ergonomic methods to assess the cumulative effects of
TRANSCRIPT
Ergonomic Methods to Assess the Cumulative Effects of
Multiple Tasks
Moving Beyond Isolated Task Analyses
Jim Potvin, PhD
Introduction
• Most ergonomics assessment tools– evaluate single subtasks in isolation
• Snook/Liberty Mutual Tables• Revised NIOSH lifting Equation• Biomechanical software
– Lumbar compression and shear forces– strength demands
• Strain Index• RULA• ACGIH TLV for HAL
• However– most jobs involve more than one subtask
• how do we assess the cumulative effect of multiple subtasks?
– repetitive strain disorders– muscle fatigue
Introduction
• Learning Objectives– Understand the basic principles related the physiology of muscle fatigue and tissue injury,
resulting from repetitive loads.
– Learn about the scientific research that has furthered our understanding of the limits of human capacity for repetitive tasks, and more complex combinations of multiple tasks.
– Gain experience with using new ergonomics tools to analyze multiple tasks to determine if muscles and/or passive tissues are getting enough time to recover.
Passive Tissue Physiology
Dense Regular Connective Tissue Elastic Connective Tissue
Passive Tissue Mechanics: Single Load
Strain (l/ lO)
Str
ess
(N
/cm
2)
Yield Point
Strength
Failure Point
Permanent Deformation
.
Acute Injury
Passive Tissue Mechanics: Repetitive Loading
40
30
20
10
0
Repetitive StrainInjury, Fatigue
Demand Capacity
Demand Capacity
Demand Capacity
Demand Capacity
Acute Injury
Passive Tissue Mechanics: Repetitive Loading
40
30
20
10
0
Repetitive StrainInjury, Fatigue
McGill (2007)
Muscle Physiology
Whole Muslce Motor Units
Tug-of-War Analogy for The Size Principle
Slow Twitch Intermediate Fast Twitch
Motor Motor Motor
Unit Unit Unit
The Size Principle
Muscle Recruitment, Fatigue & Recovery
© Jim Potvin (2016)
Low Threshold
(Slow) Intermediate
High
Threshold (Fast)
LowThreshold(slow) Intermediate
HighThreshold(fast)
Prolonged, Repetitive Task
Tissue Injury
Repetitive Forceapplications
on tissue PeripheralFatigue
Central Fatigue
Reduction in Motor Coordination
MechanicallyUnsafe technique
Shift toother tissues
Force Exceeds currentTolerance of tissue
Mechanical tissuefatigue and
progressive damage
- decreased- quality- reputation
- increased- warranty claims- errors- accidents- scrap
0
500
1000
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2000
2500
3000
3500
4000
Lu
mb
ar
Co
mp
res
sio
n F
orc
e (
N)
Time
Cumulative Loading Effects
NIOSH Compression Limit
Is this task acceptable?
Current Ergonomics ToolsThat Account for
Multiple Subtasks
Metabolic Equations
Astrand & Rhodal (1986)
Wilson & Corlett (1990)
Example: Garg et al (1978)
Stoop Lower (Kcal/lower):E = (0.01) [ (0.268)(BW)(0.81 - h1) + (0.675)(L)(h2 - h1) + (5.22)(S)(0.81 - h1) ]
Squat Lift (Kcal/lift): E = (0.01) [ (0.514)(BW)(0.81-h1) + { (2.19)(L) + (0.62)(S)(L) } (h2 - h1) ]
Arm Lift (Kcal per Lift): E = (0.01) [ (0.514)(BW)(0.81-h1) + { (2.19)(L) + (0.62)(S)(L) } (h2 - h1) ]
Walking (Kcal per Walk): E = (0.01) [ 51 + (2.54)(BW)(V2) + (0.379)(BW)(G)(V) ] t
Also, Dempsey et al (2008)
NIOSH Composite Lifting Index
F1,2 = 2 + 3 = 5 FM = 0.35
F1,2,3 = 2 + 3 + 4 = 9 FM = 0.15
FIRWL = Frequency Independent RWLSTRWL = Single Tasks RWLFILI = Frequency Independent Lifting IndexSTLI = Single Task Lifting Index
ΔFILI2 = FILI2 (1/FM1,2 – 1/FM1)
= 0.94 (1/0.35 - 1/0.65) = 1.24
ΔFILI3 = FILI3 (1/FM1,2,3 – 1/FM1,2)
= 0.35 (1/0.15 - 1/0.35) = 1.33
CLI = STLI1 + FILI2 + FILI3 = 3.17 + 1.24 + 1.33 = 5.74
Cumulative Spine Compression & Shear
Spine 1990 15(12):1311-6.
3DMatch (Jack Callaghan)
Lumbar Motion Monitor
Recent Ergonomics ToolsThat Account for
Multiple Subtasks
Cumulative Lifting Index (CULI)
• Modified the NIOSH CLI– continuous function for Frequency
Multiplier (FM)
“Conclusion: The CULI partially addressed the under-estimation of physical exposure using the TWA approach and overestimation of exposure using the peak-exposure approach.”
Jack - Task Simulation Builder
Time Histories• Joints
• moments• strength demands
• Lower Back• compression forces• shear forces
0%
20%
40%
60%
80%
100%
0 5 10 15 20 25 30 35 40 45 50 55 60
Sh
ou
lde
r D
em
an
d (
% M
ax
)
Time(s)
Recommended Cumulative Rest Allowance (RCRA)
0
10
20
30
40
50
60
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90
100
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Maxim
um
Accepta
ble
Ef
ort
(%
)
Duty Cycle (Percent)
The Maximum Acceptable Effort (MAE) Equation
Recommended Acceptable Rest Allowance (RCRA)
0.0
0.1
0.2
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0.5
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0.9
1.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Req
uir
ed
Rest
All
ow
an
ce
(as
a p
rop
ort
ion
of
the
cycle
tim
e)
Holding Time (as a proportion of the cycle time)
1%
5%
10%
15%
20%
25%
30%
35%
40%
45%
50%
55%
60%
65%
70%
75%
80%
85%
90%
95%
1%
5%
10%
15%
20%
25%
30%
35%
40%
45%50%
Max Acceptable Effort = MAE = 1 - [ DC - DCMIN ]0.24
~ 1 - DC0.24
Max Acceptable Duty Cycle = MADC = [ 1 - Effort ]4.167
Rest Allowance (per cycle) = (Holding Time)/MADC - Holding Time
Murray Gibson & Jim Potvin
MAE Decision
10% 2.00000 1.00 2.00000 0.033333 0.644691 55.8% Yes 1.10 0.55
50% 2.50000 1.50 3.75000 0.062500 0.055703 48.6% No 63.57 25.43
20% 3.00000 2.00 6.00000 0.100000 0.394652 42.5% Yes 9.20 3.07
#N/A
#N/A
#N/A
#N/A
#N/A
73.88
Total Cycle Duration (s) 60.00 48.25 Total Effort Duration (s/cycle) 11.75 1.53
Duty Cycle 0.1958
Rest
Required(s / cycle)
Rest
Required(s / effort)
Effort(% MVC)
Frequency(per cycle)
Duration of
each Effort (s)
Total
Duration (s/cycle)
Actual
Duty Cycle
(DC)
Maximum
Acceptable
DC
Is the Subtask Acceptable
in Isolation?
Total Rest Required
Total Rest Provided
RCRA Ratio
RCRA Method: Example
Rest Allowance Required = 6.00 - 6.00 = 9.20 0.395
MAE Decision
10% 2.00000 1.00 2.00000 0.033333 0.644691 55.8% Yes 1.10 0.55
50% 1.50000 1.50 2.25000 0.037500 0.055703 54.5% Yes 38.14 25.43
20% 3.00000 2.00 6.00000 0.100000 0.394652 42.5% Yes 9.20 3.07
#N/A
#N/A
#N/A
#N/A
#N/A
48.45
Total Cycle Duration (s) 60.00 49.75 Total Effort Duration (s/cycle) 10.25 0.97
Duty Cycle 0.1708
Rest
Required(s / cycle)
Rest
Required(s / effort)
Effort(% MVC)
Frequency(per cycle)
Duration of
each Effort (s)
Total
Duration (s/cycle)
Actual
Duty Cycle
(DC)
Maximum
Acceptable
DC
Is the Subtask Acceptable
in Isolation?
Total Rest Required
Total Rest Provided
RCRA Ratio
Summary
• Learning Objectives– Understand the basic principles related the physiology of muscle fatigue and tissue injury,
resulting from repetitive loads.
– Learn about the scientific research that has furthered our understanding of the limits of human capacity for repetitive tasks, and more complex combinations of multiple tasks.
– Gain experience with using new ergonomics tools to analyze multiple tasks to determine if muscles and/or passive tissues are getting enough time to recover.
