Biomechanics of Lifting

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Biomechanics of Lifting

• Graduate Biomechanics

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Biomechanics of LiftingTopics

• Lifting and Back Injury
• Biomechanics of Joint Torque and Shear
• Standards for Evaluating Lifting Tasks
• Biomechanical Factors Determining Joint Stress
• NIOSH and Evaluation of Lifting Risk

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Lifting Varied Forms
and Purposes
Component of ADLs Occupational Task Training for
Strength Enhancement Competitive Sport
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Lifting - Forms of

• Lifting Up
• Lifting Down
• Pushing
• Pulling
• Supporting
• Rising to Stand
• Sitting
• Bending

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Lifting
Why so much interest in lifting ??

• Injury

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Lifting Workplace Injury

• Incidence of Lifting-related Injury
• 2 of workers yearly
• 21 of all workplace injuries
• 33 of workplace health care cost

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Lifting-Related Injury
Economic Impact Billions
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Common Sites for Lifting Related Injury

• Incidence Rates (i.e. frequency of injury)
• 1 Low Back
• 2 Wrist and Hand
• 3 Upper Back
• 4 Shoulder
• 5 Knee
• 6 Elbow

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Low Back Pain
Lifting-related Injury is the Leading Cause of
Low Back Pain !

• Second leading cause of physician visits
• Third ranking cause of surgery (250,000 yearly)
• Fifth ranking cause of hospitalization
• 15 of adults experience episode each year

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LiftingRoles of the Clinician
What Can be Done ?
Treatment
Prevention
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Lifting Injury Prevention

• Many Issues

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Potential Areas Influencing Risk

• The Lifter
• The Load
• The Task
• The Conditions

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The LifterFactors Influencing Risk

• Anthropometrics
• Strength
• Endurance
• Range of Motion
• Technique
• Sensory
• Health Status

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The LoadFactors Influencing Risk

• Weight
• Size and Shape
• Load Distribution
• Grip Coupling

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The TaskFactors Influencing Risk

• Complexity
• Workplace Geometry
• Frequency
• Duration

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ConditionsFactors Influencing Risk

• The Workplace Environment

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Lifting Technique- Common Elements

• What do all forms of Lifting Have in Common ??
• Imposed Loads
• Motion - Inertia
• Joint Torques
• Joint Compression
• Joint Shear

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Biomechanics of Joint MotionThe Biomechanical
Model
The External Torque and intended direction of
motion determine the Internal Torque
Internal Torque
External Torque
If External Torque Internal Torque Equilibrium
If External Torque gt Internal Torque Trunk
Flexion
If Internal Torque gt External Torque Trunk
Extension
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Biomechanics of Joint MotionThe Biomechanical
Model
The External Torque is Determined by
Load - magnitude Position of Load Upper
Body Mass Position of Upper Body Inertia
External Torque
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Biomechanics of Joint MotionThe Biomechanical
Model
The External Torque is Determined by
COG
Total Load Mass of HAT External Load
Axis
Moment Arm
Line of Gravity
Torque (Total Load) (cosine of Slope Moment
Arm)
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Biomechanics of Joint MotionThe Biomechanical
Model
The External Torque is Determined by
Body Mass 150 HAT 60 of BM Load
50 Trunk Angle 60 deg Moment Arm
1.2
COG
Axis
Moment Arm
Line of Gravity
Torque (Total Load) (cosine of Slope Moment
Arm)
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Biomechanics of Joint Torque External Torque
Body Mass 150
Load 50
HAT 60 of Body Mass
COG Distance 1.2
Trunk Slope 60 deg
External Torque
Torque (Total Load) (cosine of Slope Moment
Arm)
Torque (90 50 ) (.5 1.2 )
External Torque 84 ft/lbs
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Biomechanics of Joint Torque External Torque
How Much Internal Torque is Needed to produce
Equilibrium ??
External Torque
84 ft/lbs
External Torque 84 ft/lbs
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Biomechanics of Joint Torque External Torque
Internal Torque
How Much Internal Torque is Needed to produce
Equilibrium ??
External Torque
84 ft-lbs
How hard do the extensor muscle have to work to
produce the needed internal torque ????
Muscle Moment Arm .15
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Biomechanics of Joint Torque External Torque
Internal Torque
How Much Internal Torque is Needed to produce
Equilibrium ??
External Torque
84 ft-lbs
Internal Torque MMA Muscle Force 84 ft-lbs
.15 Muscle Force Muscle Force 84 ft-lbs /
.15 Muscle Force 560 lbs
Muscle Moment Arm .15
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Biomechanics of Joint Torque Joint
Compression
Body Mass 150
Load 50
HAT 60 of Body Mass
Moment Arm 1.2
Trunk Slope 60 deg
Joint Compression
Muscle Moment Arm .15
How about Joint Compression ??
Joint Compression HAT Load Muscle
Contraction Joint Compression 90 50
560 Joint Compression 700
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Biomechanics of Joint Torque Joint
Compression
Additional Factors Motion speed of lift
Rotation Transverse Plane
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Lifting Technique
What can be done to decrease low back stress ?
COG

• Lighten the Load

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Lifting Technique
What can be done to decrease low back stress ?
COG

• Lighten the Load
• Change the position of the Load

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Lifting Technique
What can be done to decrease low back stress ?
COG

• Lighten the Load
• Change the position of the Load
• Change the position of the Body

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Lifting Technique
Bad
Good
COG
COG
Torque
Torque
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NIOSH

• National Institute for Occupational Safety and
  Health
• Work Practices Guide to Manual Lifting, 1981

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NIOSHWhat do they do ??

• Define risk associated with lifting
• Define safe lifting conditions
• Publish lifting guidelines and standards for the
  workplace
• Inspect workplace for safe lifting conditions
• Impose penalties for hazardous lifting conditions

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NIOSH - Hazardous Lifting Dependent on

• Weight of Object
• Location of Object COM at beginning of lift
• Vertical travel distance of object
• Frequency of Lift (lifts per minute)
• Duration of lifting

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NIOSH StandardsAction Limit and Maximum
Permissable Limit

• AL
• Tolerated by 99 of males and 75 of females
• L5/S1 compression below 3400N
• Energy cost below 3.5 kcals/min
• If any exceeded - some risk of injury

• MPL
• Tolerated by 25 of males and 1 of females
• L5/S1 compression above 6500N
• Energy cost above 5 kcals/min
• If exceeded severe risk of injury

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NIOSH Standards

• Below AL - Stress tolerated by most workers
• Above AL and below MPL - Risk of injury such that
  task re-design or change in worker may be
  necessary
• Above MPL - Unacceptable risk...Must re-design
  task