5 biomechanics

8/2/2019 5 Biomechanics

1/13

About 1/3 of U.S. workers perform tasks thatrequire high strength demands

Large variations in population capacity (strength)

Basis for understanding and preventingoverexertion injuries

simulate design alternatives

Wide application potential Orthopaedics

Rehabilitation

Sports science

Vehicles

maintain: D < C D: task Demands (forces and moments) C: human Capability (strength, tissue

tolerance)

ot va ues are g y var a e an cu t tomeasure and predict

"strength" = not one thing!; here, typicallyuse max. joint moment (and is a function ofposture, time, etc.)

The annual incidence of WMSDs in all industries is about

300-400 per 100,000 workers for all workers; substantial

differences between industries.

A recent NIOSH study estimated 22% of all VDT workers

a some ype o pro em.

Average Worker's Compensation cost per claim and total

costs per injury:

All WMSDs: $14,726/$20,000

Carpal Tunnel Syndrome: $29,000/$100,000

Enormous increase in complaints: 1983->1988, from 35%

to 67% of all occupational illnesses. Recent numbers have

stabilized around 25-30%.


2/13

Source Total Cost ($) Notes

In-plant med ical visitsand treatment s

14, 050 ~$50 pe r v isit

Emplo yee ab sences 127 ,905 Each 1 -week absencerequired 1 r eplacement w orker

Work restriction s 16, 192 1/2 of the work restrictions

Job c hanges initiatedby employe e

13, 9 84 E ach j ob c hange r eq uir edretraining fo r 2 workers

Tota l 172 ,131

Estimated costs associated with 93 cases of shoulder disordersreported to the plant medical department of an automobileassembly plant.

From: Punnett, L. et al. (2000) Scand J Work Environ Health

Meat Packers Thigh Boning Operations

thighs suspended 44" from floor

1 thigh every second turkey

one hand grabs thigh

X

knife hand 2-3 cuts to separate meat

knife hand 1 cut to separate tendon

7,560 turkeys per shift

Keyboard Entry Work keys/shift: very low force, very high reps

Assembly in electronics and light manufacturing

Sewing Operations

Packing Operations

X

From: Business Week, 02/23/99

De Quervain's Disease -- Stenosingtendinitis of the flexor tendons of thethumb.

From Putz-Anderson, V. (1988). CumulativeTrauma Disorders: A Manual for MusculoskeletalDiseases of the Upper Limbs. London, UK: Taylor& Francis., Fig. 3


3/13

Tendon injuries can produce localizedswelling in the area of the injury.

The inflammation associated with tendoninjuries can interfere with nerve function andeven permanently harm nerve tissue.

Carpal Tunnel Syndrome (CTS): Tendonitis ofthe flexor tendons of the fingers as they passthrough the carpal tunnel resulting incompression of the median nerve.

Carpal Tunnel Syndrome accounts for lessthan 10% of the total number of WMSD cases.

X

X

Back injuries account for nearly 20% of all injuries and illnesses in

the workplace and nearly 30% of all workers compensation

payments.

The lifetime incidence of low-back pain is estimated to be 60% with

about 4% incapacitated for over 6 months. Annual and point

prevalences are 40% and 20%.

It has been estimated that 6.5 million people stayed home each day

due to low-back injuries, with 1.5 million new injuries per month.

Chronic low-back pain affected ~35 million people.

The cost to U.S. industry includes 170 to 240 million lost work days

each year and ~$5 billion in workers compensation payments.

The total direct costs of back injuries is ~ $30 billion, with indirectcosts possibly doubling that figure.


4/13

Cervicalvertebrae

Thoracicvertebrae

Lumbarvertebrae

Sacral/Coccygealvertebrae

L5/S1

Nordic Questionnaire

Biomechanics Models

Material Handling: NIOSH Guideline forManual Lifting Task

Quick Assessment Tool

Hanya dijawab jika jawaban pada kolom 1 ya

Apakah Anda mempunyai keluhan (nyeri,ngilu, pegal) selama 12 bulan terakhir pada

anggota tubuh berikut?

Apakah dalam 12

bulan terakhir,

masalah tersebutmengakibatkan

Anda tidak dapat

bekerja secara

normal?

Apakah Anda

mempunyai

masalah yangsama dalam 7

hari terakhir?

Apakah menurut

Anda masalah

tersebutberhubungan

dengan pekerjaan

Anda di sini?

Leher tidak ya tidak ya tidak ya tidak ya

Bahu tidak ya, sebelah kanan tidak ya tidak ya tidak ya

ya, sebelah kiri

ya, keduanya

Siku tidak ya, sebelah kanan

ya, sebelah kiri

ya, keduanya

tidak ya tidak ya tidak ya

Pergelangan tangan

tidak ya, sebelah kanan

ya, sebelah kiri

ya, keduanya


Punggung Atas tidak ya tidak ya tidak ya tidak ya

Punggung Bawah tidak ya tidak ya tidak ya tidak ya

Paha tidak ya(salah satu atau keduanya)


Lutut tidak ya

(salah satu atau keduanya)


Pergelangan kaki tidak ya(salah satu atau keduanya)



5/13

Two-DimensionalStatic

Two-DimensionalDynamic

Three-DimensionalStatic

Three-DimensionalDynamic

Translational equilibrium (a = 0):

Rotational equilibrium ( = 0):

Forces = 0

Moments = 0

Free-body diagrams are schematicrepresentations of a system, identifying allforces and all moments acting on thecomponents of the system.

Here, we will differentiate between external andinternal forces and moments (see below)

Anything can be chosen as the free body!

Examples: whole body, arm, hand,

Choose wisely, and solving biomechanics

problems becomes much easier.

Resultant or External what the world does to the body

gravity

contact loading (e.g. with ground)

Reactive or Internal w a e o y oes n response

muscle activation

ligament stretch

joint contact forces

In equilibrium: Resultant + Reactive = 0

F = 0 M = 0Reactive = - Resultant (Or, internal = -external)


6/13

External Forces and Moments

F, M

Internal Forces and Moments

F, M

+Y

+X

Equilibrium:

F = 0 F + F = 0 F = -F (Translational Equilibrium)

M = 0 M + M = 0 M = -M (Rotational Equilibrium)

+Z

X

17.0 cm

35.0 cm

180 N

10 N

From Chaffin, DB et al (1999) OccupationalBiomechanics. Fig 6.2

X

From Chaffin, DB et al (1999) OccupationalBiomechanics. Fig 6.7

10 N180 N

FB?

5 cm

HANDCOMELBOW

Unknown values: Biceps and external elbow force (FB and FE), and any joint contact

force between upper and lower arms (FJT, an internal force) External elbow moment (ME), and internal ME

Lower arm selected as free body Isolates elbow forces and moments Results in a single unknown (see below)

35.0 cm17.0 cm


7/13

1. Establish coordinate system (signconvention)

2. Draw Free Body Diagram, including knownand unknown forces/moments

3. Solve for external moment(s) at joint

4. Determine net internal moment(s), and solvefor unknown internal force(s)

5. Solve for external force(s) at joint [can alsobe done earlier]

6. Determine net internal force(s), and solve forremaining unknown internal force(s)

FBD:

E H

WLA

=mLA

g=10N

FH=m

Hg=

180N

FB=??

FJT

=??

ME=??

+Y

+X

+Z

_ _ME = 0 = ME + ME -> ME = -ME

ME = MLA + MH = (WLA x maLA) + (FH x maH)

ME = (-10 x 0.17) + (-180 x 0.35) = -1.7 -

63

ME = -64.7 Nm (or 64.7 Nm CW)ME = -ME -> ME = 64.7

ME = (FJT x maJT) + (FB x maB) = FB x 0.05

FB = 1294 N (up)

_ _

Externalmoment is dueto externalforces

Internalmoment is due

to internalforces

_ = 0

FE = 0 = FE + FE -> FE = -FE

FE = WLA + FH = -10 + (-180)

FE = -190 N (or 190 N down)

FE = - FE -> FE = 190

_ _

_ _

_

Thus, an 18 kg mass (~40#) requires 1300N(~290#) of muscle force and causes1100N

(250#) of joint contact force.

Tips: 1 kg = 2.2 lb-mass; 4.45N = 1 lb-force

FE = FJT + FB

FJT = 190 - 1294 = -1104 N (down)

Links are rigid

Joints are frictionless

No motion

No out-of-plane forces (Flatland)

Known anthropometry (segment sizes andweights)

nown orces an rec ons

Known postures

1 muscle

Known muscle geometry

No muscle antagonism (e.g. triceps)

Others


8/13

Biomechanical analysis yields external moments atselected joints

Compare external moments with joint strength (maximuminternal moment)

Typically use static data, since dynamic strength data arelimited

Use appropriate strengt ata (i.e. same posture)

Two Options:

Compare moments with an individuals joint strength

Compare moments with population distributions to obtainpercentiles (more common)

Multi-joint systems & multi-axis loads:

Determine the weakest link, or where the highest (relative)loads exist

Percent of population with sufficient joint

strength

Moment demands imposed by task (M)

Mean population strength (S)

Variability of population strength (V)

Assuming a normal distribution

z = (M - S) / V

z = (X - )/

from stats table z => %-ile

z-score reflects cumulative probability P(X Xi);normalized based on mean and std. dev.

From table of cumulative probabilities of the normaldistribution (z-table), the percentile corresponding to thez-score can be found

= 40 Nm; = 15 Nm (from strength table)

z = (15.4 - 40)/15 = -1.64 (std dev below the mean)

From table, the area A corresponding to z = -1.64 is 0.95

Thus, 95% of the population has strength 15.4 Nm

If ME = 15.4 Nm, what % of the population has sufficientstrength to perform the task (at least for a short time)?

Note:Both M and S vary considerably with posture and task conditions

Quick and Easy Evaluation of Lifting Tasks


9/13

Start End

Factors affecting workload?

A load constant is the maximum recommendedweight for lifting at the standard lift location underideal conditions.

LOAD CONSTANT = 23 kg (51 lbs)

Decrease the load constant to account for theinfluence of known risk factors using 6 multipliers: vertical location (VM) vertical travel distance (DM) asymmetry (AM) frequency (FM) coupling (CM)

All Multipliers are 1

Recommended Weight Limit (RWL) =

23kg HM VM DM AM FM CM

HD

VO

VD

HO

HM = (25/H)

H = horizontaldistance (in cm)of the handsfrom themidpoint

HD

ankles.

Measure at theorigin anddestination oflift.

extreme cases HM accounts for

Low-Back Loads

HO

Example?


10/13

0.6

0.8

1

1.2

Multiplier

20 30 40 50 60 70 800

0.2

.

Horizontal Distance (cm)

If H 25, HM = 1

Relatively big, non-linear effect

25

VM = (1-(0.003|V-75|))

V = vertical distance (incm) of the hands from thefloor. Measure at theorigin and destination of

.

VM accounts foracceptability and capacity(strength) as a function ofposture

VO

VD

Example?

0.8

1

1.2

Multiplier

torso flexion overhead reach

0 20 40 60 80 100 120 140 160 1800

0.2

0.4

.

Vertical Distance (cm)

Moderate, non-linear effect

DM = (0.82 +(4.5/D))

D = vertical travel distance(in cm) between the originand destination of the lift.

D = |VD-VO|

accoun s or me a o cdemand, task dynamics, butnot lift vs. lower

D

Example?


11/13

0.6

0.8

1

1.2

Multiplier

0 20 40 60 80 100 120 140 160 1800

0.2

.

Distance Moved (cm)

Relatively small, non-linear effect

AM = (1-(0.0032|A|))

A = angle (deg) ofasymmetry angulardisplacement of theload from the

A

sagittal plane.Measure at theorigin anddestination of lift.

AM accounts fordecreased strengthand increased spineloads in asymmetricpostures.

sagittalplane

Example?

0.6

0.8

1

1.2

Multiplier

0 20 40 60 80 100 120 140 160 1800

0.2

0.4

Asymmetry Angle (deg)

Moderate, linear effect

V


12/13

V 3 likely have increased risk

Some believe that if workers are properly screened(based on the task requirements) and trained, thatthey can safely work at lift indexes greater than 1but less than 3.

What are ideal lifting conditions?? Maximize RWL (keep load close to the body, )

Manual work activities other than lifting are assumed to beminimal

The equation does not account for unpredictable situationssuch as shifting loads

A favorable ambient environment is assumed (19- 26 C or66 - 79 F)

Risk of slips not accounted for (good floor surface assumed)

Lifting and lowering tasks are assumed to pose the same riskof injury

Tasks involving one-handed lifts, lifting while seated orkneeling, or lifting in a constrained work area are not

appropriate for this model Does not account for individual anthropometric differences

Start End

H = 13.0 cm

V = 13.5 cmA = 0 deg

H = 41.5 cm

V = 89.0 cmA = 0 deg

D = 75.5 cm; F = 1/min; Couplings = Fair


13/13

HMStart = (25/13) = 1HMEnd = (25/41.5) = 0.60

VMS = (1-(0.003|13.5-75|) = 0.82VME = (1-(0.003|89-75|) = 0.96

DM = (0.82+(4.5/75.5)) = 0.88

AMS = AME = (1-(0.0032)(0)) = 1

CMS = [Fair, V

5 biomechanics

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