Seyyed Ali Moussavi-Najarkola AJE Vol.9 2009 63-78
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Rating criterion Light
Somewhat difficult
Difficult Very
difficult Near
maximal Maximal
Mean of force exertion perceived
according to Borg
0-2 3 4-5 6-7 8-10
10
Mean of force exertion in of MVC
0-20 30 40-50 60-70
80-100 100
Multiplier factor
1 3 6 9 13 17
3.1. Force Exertion FE
FE can be defined as the amount of physical effort required to perform a task or maintain control of equipment 1, 2; it depends on the type of grip, the weight of
the object, body posture, the type of activity, and the duration of the task 2. FE may depend on static or dynamic contractions 1. The need to exert force during
work-related activities may be due to handling, lifting, lowering, moving or holding objects, or keeping them in a given position 3.
The required force for performing various occupational actions is a critical factor producing WMSDs. The rate of exerted load or force on hand muscles can
exceed hundreds of pounds. FE can be estimated using the Borg CR-l0 scale or by knowing the percent of Maximum Voluntary Contraction MVC from Table 1,
which was derived from the model of CEN investigation 15 and the SI method 12.
3.2. Posture of Upper Limbs PU
Prolonged exposure to repetitive forceful exertion of the hand and wrist, especially in awkward postures, is strongly associated with tendon and nerve
damage at the wrist and hand 17. Awkward postures include repeated or prolonged reaching, twisting, turning and bending hands, working with hands or
arms, holding fixed positions, working with wrist bent for 2 hours per day 4; pressing the upper limbs against a hard or sharp edge, which can result in placing
too much pressure on nerves, tendons and blood vessels; and using the palm of hand as a hammer regularly, or typing while resting arms or wrists on the hard desk
edge 1.
UEMSDs may be attributable to occupational, non- occupational and individual factors 2. In the CEI method, the posture of upper limbs includes those of the hand
and fingers, wrist, forearm, elbow, arm, and shoulder. CEI assigns a score 4 for each of these parts and every part is divided into four awkward posture items. Thereby
every part of the upper limbs can receive a maximal score of 4. The items which must be considered include 10:
• Hand fingers: 1 Pinch grip
Chronic Exposure Index Model to Assess Ergonomic Risk Factor Related to Upper Extremity Musculoskeletal Disorders
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2 Tight grip 3 Hook grip
4 Palmar grip
• Wrist: 1 Flexion 2 Extension
3 Radial deviation 4 Ulnar deviation
• Forearm: 1 Supination
2 Pronation 3 Radial deviation
4 Ulnar deviation
• Elbow: 1 Supination 2 Pronation
3 Flexion 4 Extension
• Arm : 1 Abduction
2 Contracting arm for a long term 3 Turning inward
4 Turning outward
• Shoulder : 1 Abduction
2 Pulling forward 3 Pulling backward
4 Holding at higher than shoulder level for a long term 3.
Finally, the multiplier factor for PU of upper limbs Table 2 was extracted from 15 16 using the SI method 12.
Table 2. Parameters for determining the multiplier factor for posture of upper limbs PU
Rating criterion Very good
Good Fair
Bad Very bad
Score of upper limb posture
≤ 4 5-8 9-12 13-16
16
Multiplier factor 1 1.25 1.5 1.75 2.5
Table 3.
Parameters for determining the multiplier factor for frequency of force FF exertion
Frequency of force exertion per minute
4 4-8
9-14 15-19
19
Multiplier factor 0.5 1 1.5 2 3
Table 4. Parameters for determining the multiplier factor for duration of force exertion DF
