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Duration of force exertion
10 10-29
30-49 50-79
79
Multiplier factor 0.5 1 1.5 2 3
3.3. Frequency of Force Exertion FF
Repeating the same motions over and over again places stress on the muscles and tendons. The severity of risk depends on the frequency at which the action is
repeated, the speed of the movement, the number of muscles involved and the required force 1, 3.
The success of the psychophysical approach to assessing the risk of manual handling tasks led to the application of this approach to analyzing the effects of
repetitive motion of the hands and wrists 2. Repetitiveness can be used to characterize tasks for assessment. For this, a repetitive task for the upper limbs can
be defined as an activity of at least an unbroken hour in which the subject repeats a similar series of relatively brief actions 4. Quantifying and assessing repetitiveness
is a difficult task 18. The multiplier factor for FF Table 3 was extracted from 15 16 using the OCRA method 10.
3.4. Duration of Force Exertion DF
DF is the percentage of time that is spent exerting force per work cycle 3; it represents biomechanical and physiological stresses related to maintaining force
exertion. Therefore, both the exertion cycle and average exertion time per cycle must be determined 1. Measuring average exertion cycle time requires observing
workers performing a job for long enough to ensure that the observations correspond to job requirements 5. Average exertion cycle time is gained by
dividing the number of counted force exertions by the length of the observation period 12.
From percent force exertion duration, the multiplier factor for DF Table 4 was extracted from 15 and 16.
3.5. Task duration per day TD
TD hours represents the total time per day that a task is performed 3, 5; it represents the time that a person muscles, tendons and ligaments performs a
specified task per a shift, rather than total shift length 12. The multiplier factor for task duration Table 5 was extracted from 15, 16 using the SI method 12 and
the OCRA method 10.
Table 5.
Elements for obtaining the multiplier factor for task duration TD
Chronic Exposure Index Model to Assess Ergonomic Risk Factor Related to Upper Extremity Musculoskeletal Disorders
69
Hour 1 1-2 2-4 4-8 8
Task duration per day Minute
60 60-120
120-240 240-480
480
Multiplier factor 0.25 0.50 0.75 1 1.5
3.6. Speed of Force Exertion SF
SF is the observed velocity of a task; this speed is considered due to its effects on force exertion 12. AS FE increases, the Maximum Voluntary contraction is
diminished and range of EMG is increased 18. To achieve suitable working conditions, SF should be Very slow”, “Slow”, or “Fair” 12. The multiplier factor
for speed of force exertion Table 6 was obtained from 15 and 16 using the SI method 12.
3.7. Part Weight PW
Weight of the object PW is the most important characteristic of manual material handling; it is involved in producing UEMSDs and other cumulative
Trauma disorders 14. In manual handling systems for assessing load on upper extremities, an object or load weight between 0.5 to 4 kg is the critical range for
inducing alterations in the upper limb musculoskeletal system that can increase the incidence rate of UEMSDs 14. An object with weight 4 kg stresses the neck,
upper limbs and other upper parts of the body 18. The weighting factor for part weight Table 7 was extracted from 19, 16 and 20.
Table 6. Elements for quantifying the multiplier factor for speed of force exertion SF
Speed of force exertion
Very slow Slow
Fair Fast
Very fast
Compared to MTM
≤ 80 81-90
91-100 101-115
115
Perceived speed
Extremely relaxed pace
Taking one’s own time
Normal speed of
motion Rushed , but
able to keep up
Rushed and barely or
unable to keep up
Multiplier factor 1 1 1 1.5 2
Table 7. Elements for quantifying the multiplier factor for part weight PW
Part weight kg 0.5
0.5-1 1-2
2-4 4
Multiplier factor 1 1.5 2 2.5 3
Table 8. Elements for determining the multiplier factor for lack of recovery periods RP
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Lack of enough recovery periods hour
0 1 2 3 4 5 6 7 8
Multiplier factor 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3
Table 9. Elements for achieving the multiplier factor for age factor AF
Age rating year
≤ 40 41-50
51-60 60
Multiplier factor 0.6 0.8 0.9 1
3.8. Lack of Recovery Periods RP
A recovery period is defined as “period of time between or within cycles, during which no repetitive movements are carried out” 10. A recovery period
consists of relatively long pauses after periods of mechanical movements; during these periods muscles can recoved metabolically 10. Lack of recovery periods can
create oxygen debt and produce accumulation of lactic acid, which induces muscle fatigue 4. Therefore, providing formal and informal rest intervals between work
cycles during tasks can prevent muscle fatigue and UEMSDs 1, 3, 5. The multiplier factor for lack of recovery period Table 8 was derived from 15 and 16 using the
OCRA method 10.
3.9. Age Factor AF
Age is the most important factor in force exertion and biomechanical models 18; it has a direct relationship with FE and Maximal Aerobic Capacity in hand
activities 4. For example, a 40 year old person has the maximal power, but at later ages this capability and power decrease slowly 18. The age factor multiplier Table
9 was obtained using the European Coal and Steel Community ECSC studies that quantified the effect of age on inducing and exacerbating UEMSDs 21, 22, 23.
3.10. Effective Item EI
EIs are factors that have indirect effects on the incidence rate of UEMSDs 10.These items may be present in repetitive tasks, but not necessarily or always
10. Their type, intensity and duration lead to an increased level of overall exposure to risk of developing UEMSDs 2. These items are considered to be relevant in the
production and development of UEMSDs 3. They are always work-related, and must be considered when assessing risk exposure 5. For an item to be considered,
it must have an association with UEMSD occurrence, so that it would have a collective impact rather than an individual impact 10. To obtain EI, a score of 1 is
allocated to any item whenever that item is present 10.
• Extreme precision at working
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