Graduate Biomechanics
Biomechanics of Lifting
Biomechanics of Lifting
Topics
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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
Lifting
Varied Forms and Purposes
Component of ADL’s
Occupational Task
Training for Strength Enhancement
Competitive Sport
Lifting - Forms of
Lifting Up
Lifting Down
Pushing
Pulling
Supporting
Rising to Stand
Sitting
Bending
Lifting
Why so much interest in lifting ??
Injury
Lifting
Workplace Injury
Incidence of Lifting-related Injury
• 2% of workers yearly
• 21% of all workplace injuries
• 33% of workplace health care cost
Lifting-Related Injury
Economic Impact
*** Billions ***
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
Low Back Pain
Lifting-related Injury is the
Leading Cause of Low Back Pain !
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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
Lifting
Roles of the Clinician
What Can be Done ?
** Treatment **
** Prevention **
Lifting Injury Prevention
** Many Issues **
Potential Areas Influencing Risk
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The Lifter
The Load
The Task
The Conditions
The Lifter
Factors Influencing Risk
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Anthropometrics
Strength
Endurance
Range of Motion
Technique
Sensory
Health Status
The Load
Factors Influencing Risk
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Weight
Size and Shape
Load Distribution
Grip Coupling
The Task
Factors Influencing Risk
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Complexity
Workplace Geometry
Frequency
Duration
Conditions
Factors Influencing Risk
• The Workplace Environment
Lifting Technique- Common
Elements
What do all forms of Lifting Have in Common ??
Imposed Loads
Motion - Inertia
Joint Torques
Joint Compression
Joint Shear
Internal Torque
Biomechanics of Joint Motion
The Biomechanical Model
The External Torque and intended
direction of motion determine the
Internal Torque
External Torque
If External Torque = Internal Torque… Equilibrium
If External Torque > Internal Torque… Trunk Flexion
If Internal Torque > External Torque… Trunk Extension
Biomechanics of Joint Motion
The Biomechanical Model
The External Torque is Determined by:
External Torque
Load - magnitude
Position of Load
Upper Body Mass
Position of Upper Body
Inertia
Biomechanics of Joint Motion
The 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)
Biomechanics of Joint Motion
The Biomechanical Model
The External Torque is Determined by:
COG
Axis
Moment Arm
Body Mass = 150 #
HAT = 60 % of BM
Load = 50 #
Trunk Angle = 60 deg
Moment Arm = 1.2’
Line of Gravity
Torque = (Total Load) * (cosine of Slope * Moment Arm)
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
Biomechanics of Joint Torque
External Torque
External Torque
How Much Internal Torque
is Needed to produce
Equilibrium ??
84 ft/lbs
External Torque = 84 ft/lbs
Biomechanics of Joint Torque
External Torque
Internal
Torque
External Torque
How Much Internal Torque
is Needed to produce
Equilibrium ??
84 ft-lbs
Muscle Moment
Arm = .15’
How hard do the extensor muscle have to
work to produce the needed internal
torque ????
Biomechanics of Joint Torque
External Torque
Internal
Torque
External Torque
How Much Internal Torque
is Needed to produce
Equilibrium ??
84 ft-lbs
Internal Torque = MMA * Muscle Force
Muscle Moment
Arm = .15’
84 ft-lbs = .15’ * Muscle Force
Muscle Force = 84 ft-lbs / .15’
Muscle Force = 560 lbs
Biomechanics of Joint Torque
Joint Compression
Joint Compression
Body Mass = 150#
Load = 50#
HAT = 60% of Body Mass
Moment Arm = 1.2’
Trunk Slope = 60 deg
Muscle Moment Arm= .15’
How about Joint Compression ??
Joint Compression = HAT + Load + Muscle Contraction
Joint Compression = 90# + 50# + 560#
Joint Compression = 700#
Biomechanics of Joint Torque
Joint Compression
Additional Factors
Motion – speed of lift
Rotation – Transverse Plane
Lifting Technique
COG
What can be done to decrease
low back stress ?
(1) Lighten the Load
Lifting Technique
COG
What can be done to decrease
low back stress ?
(1) Lighten the Load
(2) Change the position
of the Load
Lifting Technique
COG
What can be done to decrease
low back stress ?
(1) Lighten the Load
(2) Change the position
of the Load
(3) Change the position
of the Body
Lifting Technique
Bad
Good
COG
COG
Torque
Torque
NIOSH
National Institute for Occupational
Safety and Health
* Work Practices Guide to Manual Lifting, 1981
NIOSH
What 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
NIOSH - Hazardous Lifting
Dependent on:
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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
NIOSH Standards
Action Limit and Maximum Permissable Limit
AL:
MPL:
Tolerated by 99% of males and
75% of females
Tolerated by 25% of males and 1%
of females
L5/S1 compression below 3400N
L5/S1 compression above 6500N
Energy cost below 3.5 kcals/min
Energy cost above 5 kcals/min
**If any exceeded - some risk of
injury
**If exceeded severe risk of injury
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 redesign task