Transcript
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
top related