potvin presentation - cre-msd fatigue conference

Jim Potvin - Department of Kinesiology

Preventing Fatigue During Repetitive Tasks:

Predicting Maximal Acceptable Efforts Using Duty Cycle

Jim Potvin, PhD Department of Kinesiology


Introduction

•  Physical ergonomists must determine acceptable demands for a variety of tasks.

•  Data exists for maximum efforts

–  many strength studies available

•  However, less data exists for prolonged and/or fatiguing tasks

–  prolonged static

–  intermittent

–  complex time-histories


Endurance as a Function of Effort

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0 10 20 30 40 50 60 70 80 90 100

Max

imum

Hol

ding

Tim

e (m

in)

Effort as a Percentage of Maximum

Manenica (1986)

Rohmert (1973)

Frey Law & Avin (2010)


Intermittent Tasks

Rohmert Rest

Allowance

Variable Demand Capacity

Strength

Joint Moment Joint Strength

Duration

Holding Time Endurance Time

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Max

imum

Hol

ding

Tim

e (m

in)


0

50

100

150

200

250

30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

Max

imum

Mom

ent (

N m

)

Angle [K] (in degrees)

MALE

FEMALE

Rest Allowance = 18 ! Holding TimeHolding Time

Endurance Time

1.4 Strength DemandMaximum Strength

-0.15


Intermittent Tasks

Rohmert Rest

Allowance

20% MVC

30%

40%


Rohmert Rest Allowance

•  Limitations –  Task must always be

On/Off

–  Assumes <15% MVC can be held indefinitely

•  physiological data to the contrary

•  even for 1 hour

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0 10 20 30 40 50 60 70 80 90 100

Max

imum

Hol

ding

Tim

e (m

in)


Time (s)

30% MVC


Intermittent Tasks

•  Psychophysics has shown promise in Ergonomics –  Manual Materials Handling Tasks

•  Liberty Mutual Texas Tech –  Snook and Ciriello Ayoub


Psychophysics in Ergonomics

•  The power law in psychophysics Stevens (1957) –  exponential relationship between

sensations and their physical stimuli –  raised to some power

•  these powers vary for different stimuli.

•  Snook & Irvine (1967) –  First published psychophysical study

in physical ergonomics •  determine acceptable demands for

repetitive manual materials handling tasks

•  Snook et al (1995) –  began psychophysical studies of the

upper limbs


Psychophysical Methodology

•  Snook, Ciriello et al •  Subject controls maximal acceptable value

–  typically resistance •  could also be frequency, rest, duration

•  Procedure –  repetition rate controlled –  task requirements

•  gripping, screwdriver, wrist ulnar deviation etc

–  subject controls the amount of resistance •  by moving an unmarked dial

–  resistance changed every 14 minutes •  randomly selected direction and amount

–  subjects instructed to “work as if they were on an incentive basis, getting paid for the amount of work they performed. They were asked to work as hard as they could without developing unusual discomfort in the hands, wrists, or forearms”

•  symptoms were monitored

•  Subject Training –  4-5 days ramping from 2 to 7 hours/day –  testing over another 3 weeks


Example: Ciriello et al (2002)

- Screw driving - Ulnar deviation - Handgrip


Moore & Wells (2005)

-  Wrist Extension -  Simulated in-line screw running


Potvin et al (2006)

Manual Electrical Connections

Description –  mating of electrical

connectors in automotive assembly

Study –  Biomechanics and

Psychophysics –  18 hours of training –  9 hours of collection


Methods: Electrical Connectors

•  Subjects –  24 females

•  20 to 59 years

•  Independent Variables –  3 Frequencies

•  2, 7, 12/min

–  3 Grips •  Oblique Grasp, Pulp Pinch, Finger Press

–  3 Wrist postures •  Neutral, Extended, Ulnar Deviated

•  Dependent Variables –  Maximal Acceptable:

•  Peak Force (N) •  Force Impulse (N s)

–  Maximum Voluntary Contraction (MVC)


Methods: Simulation Device

Spring

Frictionless Gliding Blocks

Force Transducer

Tension Adjusting

Dial

Removable Connector

Fixture Travel Distance of 13 mm

Frictionless Sliding Blocks


Sample Data7/min, Finger Press, Neutral Wrist

0

2

4

6

8

10

12

0 0.05 0.1 0.15 0.2 0.25

Time (s)

Fo

rce

(lb

s)

Average Peak Force8.5 ± 0.7 lbs (8.0% CV)

Average Force Impulse(Area under Curve)

0.907 ± 0.064 lbs s (7.0% CV)

Results: Sample Data


Estimating Duration from Impulse and Peak Force

0

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25

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35

40

45

0 0.05 0.1 0.15 0.2 0.25

Time (s)

Fo

rce

(N)

Peak Force = 40 N

Impulse = 4.4 Ns

Rectangular Estimate of Duration

= Impulse / Peak Force = 4.4 / 40 = 0.11 s

Mean duration = 0.16 s

Methods: Duration


Methods: Peak Force Time-History

Sample Force Time History(S03, OG, Ulnar Dev, 12/min)

0

2

4

6

8

10

12

14

16

18

20

0 15 30 45 60

Time (min)

Peak

For

ce (l

bs)

Mean = 12.6 lbsStDev = 1.0 lbsCV = 8%


•  Hose Insertions

•  Subjects –  15 females

•  21 to 59 years

•  Independent Variables –  3 Frequencies

•  1, 3, 5/min

–  5 Postures

•  Dependent Variables –  Maximal Acceptable

•  Peak force (N) •  Force Impulse (N s)

–  MVC

Andrews et al (2007)


Integration of Psychophysical Results


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6s 6s (x 5/min)

Frequency = 5/min Duty Cycle = 30 s / 60 s = 0.50

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1s 1s (x 15/min)

Frequency = 15/min Duty Cycle = 30 s / 60 s = 0.50

Duty Cycle

Duty Cycle = Work Time .

Work + Rest Time

7 Studies Used

Duty Cycle =

Freq x Duration Time

n = 69

Subjects Freq(/min)

Duration (s)

Duty Cycle

(s/min)

Duty Cycle (%)

Tasks

2 0.810 1.62 2.7%5 0.810 4.05 6.8%

10 0.810 8.10 13.5%15 0.810 12.15 20.3%20 0.810 16.20 27.0%

5 days/week 14 15 0.810 12.15 20.3% - Wrist flexion: power grip (n = 1)15 1.120 16.80 28.0%20 1.050 21.00 35.0%25 1.080 27.00 45.0%15 0.640 9.60 16.0%20 0.610 12.20 20.3%25 0.550 13.75 22.9%15 1.080 15.00 27.0%20 1.080 20.00 36.0%25 1.080 25.00 45.0%15 1.215 15.00 30.4%20 1.215 20.00 40.5%25 1.215 25.00 50.6%3 5.000 15.00 25.0%5 3.000 15.00 25.0%

10 1.500 15.00 25.0%3 10.000 30.00 50.0%5 6.000 30.00 50.0%

10 3.000 30.00 50.0%20 1.500 30.00 50.0%3 16.667 50.00 83.3%5 10.000 50.00 83.3%

10 5.000 50.00 83.3%20 2.500 50.00 83.3%2 0.160 0.32 0.5%7 0.160 1.12 1.9%

12 0.160 1.92 3.2%1 0.685 0.69 1.1%3 0.717 2.15 3.6%5 0.721 3.61 6.0%

- Ulnar deviation: power grip (n = 3)

- Wrist extension: pinch grip (n = 3)

- Wrist extension: in-line powered srcrewdriver (n = 11)

- Wrist flexion: power grip- Wrist flexion: pinch grip- Wrist extension: power grip (n = Total of 15 tasks)

- Push: pulp pinch- Push: finger tip (n = 6)

- Hose insertions: 5 hand locations (n = 15)

- Ulnar deviation: power grip (n = 3)- Supination: 31 mm screwdriver- Supination: 40 mm screwdriver- Supination: 39 mm yoke- Pronation: 31 mm screwdriver (n = 12)

Duty Cycle = 83.3%

Duty Cycle = 25%

Duty Cycle = 50%

Potvin et al (2006)

Andrews et al (2008)

Moore & Wells (2005)

Ciriello et al

(2002)

24

15

8

Ulnar Deviation

Pronation & Supination

Study

15

11

20

10

Snook et al

(1995)

2 days/week

Snook et al (1997)

Snook et al (1999)

0%"

10%"

20%"

30%"

40%"

50%"

60%"

70%"

0 5 10 15 20 25

Max

imum

Acc

epta

ble

Effo

rt (%

)!

Frequency (Efforts per Minute)!

Snook et al (1995) 2D - Wrist Flexion (Power Grip)"

Snook et al (1995) 2D - Wrist Flexion (Pinch Grip)"

Snook et al (1995) 2D - Wrist Extension (Power Grip)"


Snook et al (1997) Ulnar Deviation"

Snook et al (1999) Wrist Extension (Pinch Grip)"

Ciriello et al (2002) Supination (31 mm handle)"


Ciriello et al (2002) Supination (39 mm yoke)"

Ciriello et al (2002) Pronation (31 mm handle)"

Ciriello et al (2002) Ulnar Deviation"

Moore & Wells (2005) Wrist Extension - DS = 0.25"



Potvin et al (2006) Pulp Pinch Push"

Potvin et al (2006) Finger Push"

Andrews et al (2007) Hose Insertion - (Medial Far)"

Andrews et al (2007) Hose Insertion - (Medial Near)"

Andrews et al (2007) Hose Insertion - (Push Forward)"

Andrews et al (2007) Hose Insertion - (Pull Back)"

Andrews et al (2007) Hose Insertion - (Push Down)"

0%"

10%"

20%"

30%"

40%"

50%"

60%"

70%"

0%" 10%" 20%" 30%" 40%" 50%" 60%" 70%" 80%" 90%"

Max

imum

Acc

epta

ble

Effo

rt (%

)!

Duty Cycle (Percent)!


Snook et al (1995) 2D - Wrist Flexion (Pinch Grip)"

Snook et al (1995) 2D - Wrist Extension (Power Grip)"


Snook et al (1997) Ulnar Deviation"

Snook et al (1999) Wrist Extension (Pinch Grip)"



Ciriello et al (2002) Supination (39 mm yoke)"

Ciriello et al (2002) Pronation (31 mm handle)"

Ciriello et al (2002) Ulnar Deviation"

Moore & Wells (2005) Wrist Extension - 3/min"




Potvin et al (2006) Pulp Pinch Push"

Potvin et al (2006) Finger Push"

Andrews et al (2007) Hose Insertion - (Medial Far)"

Andrews et al (2007) Hose Insertion - (Medial Near)"

Andrews et al (2007) Hose Insertion - (Push Forward)"

Andrews et al (2007) Hose Insertion - (Pull Back)"

Andrews et al (2007) Hose Insertion - (Push Down)" 0

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50

60

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90

100

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Duty Cycle (Percent)

Max

imum

Acc

epta

ble

Effo

rt (%

)

Upper Extremity Psychophysical Data

Equation

r2 = 0.87 RMS Error = 7.2% MVE

MADC = (1 – Effort)4.167 – 1/28,800

DC = 0.12 (~7 s/min) Effort = 0.40 (40% MVC)

0

10

20

30

40

50

60

70

80

90

100

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%


Max

imu

m A

ccep

tab

le E

ffo

rt (

%)

EquationRohmert (1973) - rest allowance equation for 360 s cycleRohmert (1973) - rest allowance equation for 30 s cycleJonsson (1982) - recommended for 8 hoursBjorksten & Jonsson (1977) - fatigue within 1 hourSjogaard et al (1986) - fatigue within 1 hourJorgensen et al (1988) - fatigue within 1 hourBystrom & Kilbom (1990) - fatigue within 1 hourBystrom & Fransson-Hall (1994) - fatigue within 1 hour


Frequency Effects: NIOSH Lifting Equation


Frequency Effects: ACGIH TLV

Tolerance Limit Value (TLV)

0

1

2

3

4

5

6

7

8

0 1 2 3 4 5 6 7 8 9 10

Hand Activity Level (HAL)

Eff

ort

(90th

perc

en

tile

)

TLV (Controls Needed)

Action Limit (General Controls)

.


Lifting & Lowering Curves

30

40

50

60

70

80

90

100

0% 1% 2% 3% 4% 5% 6% 7% 8% 9% 10%


Max

Acc

epta

ble

Wei

ght o

f Lift

/Low

er (%

of 1

per

day

)

Lift (Floor to Knuckle)Lift (Knuckle to Shoulder)Lift (Shoulder to Reach)Lower (Knuckle to Floor)Lower (Shoulder to Knuckle)Lower (Reach to Shoulder)

Snook & Ciriello (1991)


Lifting & Lowering Fit with Equation

•  0.24 best coefficient for Lifting & Lowering

•  RMS Diff = 5.0% MAE •  r-squared = 0.812

0

10

20

30

40

50

60

70

80

90

100

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%


Max

imum

Acc

epta

ble

Load

(%M

VC)

40

50

60

70

80

90

100

0% 1% 2% 3% 4% 5% 6% 7% 8% 9% 10%Duty Cycle (Percent)

MA

L%


Snook & Ciriello (1992) Lift & Lower MAWLs

Fitted Equation


Example: Maximum Acceptable Effort

Greig & Wells (2004) Assume:

•  2.3 efforts/min •  0.80 s/effort •  DC = (2.3 x 0.80) / 60 •  DC = 0.031 (3.1%)

MAE = 1 – (0.031 – 1/28,800)0.24

= 1 – 0.433 = 0.567 (56.7%) 50th Female Power Grip

•  mean = 162.3 N •  MAF = 162.3 x MAE •  = 162.3 x 0.567 = 92.0 N

25th Female Lateral Pinch •  mean = 64.8 N •  25th = 64.8 – (13.2 x 0.67) = 56.0 N •  MAF = 56.0 x 0.567 = 31.7 N


Maximum Acceptable Effort = 70.6%


Limitation of MAE Equation:

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30

35

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0

5

10

15

20

25

30

35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Future Directions:

•  More research needed –  intermittent tasks –  non On/Off tasks –  complex time-histories –  combined tasks

Sonne (2012)


MinMax 1 Percent

Isometric Sinusoidal On/Off

Fatigue Response Continuum

Most Fatiguing Least Fatiguing

Yung & Wells (2012)


Wells et al (2011)

50%

60%

70%

80%

90%

100%

110%

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

50%

21 42 63 84 105 126 147 168 199 220 241 262 283 304 325 346 367 388

Forc

e pr

oduc

ed a

fter

pla

teau

e (%

MV

C)

Pla

teau

Inte

nsity

Lev

el (%

MV

C)

Time at End of Each Plateau (s)

Changes in Force Production Between Plateaus (n = 12)


Wells et al (2011)

-15%

-10%

-5%

0%

5%

10%

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

50%

42 63 84 105 126 147 168 199 220 241 262 283 304 325 346 367 388

Cha

nge

in M

VC

For

ce P

rodu

ctio

n B

etw

een

Cur

rent

and

Pre

viou

s Fo

rce

Pla

teau

(C

olou

red

Bar

s) (%

MV

C)

Pla

teau

Inte

nsity

Lev

el (B

lue

Bar

s) (%

MV

C)

Time at End of Each Plateau (s)

Changes in Force Production Between Plateaus (n = 12)


Mike Sonne PhD Thesis

•  High duty cycle psychophysics study

0

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100

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%


Max

imum

Acc

epta

ble

Effo

rt (%

)


Equation


Mike Sonne PhD Thesis

•  Peripheral vs central fatigue in complex tasks –  flexor pollicis longus –  complex intermittent task –  voluntary vs stimulation maximums

•  Xia & Frey-Law (2008) Model –  further development –  motor unit model


Take Home Message

1.  Most tasks are repetitive, but most of the research data relates to single effort strengths

2.  Psychophysics can provide valuable data related to acceptable forces and torques during repetitive tasks

3.  Duty cycle can be used to estimate maximum acceptable efforts for repetitive, intermittent tasks with no associated psychophysical data

4.  Rohmert Rest Allowances appear to overestimate acceptable demands

5.  More research is needed for tasks that are: –  intermittent –  complex –  combined


Acknowledgements


Contact Information

Jim Potvin Associate Professor Department of Kinesiology, McMaster University 1280 Main Street West, Hamilton, Ontario, L8S 4K1 phone: (905) 730-9473 email: [email protected] website: http://www.science.mcmaster.ca/kinesiology/people/faculty/293-Jim%20Potvin.html

potvin presentation - cre-msd fatigue conference

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