Transcript BIOMECHANICS OF WORK

BIOMECHANICS OF WORK
The Musculoskeletal System
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Bones, muscle and connective tissue
supports and protects body parts
maintains posture
allows movement
generates heat and maintains body
temperature
Bones
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206 bones
Body “framework”
Protective: rib cage and skull
Provide for action: arms, legs
linked at joints by tendons and ligaments
Tendons: connect bone to muscle
Ligaments: connect bone to bone
Joints
• Connection of two or more bones
• Movement
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no mobility joints
hinge joints (elbow)
pivot joints (wrist)
ball and socket joints (hip and shoulder) 3DOF
Muscles
• 400 muscles
• 40-50% of your body weight
• half of your body’s energy needs
Muscles
Muscle Composition
• bundles of muscle fibres, connective tissue
and nerves
• fibres are made of long cylindrical cells
• cells contain contractile elements
(myofibrils)
• both sensory and motor nerves
• motor nerves control contractions of groups
of fibres (motor unit)
Muscle Contraction
• Concentric: muscle contracts and shortens
• Eccentric: muscle contracts and lengthens
(overload)
• Isometric: muscle contracts and stays the
same length
Muscle Strength
• proportional to muscle cross-section
• usually measured as torque
– force applied against a moment arm (bone) to
an axis of rotation (joint)
• Static strength: measured during isometric
contraction
• Dynamic strength: measured during
movement
Basic Biomechanics
• Statics model (F=0,  Moments=0),
isometric contraction
• Force at the point of application of the load
• Weight of the limb is also a force at the
center of gravity of the limb
• F can be calculated
Problem in Text
20kg
Person holding a 20kg
weight in both hands.
What are the force and
moment at the elbow?
Given:
Mass =20kg
Force of segment = 16N
Length of segment = .36m
Assume:
COG of segment is at the
midpoint!
Problem in Text
1. Convert mass to Force
20kg*9.8 m/s2 = 196 N
2. Divide by # of hands.
196N/2 hands = 98N/hand
98 N
Problem in Text
1. Convert mass to Force
20kg*9.8 m/s2 = 196 N
Felbow
2. Divide by # of hands.
16 N
196N/2 hands = 98N/hand
98 N
3. Calculate F elbow.
F=0
Felbow – 16N – 98N = 0
Felbow= 114N [up]
Problem in Text
1. Convert mass to Force
Felbow
20kg*9.8 m/s2 = 196 N
.36m
.18m
2. Divide by # of hands.
196N/2 hands = 98N/hand
16 N
3. Calculate F elbow.
F=0
98 N
Felbow – 16N – 98N = 0
Felbow= 114N [up]
4. Calculate M elbow.
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elbow-16N*.18m +(-98N)*.36m=
elbow=38.16N*m
Multi-segment models
• Repeat for each segment, working the
forces and moments back
• How would you work out the Force and
Moment in the shoulder?
• What information would you need?
Lower Back Pain
• estimated at 1/3 of worker’s compensation
payments
• may affect 50-70% of the population in
general
• Both in high lifting jobs and jobs with
prolonged sitting
Biomechanics of Lower Back
Pain
• Calculation in text
• Back must support many times the lifted
load, largely due to the moment arms
involved
• Calculation of compressive forces vs.
muscle strength can identify problems
NIOSH Lifting Guide
• Sets numbers that are associated with risk of
back injury
• Two limits (single lifts)
– Action limit (AL): small proportion of the
population may experience increased risk of
injury
– Maximum permissible limit (MPL): Most
people would experience a high risk of injury.
3xAL
NIOSH Lifting Guide
• Recommended Weight Limit (RWL): a load
value that most healthy people could lift for
a substantial period of time without an
increased risk of low back pain
• Biomechanical criteria
• Epidemiological criteria
• Physiological criteria
Lifting Equation
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RWL=LCxHMxVMxDMxAMxFMxCM
LC: load constant, maximum recommended weight
HM: horizontal multiplier, decreases weight with distance from spine
VM: vertical multiplier, lifting from near floor harder
DM: distance multiplier, accommodates for vertical distance that must
be lifted
• AM: assymetric multiplier, reductions for torso twisting
• CM: coupling modifier, depends on whether loads have handles for
lifting
• FM: frequency modifier, how frequently is the load lifted
Lifting Equation
• Multipliers can all be obtained from tables
(11.1, 10.2, 10.3, 11.2, 11.3)
• Multipliers are unitless
• Multipliers are always less than or equal to
1 (they reduce the maximum load or load
constant)
Example in the Text
• A worker must move boxes from 1
conveyor to another at a rate of 3
boxes/minute. Each box weighs 15lbs and
the worker works for 8 hours a day. The
box can be grasped quite comfortably. The
horizontal distance is 16 inches, the vertical
is 44 inches to start and 62 inches to finish.
The worker must twist at the torso 80
degrees.
Information
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h=16”
v=44”
d=18”
A=80degrees
F=3 lifts/minute
C=good
job duration = 8 hours/day
weight = 15lbs
Multipliers
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HM (T11.1): 10/h=10/16=.625
VM (T11.1):(1-.0075|v-30|)=.895
DM (T11.1): (0.82+1.8/d)=0.82+1/8/18=.92
AM (T11.1): 1-.00032a=1-.00032x80=.744
FM(T11.2): 0.55 (v<75, work 8hrs, 3lifts)
CM (T11.3): 1 (good, v<75cm)
Calculation of RWL
• RWL=LCxHMxVMxDMxAMxFMxCM
• RWL=51lbx.625x.895x.92x.744x.55x1
• RWL= 10.74lbs
• The load is greater than the RWL so there is
a risk of back injury.
Designing to avoid back pain
• More importantly, NIOSH equation gives
ways to reduce injury
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reduce horizontal distance
keep load at waist height
reduce distance to be travelled
reduce twisting
add handles
reduce frequency of lifts