Civil Engineering Reference
In-Depth Information
meat away from the bone. Although the force peaks are not as conspicuous, four exertions per thigh are
required for each 7.6 sec cycle. The left hand can rest between successive thighs. The average exertion
frequency for the left hand is 0.5 exertions per second.
Other examples of an exertion include swinging a hammer, twisting a screwdriver, driving a threaded
fastener with a power screwdriver, getting and placing a part, folding the flap of a box, ejecting the con-
tents of a pipette, activating a control on a machine, pressing a key on a keyboard, etc. All of these
examples entail a single movement or exertion of force. It is not always necessary to make a plot as
shown in Figure 41.3. Exertions can be counted from detailed job descriptions used by engineers to estab-
lish production standards or from video recordings.
In addition to exertion frequency, the force plots also provide information about recovery time so that
Table 41.1 can be used to estimate the HAL. There is essentially no recovery time for the right hand
because of the exertion to hold the knife in between successive cuts and thighs. The recovery time can
be calculated by adding up the times that the hand is at rest or estimated from job observations:
Rest: (2
:
0
0)
þ
(10
:
6
5
:
6)
¼
7 sec
Work: 2
:
0
5
:
6; 10
:
6
15
¼
(5
:
6
2
:
0)
þ
(15
10
:
6)
¼
8 sec
In this case the recovery time was found to be 47%.
Using Table 41.1, the HAL can be estimated for the right hand with 0.66 exertions per sec and
0% recovery time as 6, and for the left hand with 0.5 exertions per sec and 47% recovery time as 5.
HALs also can be estimated using the visual analog scale shown in Figure 41.1a (Latko, 1997). The
HAL of six for the right hand corresponds to “slow steady motion per exertions frequent brief
pauses.” The HAL of five for the left hand is in between six “steady motion per exertion infrequent
pauses” and four “slow steady motion per exertions frequent brief pauses.”
41.4.3 Peak Finger Force
The finger force varies with time and work elements as shown in Figure 41.3a and Figure 41.3b for the
turkey boning job. Peak finger force is expressed as a 90th percentile on a relative zero to ten scale
(see Figure 41.1b). The upper percentiles are more sensitive to jobs with high force variations than
are average values, but are not as sensitive to random values as are absolute peaks. The relative zero to
ten scale adjusts for posture variations and can be assessed using the Borg (1990) scale or observer
ratings (as shown in Figure 41.1b. In cases where force is measured in conventional force units, it is
necessary to divide by the posture-specific strength. Peak finger force on a zero to ten scale can be
calculated based on a 90th percentile finger force and posture-specific hand strength. Figure 41.4
shows frequency histograms for the force plots shown in Figure 41.3a and Figure 41.3b. Using the cumu-
lative frequency curve the 90th finger forces are estimated for the right and left hands as 118 and 52 N,
respectively. The can be expressed on a zero to ten scale by dividing them by the posture-specific strength
and multiplying the result by ten:
Peak finger force (0-10)
¼
90th percentile finger force (N)
=
strength (N)
10
The knife and thigh in the thigh boning example are held in a power grip posture. Based on a survey of
40-45-yr-old suburban and rural females by Mathiowetz et al. (1985), average right and left hand grip
strengths are estimated as 314
+
60 and 278
+
61 N respectively:
Right hand: peak finger force
¼
118N
=
314
10
¼
3
:
8
Left hand: peak finger force
¼
52 N
=
278
10
¼
1
:
9
Strength can be adjusted from males or females, young or old workers, or higher or lower percentiles,
depending on the population of interest.
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