Work-Related Musculoskeletal Disorders1-ST

8/7/2019 Work-Related Musculoskeletal Disorders1-ST

http://slidepdf.com/reader/full/work-related-musculoskeletal-disorders1-st 1/52

Work-Related

Musculoskeletal Disorders

IME 549

Industrial Ergonomics

Spring 2011

Dr. Jorgensen



Material and Learning Objectives

• Book – Chapter 2, page 25

– Chapter 12, pages 215-240

• Understand how external exposures in theworkplace increase the risk of injury tointernal structures of the body

• Identify different musculoskeletal disordersand the occupational and non-occupationalcontributors



Work Related Musculoskeletal

Disorders (WMSDs)• A disorder of the muscles, nerves, tendons, ligaments,

joints, cartilage, blood vessels, or spinal discs

• WMSDs may include muscle strains and tears, ligamentsprains, joint and tendon inflammation, pinched nerves,

and spinal disc degeneration

• WMSDs include such medical conditions as: low backpain, tension neck syndrome, carpal tunnel syndrome,rotator cuff syndrome, DeQuervain’s syndrome, sciatica,

epicondylitis, tendonitis, Raynaud’s phenomenon, hand-arm vibration syndrome (HAVS), carpet layer’s knee,and herniated disc

Occupational Safety and Health Administration, Ergonomics Program Final

Rule, Federal Register, Nov 14, 2000



Magnitude of Causal Contributions to WMSDs

Biomechanical

Individual

Organizational

Social

Source: National Academy of Science (1999), Work-Related Musculoskeletal Disorders

Biomechanical – external loads acting on the body

Individual – age, gender, strength, previous injuries, etc.

Organizational – overtime, incentive systems, shiftwork, rest breaks, etc.

Social – relationships with peers, supervisors, etc.



External Exposure(Workplace Factors)

Internal Exposure(Soft Tissues)



Biomechanical Load-Tolerance

When a load exceeds a structuretolerance injury occurs

Tolerance

Ti

ssu

e

Lo

ad

Loading Pattern

Time



Biomechanical Load-Tolerance

Loading PatternTissu

e

Lo

ad Tolerance

Time

Tissue tolerance can decrease due to:

fatigue, tissue degeneration, age, etc.

Injury




Disorders

• Types of Disorders – Strain

• Injury to a muscle or tendon

•Muscles – stretched excessively, torn

fibers

• Tendons – stretched excessively

– tensile forces, torn

collagen fibers – scar tissue, surface

become rough, irritateadjacent structures




Disorders

• Types of Disorders

– Sprain

• Joint is displaced beyond

its regular range

• Ligament fibers are

stretched excessively, torn,

or pulled from the bone

• Single trauma or repetitive

loading• Heal slowly due to poor

blood supply




Disorders

• Types of Disorders

– Nerve Compression

• Pressure by bones,

ligaments, tendons,

muscles, discs (internal

sources)

• Pressure from swelling

of adjacent structures

(e.g., tendon sheaths)

(internal source)

• Hard surfaces and

sharp objects (external

sources)



• Frequency of exertion – Repetitive joint motion

• Forceful exertions – Pinch grips, forceful grips

• Segmental vibration – Power hand tools

• Repetitive forceful exertions – Combined risk factors

• Repetitive awkward postures – Combined risk factors

• Sustained awkward postures – Static loading

Upper Extremity WMSD

Risk Factors



Upper Extremity WMSD Risk FactorsRepetitive Forceful Exertions

Pinch Grips

• Desirable for precision tasks• Results in more force on tendons than power

grips to perform the same task

Power Grips• More desirable than pinch grips when force is necessary

• Results in less force on tendons than pinch grips to perform the same task



Upper Extremity WMSD Risk Factors

Segmental Vibration• Use of power tools (chain saws,

power grinders, pneumatichammers)

• Vibration increases grip force – Additional effects of gloves and

handle surface

• Increases force on tendons

• May reduce time to muscle fatigue

• Reduced diameter of arteries,impede blood supply – Vibration White Finger (Raynaud’s

phenomenon)

• Exposure to cold can aggravatesituation

• Dependent upon duration of exposure, vibration amplitude and

frequency

High frequency – low impact:

absorbed in the hand (fingers and

palm)

Low frequency – high impact:transmitted u the arm




Segmental Vibration

• Vibration absorptionand transmission

dependent on: – Frequency and

magnitude of vibration

• Drills, orbital sanders

– Low impact, high

frequency (>60 Hz)

• Rivet guns andbucking bars:

– High impact (m/s2),

low frequency (<40

Hz)

Vibration energy mostly absorbed in fingers and palm




Segmental Vibration

• Vibration absorptionand transmission

dependent on: – Frequency and

magnitude of vibration

• Drills, orbital sanders

– Low impact, high

frequency (>60 Hz)

• Rivet guns andbucking bars:

– High impact (m/s2),

low frequency (<40

Hz)

Vibration energy transmitted mostly unattenuated through wrist to elbow




Segmental Vibration

• Vibration absorption andtransmission dependent

on:

– Grip Force

• Larger grip forces increasetransmissibility and

absorption of energy

• Larger accelerations

increase grip force

– Push Force

• Higher push forces

increase transmissibility

and absorption of energy

Vibration Energy



Upper Extremity WMSD Risk FactorsRepetitive Awkward Postures

Source: Cumulative Trauma Disorders, Putz-Anderson 1988



Shoulder Flexion Shoulder Extension

Shoulder Abduction Shoulder Adduction

Elbow Flexion Elbow Extension

Upper Extremity WMSD Risk FactorsRepetitive Awkward Postures




Sustained (Static) Awkward Postures

• Vascular Compression – Sustained muscle contractions

– Increased intramuscular pressure

– Reduced blood flow through the

supplied area (ischemia) – Reduced supply of oxygen and

nutrients to the working muscles

– Reduction in removal of metabolicwaste produces

– Limits duration of muscle contraction – Decrease ability of muscle to

generate muscle force

– Impairs recovery of fatigued muscles

– Neck, shoulder, arm, hand, low back

Source: Occupational Biomechanics, Chaffin, Andersson and Martin, 2006

Time to reach significant shoulder fatigue as

A function of shoulder abduction angle



• Carpal Tunnel Syndrome• Tendinitis• Tenosynovitis

• De Quervains Tenosynovitis• Epicondylitis• Vibration White Finger

• Shoulder Tendinitis• Low Back Pain/Disorders

Work-Related Musculoskeletal

Disorders



Upper Extremity WMSDs

• Carpal Tunnel Syndrome

• Medical Definition – Reduction of conduction

velocity of electric impulsesalong the affected section of the median nerve

• Symptoms – Pain and burning, numbness,

and tingling of the thumb andtips of the first three fingers

• Internal Factors – Increased pressure in the

carpal tunnel

– Reduction of nerveconduction velocity

Hitchcock and D'Silva, 2000




• Carpal Tunnel Syndrome

– Carpal bones

– Transverse carpal ligament

• flexor retinaculum

– Flexor tendons of the fingers

• flexor digitorum superficialis

• flexor digitorum profundus

–Flexor tendon of the thumb• flexor pollicis longus

– Median nerve



Upper Extremity WMSDs• Carpal Tunnel Syndrome (CTS)

Occupational Risk Factors

– CTS can occur when the median nerveis repeatedly compressed duringmovements of the hand/wrist

• direct compression from tendons,increased pressure in carpal tunnel dueto swelling of tendon synovial sheaths

– Repetition – in combination with other risk factors

– Wrist flexion/extension - increasedcarpal tunnel pressure, tensile/normalforces on tendons

– Forceful gripping – forceful pinchgrips: force on tendons, increased

carpal tunnel pressure – Wrist acceleration - force on tendons – Segmental vibration - force on

tendons, reduced blood flow, decreasenerve conduction velocity

– Multiple causation – combination of risk factors



Upper Extremity WMSDsEpidemiological Evidence for Causation

Risk Factor StrongEvidence

Evidence InsufficientEvidence

Posture X

Repetition X

Force X

Vibration X

Combination X

Carpal Tunnel Syndrome

Bernard et al., 1997



Upper Extremity WMSDs• Tendinitis – Inflammation (“itis”)

of a tendon (Kroemer et al. 2000)

• Common locations

– Shoulder: rotator cuff, bicepstendonitis

– Elbow: lateral, medialepicondylitis

– Wrist: flexor, extensor tendons

• Repeated strain of a tendon (physicaldisruption of tendon collagen fibers)

• Highly repetitive work in combination

with other job risk factors

• Exposure to awkward wrist postures in combination with other job riskfactors

• Exposure to forceful exertions in

combination with other job risk factors

Hitchcock and D'Silva, 2000






Posture X

Repetition X

Force X

Combination X

Tendinitis





• Tenosynovitis

– Inflammation of a tendon sheath

– Tendon surfaces can become

irritated, rough, bumpy

– Repetitive movement of the

tendon with irregular surface

may irritate the tendon sheath

– Movement of a tendon inside

the tendon sheath may be

impeded




• DeQuervain’s Tenosynovitis

– Tenosynovitis of the tendon

sheath of the abductor pollicis

longus and extensor pollicis

brevis – Prolonged or repeated

movements or deviations of the

wrist with the thumb fixed on an

object

• Moore (1992) – Combined forceful gripping and

hand twisting

• Kroemer et al. (2000)




• Epicondylitis – Irritation of

tendons attaching to

epicondyles of the humerus.

– Lateral epidondylitis – related tomotions that tense the wrist’s

extensor and supinator muscles

– Repeated gripping

– Repeated forceful

pronation/supination

– Forceful wrist extension

– Impact of jerky throwing motions






Posture X

Repetition X

Force X

Combination X

Elbow





• Vibration White Finger

– Insufficient blood supply

to digits

– Prolonged gripping of vibrating tools, especially

in cold environments






Vibration X

Hand-arm Vibration Syndrome





• Shoulder tendonitis – Rotator cuff tendonitis –

• Rotator cuff: shoulder

abduction, medial/lateral

shoulder rotation

• Tendonitis or tear of thecommon tendon that

makes up the rotator cuff

in the shoulder

– Repeated or prolonged

elevation of the arm• Shoulder abduction

• Shoulder flexion

– Impact motions

• Throwing objects






Posture X

Repetition X

Force X

Vibration X

Shoulder




0

100

200

300

400500

600

1998 1999 2000 2001 2002 2003

Private Industry WMSD Lost D

Cases (thousands)

WM SDs Back Injury CTS Tendinitis

Source: Bureau of Labor Statistics Annual Survey



Low Back Disorders

Psychosocial Factors

• Job satisfaction

• Job control –

decision latitude• Relationships at work

– Co-workers and

supervisors

• Mental concentration

• Mental stress

• Increased paraspinal

muscle activity

• Increased complexspinal loading

• Release of pain

producing substances

• Increased likelihood of

reporting LBP?



Low Back Disorders


• Manual materials handling – Lifting, Lowering, Pushing, Pulling, Carrying

• Moment – Increase in the weight of the load, the distance of the load from

the body, or both during handling

• Awkward torso postures – Forward and side bending – Twisting – Duration of awkward postures

• Torso motion – Velocity of torso motion

• Vibration – Exposure to whole body vibration



Spinal Loading

Torso muscles – Various muscles within thetorso provide the forcesnecessary for motion

– Torso muscles develop

force vectors in differentdirections – Magnitude of muscle force

depends on size of themuscle, direction of motion,

weight of load, dynamics,and posture – Torso muscles generate

compressive and shearingforces on the intervertebraldiscs



Spinal Loading

• Torso muscles – Erector spinae (major extensor muscle)

– Latissimus dorsi (extensor, twist, lateral bending)

– Rectus abdominis (major flexor muscle)

– External obliques (flexor, extensor, twist, lateralbending)

– Internal obliques (flexor, extensor, twist, lateralbending)

• Biomechanical models predict muscle forcesand direction of the forces, then estimate theforces on the intervertebral discs





Erector Spinae Group



Torso Muscles

Rectus Abdominis External Obliques Internal Obliques

Low Back Disorder



Low Back Disorder


• External Moment

– Most significant occupationalLBD risk factor

– Increase in weight, distanceof load from the body, or both

– Increase the internallygenerated moment

– Increased muscle coactivity

– Increased forces on the spine

Internal Force External Force



Low Back Disorder


Torso Flexion

T Fl i I t l St t



Torso Flexion – Internal Structures• Lumbar spine flattens, hip flexion

then pelvis rotation

• Vertebral bodies rotate

• Posterior spinal ligaments

stretched

• Strain on posterior annulus

fibrosis fibers

• Torso extensor muscles lengthen – Macintosh et al. 1993

• Muscle fibers align with spinal

column – Macintosh et al. 1993

– McGill et al. 2000

• Moment arm at L5/S1 decreases

– Macintosh et al. 1993

– Jorgensen et al. 2003



Muscle Length-Tension

Source: Occupational Biomechanics, Chaffin, Andersson and Martin 1999

L B k Di d



Low Back Disorder

Occupational Risk Factors• Torso Flexion

– Forward bending of thetorso – lumbar spineflattens

– Torso muscleslengthen

– Posterior spinalligaments stretched

– Strain on posterior annulus fibrosis fibers

– Increase the internallygenerated moment

– Increased musclecoactivity

– Increased compression

and shear forces onthe s ine

Low Back Disorder



Low Back Disorder


• Torso Twisting – Compression and

tension on facet joints

– Increased torso musclecoactivity

– Increased compressionand shear forces on thespine

Low Back Disorder



Low Back Disorder


• Torso Motion

– Fast torso motionduring material handling

– Increased torso muscle

coactivity – Increased compression

and shear forces on thespine



Low Back Disorder


• Unexpected loading – Sudden unexpected

loadings of the body

may lead to overexertion

injury – Large muscle forces as

a result of unexpected

loading

– May include lifting an

unexpectedly light or heavy load



Low Back Disorder


• Static Awkward Postures – Increases fatigue of torsomuscles

– In forward flexed postures,ligaments become lax –increases the loading placed

on the intervertebral discs

• Whole Body Vibration – Spine wants to remain stable

– WBV increases torso musclecoactivation to maintain

stability of the spine – Muscle coactivation Increases

loading on the intervertebraldiscs

– Continued WBV exposure canresult in torso muscle fatigue



Low Back WMSDsEpidemiological Evidence for Causation



Lifting/Forceful

Movement

X

Whole bodyvibration

X

Awkward posture X

Heavy physicalwork

X

Static workposture

X

Low Back

Documents

Work-Related Musculoskeletal Disorders1-ST