Biomechanics of Lifting

Graduate Biomechanics

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

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

Injury

Why so much interest in lifting ??

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

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

LiftingRoles of the Clinician

** Treatment **

What Can be Done ?

** Prevention **

Lifting Injury Prevention

** Many Issues **

Potential Areas Influencing Risk

• The Lifter

• The Load

• The Task

• The Conditions

The LifterFactors Influencing Risk

• Anthropometrics

• Strength

• Endurance

• Range of Motion

• Technique

• Sensory

• Health Status

The LoadFactors Influencing Risk

• Weight

• Size and Shape

• Load Distribution

• Grip Coupling

The TaskFactors Influencing Risk

• Complexity

• Workplace Geometry

• Frequency

• Duration

ConditionsFactors 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

Biomechanics of Joint MotionThe Biomechanical Model

External Torque

The External Torque and intended direction of motion determine the Internal Torque

Inte

rnal

Tor

que

If External Torque > Internal Torque… Trunk FlexionIf Internal Torque > External Torque… Trunk Extension

If External Torque = Internal Torque… Equilibrium

Biomechanics of Joint MotionThe Biomechanical Model

Load - magnitude Position of Load Upper Body Mass Position of Upper Body Inertia

External Torque

The External Torque is Determined by:

Biomechanics of Joint MotionThe Biomechanical Model

The External Torque is Determined by:

COG

Axis

Line of Gravity

Moment Arm

Torque = (Total Load) * (cosine of Slope * Moment Arm)

Total Load = Mass of HAT + External Load

Biomechanics of Joint MotionThe Biomechanical Model

The External Torque is Determined by:

COG

Axis

Line of Gravity

Moment Arm

Torque = (Total Load) * (cosine of Slope * Moment Arm)

Body Mass = 150 # HAT = 60 % of BM Load = 50 # Trunk Angle = 60 deg Moment Arm = 1.2’

Biomechanics of Joint TorqueExternal Torque

Body Mass = 150#Load = 50#HAT = 60% of Body MassCOG Distance = 1.2’Trunk Slope = 60 deg

Torque = (Total Load) * (cosine of Slope * Moment Arm)

Torque = (90# + 50# ) * (.5 * 1.2’ )

External Torque = 84 ft/lbs

External Torque

Biomechanics of Joint TorqueExternal Torque

External Torque = 84 ft/lbs

External Torque

How Much Internal Torque is Needed to produce Equilibrium ??

84 ft/lbs

Biomechanics of Joint TorqueExternal Torque

External Torque

How Much Internal Torque is Needed to produce Equilibrium ??

84 ft-lbs

How hard do the extensor muscle have to work to produce the needed internal torque ????

Muscle Moment Arm = .15’

Internal Torque

Biomechanics of Joint TorqueExternal Torque

External Torque

How Much Internal Torque is Needed to produce Equilibrium ??

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’

Internal Torque

Biomechanics of Joint Torque Joint Compression

Body Mass = 150#Load = 50#HAT = 60% of Body MassMoment Arm = 1.2’Trunk Slope = 60 degMuscle Moment Arm= .15’

Joint Compression = HAT + Load + Muscle ContractionJoint Compression = 90# + 50# + 560#

Joint Compression = 700#

Joint Compression

How about Joint Compression ??

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

TorqueTorque

Bad Good

COG

COG

NIOSH

National Institute for Occupational Safety and Health

* Work Practices Guide to Manual Lifting, 1981

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

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

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

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