Transcript Chapter 6

Chapter 6
Occupational
Biomechanics
Models
Why Model?
• Models
• simple representation of complex systems
• improve understanding
• even with gross simplifications and assumptions
• rigid links vs complex anatomy of segment
• SEM vs multiple muscles of the group
• If error in the model is too large
• improve our model parameters
• % TBM in segment, CofM location
• Increase complexity
• individual muscles
Why Model?
• Biomechanical model allows to predict
potentially hazardous loading
conditions on NMS components
without actual subject risk
• manipulate parameters (loading,
geometry)
• Measure response
• compare model prediction to real world
• refine the model (limit interpretation)
Why Model?
• Biomechanical model allows to predict
potentially hazardous loading
conditions on NMS components
without actual subject risk
• Provide understanding of Guidelines to
improve efficiency and safety in the
workplace
Our application:
• estimate forces acting on different
components of the body.
• F ==> stress ==> injury
• understanding risk
• compare force (stress) predicted from
model to force (stress) known to cause
fatigue and/or injury
• Identify dangerous situations and
success of interventions.
Planar Static Biomechanical
Models
Single-body segment static model • used when no movement
no linear
or angular acceleration is present.
• standing with a load in the hands.
• moving at constant velocity (isokinetic)
• Planar analysis is limited to a 2D
analysis.
Planar Static Biomechanical
Models
• KEY: identify the magnitude of the
external forces acting on the stationary
mass
• Always ==> gravity
W = mg
• where:
• weight measured in Newtons;
• mass measured in kg;
• g is the gravitational acceleration (-9.8 m.s-2)
Forces is a vector quantity, and
has 4 characteristics:
• 1. Magnitude.
Vector
• 2. Direction (-)
• 3. Line of action.
• 4. Point of application.
Planar Static Biomechanical Models
• Consider 10kg load
in one-handed lift
• Since a=0,  forces
on the body = ???
• Some other external
force must be acting
to counter the
weight
• Obviously 2nd force
is provided by the
hand pulling up the
load
Planar Static Biomechanical
Models
• Since the object is not moving, it is
defined to be in static equilibrium.
• This means that the additive effect of all
external forces acting on a mass is zero
F = m a
dir
dir
Planar Static Biomechanical
Models
• Since the object is not moving, it is
defined to be in static equilibrium.
• This means that the additive effect of all
external forces acting on a mass is zero
F=0
dir
Planar Static Biomechanical
Models
• Since the object is not moving, it is
defined to be in static equilibrium.
• This means that the additive effect of all
external torques acting on a mass is
zeroα
T = I α
axis
axis
axis
Planar Static Biomechanical
Models
• Since the object is not moving, it is
defined to be in static equilibrium.
• This means that the additive effect of all
external torques acting on a mass is
zero
T =0
axis
Free body diagrams:
• Force vectors are scaled in the drawing
to indicate magnitude
• Vectors are orientated in the direction
of the force (tip)
• Vectors are aligned on the body to
indicate point of application and line of
action.
Solve for force in EACH hand
F
vert
=0
Identify the forces to be summed:
Weight + 2 hands = 0
2 hands = - Weight
Each hand = - Weight / 2
Each hand = - (-98) / 2
Each hand = + 49 N
Planar Static Biomechanical Models
• Determine force on each
hand to hold a 10-kg mass
in static equilibrium
• Extend the planar static
analysis to estimate the
elbow forces and moments
(torques) with forearm
horizontal
• Assumptions
• Average Anthro & Inertia
• Load applied at CofM of
hand
• Forearm/hand is a single
segment
Calculate Force at Elbow:
1st condition of equilibrium
F=0
vert
W load + W f&h + R elbow = 0
R elbow = -W load - W f&h
R elbow = - (-49) - (-15.8)
R elbow = + 64.8 N
Joint Reaction Force
R elbow = + 64.8 N
The NET tensile force created by
ligaments and muscles holding the joint
together.
MUST be present to give a = 0 (no
translational acceleration)
Forces acting on the system will
cause torque
•If eccentric to an axis
•W f&h & W load
•Not if centric to an axis
• R elbow
Calculate torque at elbow:
2nd condition of equilibrium
T=0
axis
T load + T f&h + T elbow = 0
T elbow = - (T load) - (T f&h)
T elbow = - (-49 x .355) - ( -15.8 x .172)
T elbow = - (-17.4) - (-2.7) = 20.1 Nm (CCW, flexion)
Net Joint Torque
(net moment of force)
T elbow = 20.1 Nm (CCW, flexion)
Present at EACH elbow
The NET torque created at the elbow joint by
muscles.
Which muscles?
Ignores co-contraction
MUST be present to give α = 0
(no angular acceleration)
On own:
• Calculate Rf at elbow and torque at
elbow for the segment held at the
horizontal without the hand held load.
Arm not at horizontal (20º below)
• Reaction force at
elbow is the same
because ...
• Muscle torque at
elbow is decreased
because .....
Trig? SOH CAH TOA
Two-body segment static model:
• Start at segment
with known external
force (or only one
unknown force)
• FH: W f&h, R elbow, T
f&h, T elbow
• Horizontal position
Two-body segment static model:
• Start at segment with
known external force
(or only one unknown
force)
• FH: W f&h, R elbow, T f&h, T
elbow
• Horizontal position
• Non-horizontal position
• Note elbow load from
task & posture
Extend model to nonparallel forces
• Preceding egs. have
considered gravity as
the only source of
external forces
(parallel force
systems)
• What if person is
pushing or pulling on
a load???
• Resolve force to
orthogonal components
• horizontal
• vertical
Planar static analysis of internal forces:
• Extend the model
technique to estimate
the force on various
musculoskeletal tissues
• tension within a muscle
(SEM) that creates the
observed moment of
force
• bone on bone force (not
just JRF) that accounts for
the tension in the muscle
Planar static analysis of internal
forces:
• Needed: the point of application and
the line of action of muscle(s) tendons
within the musculoskeletal structure
• Our simplification:
• concept of Single Equivalent Muscle (SEM)
• only Biceps Brachii acting at the elbow
joint, inserting 0.05 m from the axis
Solve for Biceps Muscle Force
Earlier: T load + T f&h + T elbow = 0
Becomes: T load + T f&h + T biceps = 0
Isolate:
T biceps = - (T load) - (T f&h)
Expand to:
F bi x MA bi = - (T load) - (T f&h)
F bi = (- (T load) - (T f&h)) / MA bi
Substitute:
F bi = (- (-49 x .355) - ( -15.8 x .172)) / 0.05
Solve:
F bi = 20.1 Nm / 0.05 m
F bi = 402 N
Solve for Biceps Muscle Force
Earlier: T load + T f&h + T elbow = 0
Becomes: T load + T f&h + T biceps = 0
Isolate:
T biceps = - (T load) - (T f&h)
Expand to:
F bi x MA bi = - (T load) - (T f&h)
F bi = (- (T load) - (T f&h)) / MA bi
Substitute:
F bi = (- (-49 x .355) - ( -15.8 x .172)) / 0.05
Solve:
F bi = 20.1 Nm / 0.05 m
F bi = 402 N ========> 8x > HHW
Solve for Joint Reaction Force
Earlier: W load + W f&h + R elbow = 0
Becomes: W load + W f&h + F bi + R elbow = 0
R elbow = -W load - W f&h - F bi
R elbow = - (-49) - (-15.8) - (402)
R elbow = + 64.8 - 402 N
R elbow = - 337.2 N
Joint Reaction Force
Previous: R elbow = + 64.8 N (no Biceps)
Now: R elbow = - 337.2 N
The NET compressive force pushing DOWN on
the forearm from the humerus (created by
muscle squeezing the joint together)
Minimum MUST be present to give a = 0 (no
translational acceleration)
ignores potential co-contraction
Planar static analysis of internal forces
• Technique more
complicated if
• consider > 1 muscle
Planar static analysis of internal forces
• Technique more
complicated if
• consider > 1 muscle
• Overhead Scott &
Winter, MSSE, 1991
• determine each
muscle %
contribution
• move through the
ROM (L/T)
Models used to
calculate Forces
Results
Planar static analysis of internal forces:
• Compare relative lengths of MAs
•Hand held load > muscle
•To generate equal but
opposite torque, force in
muscle must be greater
• mechanical disadvantage
Multiple-link coplanar static modeling
• Posture has no effect on
calculated JRF, but has a very
large effect on calculated JMF
•
•
•
•
• note JRF constant at 549 N
while moment increase is
approximately 10x
• Note: has not considered the
increase in JRF from muscle
tension to provide the
moment (very complex
musculature)
What muscle group active in a), b), c)??
What happens to alignment of
vertebrae?
What happens if load added to hands?
What happens if arm/ab used for
support?
Qualitative
Low Back Load
Importance:
• Since skeletal muscle responds to the
load moments (creates the calculated
net JMF)...
• simple static models give insight into
what postures require
• specific muscle groups to be active
• to what relative magnitude each specific
muscle group must be active
Dynamic Biomechanical Models
HPR 482: Advanced Biomechanics
• The introduction of motion into
biomechanical models introduces two types
of complexity
• kinematics must be quantified
• position, velocity, acceleration
• linear and angular
• must account for inertial force and inertial torque
in calculations
• F = ma when a <> 0
• T = I (alpha) when (alpha) <> 0
Dynamic biomechanical model:
• Analysis indicates the increased hazard of
performing dynamic movements.
• greater force for linear acceleration
• speed up or slow down
• greater torque for angular acceleration
• speed up or slow down
• Musculoskeletal load increases as speed of
movement increases
• greater accelerations
• Add additional mass ???
• Additional segments????
Summary of dynamic biomechanical
models:
•Prudent to encourage workers
to develop smooth movements
that reduce accelerations and
decelerations, especially if
heavy loads are being
manipulated.
Coplanar Biomechanical Models of Foot
Slip Potential While Pushing a Cart.
• Common activity in workplace
• mailroom, scrap, TVs, luggage
• Load may approach max strength
• Common time to slip
• What causes slipping???
Coplanar Biomechanical Models of Foot
Slip Potential While Pushing a Cart.
• Common activity in workplace
• mailroom, scrap
• Load may approach max strength
• Common time to slip
• What causes slipping???
• Low Friction between sole and surface
• What prevents slipping???
Coplanar Biomechanical Models of Foot
Slip Potential While Pushing a Cart.
• Common activity in workplace
• mailroom, scrap
• Load may approach max strength
• Common time to slip
• What causes slipping???
• Low Friction between sole and surface
• What prevents slipping???
• Adequate friction between sole and surface
Coplanar Biomechanical Models of Foot
Slip Potential While Pushing a Cart.
• What is friction??
• Force that tends to resist slipping
• Reflects nature of TWO surfaces
Friction = u N
“mu”
Coefficient of Friction
u=

Max Limiting Friction
Normal Reaction Force
property of two materials placed in contact.

0.2 smooth & wet
0.9 rough & dry
Coefficient of Friction
Peak Shear Force
u=
Max Limiting Friction
Normal Reaction Force
Peak Normal Force
Materials in Contact
from http://www.fearofphysics.com/Friction/frintro.html
Typical Coefficients
Sticky
Slippery
Extent of
problem of falls
Pedestrian-fall accidents
have been the second
largest generator of
unintentional workplace
fatalities, accounting for
nearly 11% and 20%,
respectively, of all fatal
and non-fatal occupational
injuries in the USA.
Redfern et al (2001). Biomechanics of
slips. Ergonomics, 44:13:1138-1166.
Workplace
falls
Friction Manipulation
Occupational Health & Safety E-News 03-17-03
Our Safety Tip of the Week is courtesy of
Manuel (Mel) Rosas, a safety consultant for
Carolinas Associated General Contractors. "I
consistently find employers do not have
procedures in place to inspect the soles of the
shoes their employees wear to work. During
walk-around inspections I ask employees to lift
up and show me the bottom of the work shoe
(boot), and I find many with worn or nearly slick
soles. Employers should address this issue to
reduce the risk of injury due to a worn shoe or
boot."
Friction Manipulation
floor design
Friction Manipulation
PVC dots on
palmar surface
Coplanar Biomechanical Models of Foot
Slip Potential While Pushing a Cart.
• To prevent slipping:
• What are the peak normal and shearing
forces expected at the foot/floor contact
point?
• Design footwear and floor surfaces that
will provide friction under these
circumstances.
• ie provide greater coefficient of friction
How does
employee alter
pushing action as
the load gets
heavier?
What does this do
to friction
requirements?
Special Purpose Biomechanical
Models of Occupational Tasks:
• Model specific areas that are prone to
overuse and/or traumatic injury
• low back
• wrist/hand
• knee
• shoulder
Low-back biomechanical models:
• NIOSH suggests using the load moment
about the lumbosacral disc (L5/S1) as the
basis for limits when
• lifting
• carrying loads
• Why L5/S1:
• 85% to 95% of all disc herniations
• loads in hands have the largest moment arms
relative to this axis
Earlier, showed
• Large increase in Net
moment at low back
with lifting
• What will happen with
load in hands?
• What about anatomical
moment??
• Muscle force: erector
spinae
• Moment arm: 0.05 m
• Abdominal pressure
• pushes torso into
extension
• Role of abdominal
muscles??
Resultant effect on vertebral column
• HUGH compressive forces
• some from the load itself (posture)
• greatest % from the muscle force
• STRESS on vertebral disk = Force / area
• effect of posture
• increase force
• decrease area
Low-back biomechanical models:
• Initial calcs: used simple back
model
• Cadavers: compression forces
created micro-fractures on
intervertebral disks
• weak spot for potential
herniation?
• Recent models incorporate:
• more muscle groups
• corrected moment arms
• relative contribution of each
muscle group
• effect on compressive force?????
Low-back biomechanical models:
• NIOSH (1981)
• recommended that predicted L5/S1 compression
values
• above 3400 N be considered potentially hazardous for
some workers.
• above 6400 N be considered hazardous for most
workers.
• Basis: repeated, large compression force may
increase risk of disc degeneration & chronic
low-back symptoms.
• NIOSH (1993): to be discussed
Summary
• Models vary in complexity
• All based on Newton II
• Require adeptness with Tables
• Require logical thinking
• CM locations, Moment Arm length
• Provide insight to joint & muscle
loading
• Underly postural & load guidelines