Force Exertion FE Introduction of variables involved in calculating the CEI

Seyyed Ali Moussavi-Najarkola AJE Vol.9 2009 63-78 68 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