Job Strain Index (JSI)
Job Strain Index (JSI)
Moore and Garg (1995)
Focuses on:
A “semi-quantitative” job analysis
methodology developed to address
upper extremity disorders.
Intensity of Exertion
Duration of Exertion per Cycle
Efforts per Minute
Wrist Posture
Speed of Work
Duration of Task per Day
A problem for evaluating strain
Job Strain Index (JSI)
Individual Differences in Capability
[multiple curves]
Time
The Strain Index (SI) Score is the product of the six
multipliers:
SI = Intensity * Duration * Efforts * Posture * Speed * Duration
Force Exertion
Normalized by Individuals Own Capability
Normalized Curve
Time
Time
[one curve}
25−35 %
15%
% of Maximum Force (f/Fmax)
Normal Breaks
are Sufficient
% of Maximum Force (f/Fmax)
1
Important Point !!
Work/Recovery Cycles
WT = (40 / % Fmax) - 39
The amount of strain experienced is
related to the persons capabilities
RT = 8.8 ln (% Fmax- 0.5) + 25
One size does NOT fit all
Fmax = Fraction of the maximum capacity
WT = Work Time
RT = Rest Time
Job Strain Index
Job Strain Index
Also uses a modification of the Borg Scale of “perceived exertion”
Evaluates the “Intensity of Exertion”
relative to the persons
maximum strength (MS)
Light
Somewhat Hard
Hard
Very Hard
Near Maximal
Rating
0
0.5
1
2
3
4
5
6
7
8
9
10
< 10% MS
10 – 30% MS
30 – 50 % MS
50 – 80 % MS
> 80 % MS
Verbal Anchor
Nothing at all
Just noticeable
Very weak
Weak / light
Moderate
Strong / heavy
Very strong
Almost Maximum
Job Strain Index
Job Strain Index
Intensity of Exertion Factor
Intensity of Exertion Factor
Perceived Effort
Rating
Intensity
IE Multiplier
Light (<
< 2)
Barely noticeable or relaxed effort
Somewhat Hard ( 3)
Noticeable or definite effort
1
Light
1
Hard (4-5)
Obvious effort; unchanged facial expression
2
Somewhat Hard
3
Very Hard (6-7)
Substantial effort; changes in facial expression
3
Hard
6
Near Maximal (>
>7)
Uses shoulder or trunk to generate force
4
Very hard
9
5
Near Maximal
13
2
Job Strain Index
Job Strain Index
Duration of Exertion Factor
Duration of Exertion Factor
The duration of the exertion (DE) factor pertains to the
proportion of the cycle time during which the exertion is
exerted.
Rating
The proportion of time spend exerting is equal to:
Total duration of exertions during observation period (sec.)
Total observation time (sec.)
Duration within cycle
DE Multiplier
1
< 10 %
.5
2
10 – 30 %
1
3
30 – 50 %
1.5
4
50 – 80 %
2
5
80 – 100 %
3
The % DE = 100 * (proportion of time)
Job Strain Index
Job Strain Index
Efforts per Minute Factor
Efforts per Minute Factor
The efforts per minute (EM) factor pertains to the average
frequency of the exertion and is related to the repetitiveness
of the task.
The average efforts per minute is equal to:
Total number of exertions during the observation time Total
observation time (min.)
Rating
Efforts per Minute
EM Multiplier
1
<4
.5
2
4 – 8
1
3
9 – 14
1.5
4
15 – 19
2
5
> 19
3
Job Strain Index
Job Stain Index
Wrist Posture Factor
Wrist Posture Factor
The posture of the wrist (WP) factor relates the
deviation of the wrist relative to the forearm. The
categories of deviation are:
Flexion
- toward the palm of the hand
Extension
- toward the back of the hand
Ulnar deviation - toward the little finger
Rating
Extension
Flexion
Ulnar
Deviation
Perceived
Posture
Multiplier
Very
Good
0 – 10 deg
0 – 5 deg
0 – 10 deg
Perfectly
neutral
1
1
Good
11 - 25 deg
6 – 15 deg
11 - 15 deg
Near
neutral
Fair
26 - 40 deg
16 – 30 deg
16 – 20 deg
Non-neutral
1.5
2
3
Bad
41 - 55 deg
31 - 50 deg
21 – 25 deg
Marked
deviation
Very
Bad
> 60 deg
> 50 deg
> 25 deg
Near
extreme
3
Job Strain Index
Job Stain Index
Speed of Work Factor
Speed of Work Factor
The speed of work (SW) factor addresses the perceived
pace of the operation. The pace is subjectively estimated
by the analyst. The pace can also be estimated by using the
MTM-1 work measurement system.
The categories for pace are:
Very slow
Slow
Fair
Fast
Very fast
Compared to
MTM – 1
Rating
Criterion
1
Very slow
2
Slow
3
Fair
90 – 100 %
4
Fast
5
Very fast
Job Strain Index
Perceived Posture
Multiplier
< 80 %
Extremely relaxed pace
1
80 – 90 %
“Taking one’s own time
1
“Normal” speed of motion
1
101 – 115 %
Rushed, but Able to keep up
1.5
> 115 %
Rushed and barely or unable
to keep up
2
Job Stain Index
.25
2
1 – 2 hours
.5
3
2 – 4 hours
.75
4
4 - 8 hours
1
5
> 8 hours
1.5
Duration
< 1 hour
Speed
1
Wrist Posture
EM Multiplier
Efforts/Min
Efforts per Minute
1
Light
[1]
< 10 %
[0.5]
<4
[0.5]
Very Good
[1]
Very slow
[1]
< 1 hr
[0.25]
2
Somewhat
hard [3]
10 – 30 %
[1]
4–8
[1]
Good
[1]
Slow
[1]
1 – 2 hr
[0.5]
3
Hard
[6]
30 – 50 %
[1.5]
9 - 14
[1.5]
Fair
[1.5]
Fair
[1]
2 – 4 hr
[0.75]
4
Very hard
[9]
50 – 80 %
[2]
15 - 19
[2]
Bad
[2]
Fast
[1.5]
4 – 8 hr
[1]
5
Near
Max. [13]
80 – 100 %
[3]
> 20
[3]
Very Bad
[3]
Very Fast
[2]
> 8 hr
[1.5]
Duration
Rating
Intensity
The duration per day (DD) factor reflects the total time
that the task is performed per day. The duration is
expressed in hours.
Rating
Duration of Task per Day Factor
Job Stain Index
Job Stain Index
Intensity
IE
Duration
DE
Efforts per
EM
Posture
WP
Speed
SW
Duration
DD
1
Light
[1]
< 10 %
[0.5]
<4
[0.5]
Very
Good [1]
Very slow
[1]
< 1 hr
[0.25]
Rating
Intensity
Duration
Efforts/Min
Wrist
Posture
Speed
Duration
2
Somewhat
hard [3]
10 – 30 %
[1]
4–8
[1]
Good
[1]
Slow
[1]
1 – 2 hr
[0.5]
Exp
Somewhat
Hard
40 %
6
Fair
Fair
8 hr
3
Hard
[6]
30 – 50 %
[1.5]
9 - 14
[1.5]
Fair
[1.5]
Fair
[1]
2 – 4 hr
[0.75]
Mult
3
1.5
1
2
1
1
4
Very hard
[9]
50 – 80 %
[2]
15 - 19
[2]
Bad
[2]
Fast
[1.5]
4 – 8 hr
[1]
5
Near
Max. [13]
80 – 100 %
[3]
> 20
[3]
Very Bad
[3]
Very Fast
[2]
> 8 hr
[1.5]
SI = 3 * 1.5 * 1 * 2 * 1 * 1 = 9
SI = IE * DE * EM * WP * SW * DD
4
Job Strain Index
Evaluation of the SI
Higher values of SI indicate higher risk of disorders
The authors indicate that:
•
SI scores less than or equal to 3 are probably “safe”
•
SI scores greater than or equal to 7 are probably
“hazardous”
5