basis of Movement

Kinetic Concepts for Analyzing Human Movement

Goals for Chapter 3

Know units of measure for mass, force, weight, pressure, volume, density, specific weight, torque, and impulse.

Know the mechanical loads placed on the body.

Be familiar with equipment used for measuring kinetic quantities.

Goals for Chapter 3

Know the difference between vectors and scalars.

Solve quantitative problems using vectors and trigonometry.

Why study kinetics?

The human body makes forces and resists forces during daily activities and sporting events.

Basic Concepts Related to Kinetics

Biomechanists must understand the concepts of mass, inertia, weight, pressure, volume,…..to understand the effects of forces.

Mass and Inertia

Mass-

Units: kg, slug

Inertia-

Dependent on

Force

Force: a push or pull; mechanical disturbance or load that tends to produce an acceleration of a body in the direction of its application. Forces may deform an object, change its state of

motion, or both. the product of mass and acceleration. magnitude,

direction, and point of application: F=ma; units: N, lb 1lb = 4.45 N

Force Example

A 3kg mass is accelerating at 2m/s2. How much force is pushing the mass?

Net Force

Net force-

net force=0

net force0

Free Body Diagram Free body-

Free body diagram-

Weight

Bat Force Air Resistance

Help identify the forces and moments acting on individual parts of a system to ensure the correct use of the equations of mechanics to analyze the system.

Free Body Diagram

Free Body Diagram

Weight

Weight-

Wt=mag

ag=-9.81m/s2 (negative denotes direction)

units: N, lb

Weight Example

If Bill has a mass of 91kg. How much does he weigh (in Newtons)?

Weight Example

Bill has a mass of 91kg. How much does he weigh in pounds?

Weight Example

A lineman weighs 1400N. What is his mass (in kg)?

Center of Mass

COM

COM COG

Pressure

Pressure-p=F/Aunits: N/cm2, Pa

Example:Rearfoot strikers land with 500lb of force for each

stride, and over a small contact area- HIGH PMidfoot strikers land with the same force over a

larger contact area- LOWER P

Pressure Example

I weigh 854.7N. If I stand on one foot and the area on the bottom of my shoe is 230cm2, what is the pressure under my shoe?

Solution

Volume

Volume-units: length3 (m3, cm3, in3)

Density and Specific Weight

Density-=mass/volume

units: kg/m3

Specific weight-Spec wt=weight/volume

units: N/m3, lb/ft3

Torque

Torque-Also called

T=Fd

units: Nm

Impulse

Impulse-

Impulse=Ft

units: Ns

Impulse Example

A Biomechanics student (tired from studying) takes a nap and uses his 10N book as an eye-shade. His head has a mass of 7kg. If the student sleeps for an hour with the book on his face, what is the impulse applied?

Solution

Another Impulse Example

A car accident creates an impulse of 36000Ns. If the impact takes 0.5s, how much force is applied to the passenger?

Stress

Stress-

Internal pressure

stress=F/A

normal values:10-20N/cm2

Axial/ Normal stress (σ):

Tensile stress:

Compressive stress:

Shear stress (τ):

Stress Example

How much compressive stress is on my L1,L2 intervertebral disc?45% of my body weight is supported by the disk

body weight=892.7N

area of the disc is 25cm2

stress=F/A

Solution

Another Example

I was doing yard work a couple of weeks ago (spreading rocks). How much stress is on the L1,L2 intervertebral disc when I was carrying a 178N bag on each shoulder?45% of my body weight is supported by the disk

body weight=892.7N

area of the disc is 25cm2

Another Solution

Strain:

Normal (ε):

ε= Δl / l

P= proportionality limit Stress and strain

proportional E= elastic limit Y= yield point U= highest stress point

(ultimate strength) R= failure point

Mechanical Loads on the Human Body

Compression Tension Shear Stress Bending

LOAD=

Axial Torsion Combined Repetitive Acute

Compression

Compression-

Tension

Tension-

Shear

Shear-

Bending Bending-

Failure on

Axial

Axial-

Same as

Torsion

Torsion-

Combined Loading

Combined loading-

The Effects of Loading

material properties important

Load-deformation Curve

Deformation

PlasticRegion

UltimateFailurePoint

YieldPoint

ElasticRegion

Lo

ad

Deformation

PlasticRegion

UltimateFailurePoint

YieldPoint

ElasticRegion

Lo

ad

Basic Concepts

Elasticity:

Linear elastic material: stress is

linearly proportional to strain.

Young’s Modulus (E) :

ε

σ

Plasticity -

In general, materials undergo plastic deformations following elastic deformations when they are loaded beyond their yield points.

Viscoelasticity:

Elastic materials

Stress is a function of strain only. No time dependent behavior.

Viscoelastic materials:

Viscoelastic response:

Extent of Deformation:

Ductile-

Brittle -

Toughness:

Larger area: tougher.

Resilience:

Repetitive Loading

Repetitive loading-

Acute Loading

Acute loading-

Repetitive vs Acute:Likelihood of Injury

Likelihood of Injury

Frequency of LoadingL

oad

Ma

gnitu

de

Over-use injuries Single force

macrotrauma REPETITIVE

Shin splints Stress fractures Tendonitis

MACROTRAUMA Ligament tear Bone Fractures

Tools for Measuring Kinetic Quantities

EMG Does it belong here? Measures

Dynamography

Plantar Pressure

Measures

Applications:

Plantar Pressure

Practice problems for exam 1

Chapter 1 Additional: 5-7,8(a),10

Chapter 3 Introductory: 1,2,6 Additional: 1,3-6

Chapter 4 Introductory: 8,9