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3. Transfer
library to PDA 4. Perform observation
Print 5. Transfer
data to PC 6. Edit Process Link Calculate
2 3 4
5 Save to file
1
1. Observation
by camera 2. Create library
6
repetitions. Step 2 draws on the analysis of the video to arrange the tasks, activities, within activities, postures and extra information into a tree-diagram (the library). The accurate definition of each of these elements is an important aspect of the creation of this library. The elements are hierarchically structured in 6 blocks; first block will be the description of the task it self (for instance land mowing), the second block will be a description of the activities during land mowing (for instance sitting, walking, and running). The 3rd block will be within activities (for instance with walking one can actually be pushing or lifting weights). The 4th block describes which body postures the within activities withheld, (for instance while walking and pushing ones rotating his hips 45 degrees). Block 5 gives us extra info about the weight of the object which is being pushed and block 6 is an indication about the duration of the activity and the within activity. Accurate definition of the different blocks helps to ensure high intra-observer and inter-observer reliability. An example of this accurate definition of a task for a driver might be, transport = all activities with the goal of moving load A from place X to place Y. Step 3 involves the transfer of the library to the Personal Digital Assistant (PDA). Step 4 consists of the actual worksite observation, and the observed data are
transferred to the computer in Step 5. Step 6 involves the final editing, processing, linking and calculation of the duration, frequency and intensity of the physical demands in the assessed observations. A full description of the Trac system has been published earlier.18
Figure 1: Different steps involved in the use of the PalmTrac system
Simultaneous measurement
To ensure that the PalmTrac system and the WBV measurement would work simultaneously, the start of the measurement was synchronized. A measurement of the WBV magnitude according to the requirements of the international ISO standard 2631-1 should lasts at minimum 3 minutes and should represent as much as possible a complete work cycle of the
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Chapter 2
________________________________________________________________________ 22 vehicle of interest. The WBV measurements in this study lasted between 15 and 25 minutes, the exact measurement time depends on the representative work cycle of the specific vehicle.
Observation by PalmTrac lasted approximately four hours. Due to practical matters, the exact time of the PalmTrac observations will vary.
Self-administered questionnaire
Data on physical work demands were collected using the VIBRISKS Whole-body vibration, Initial Assessment, Self-Administered Questionnaire.22 This questionnaire was elaborated from an earlier version that was tested in several populations.8,23 And was used in the baseline measurement from the VIBRISKS study. For this study only those questions that concern physical work demands (Section 2) were included. Specifically this concerns question 8a, 9a,13, 16 and 15 (see appendix 1). Five physical work demands were defined according to the questions and its questionnaire items, respectively: (1) walking + standing, (2) bending over (straight or twisted), (3) sitting- while driving, (4) sitting twisted/ leaning forward- while driving and (5) lifting. The data that were gathered with the self-administered questionnaire were stored in a Microsoft Access (2002) database.
Analyses
WBV measurement
The WBV measurement data were exported and stored in an Excel spreadsheet, using the Noise Explorer type 7815 version 4.7 software. For further analysis, the first two minutes of the total recorded time were deleted, as it was necessary to install both the equipment and the driver properly. Zero values that might appear in the WBV signal of the actual
measurement, due to very short lost signal, were deleted and extrapolated. This was done by using the average of the values one second before and one second after the zero value, as the WBV signal was logged every second.
The vibration vector sum (total value) of the weighted root-mean-square (r.m.s.) accelerations (awv) in m/s2was calculated according to formula 1:
(1)
(whereawx is the frequency weighted r.m.s. for the x-axis, awyis the frequency weighted r.m.s.
for the y-axis, and awz is the frequency weighted r.m.s. for the z-axis). We calculated for each 1.4awx² + 1.4awy² + awz²
awv = Chapter 2
________________________________________________________________________ 23 driver the daily r.m.s. acceleration normalized to an eight hour working day (a(8)). This a(8) was calculated according to formula 2:
(2)
whereT0is the duration of eight hours expressed in seconds, awv2 is the squared vector sum awv, and Tiis the WBV measurement time expressed in seconds.
PalmTrac data
Results from the ten drivers were used for the comparison between the self-reported and the observational assessments of physical work demands. All of the questions in the self-administered questionnaire were related to an entire working day (8 hours). PalmTrac data were recorded for approximately four hours. To standardize the PalmTrac data also to an entire working day, the exact PalmTrac recording time (in minutes) was calculated as a proportion of eight hours (480 minutes). This proportion was multiplied by the total recorded time for each of the five physical work demands.
Self-administered questionnaire data
The self-administered questionnaire included a response item (question 16, see 2.6) with an ordinal (never- seldom- often) scale. The physical work demand that was defined according to this question is ‘sitting twisted/ leaning forward’. To make a comparison possible between the self-reported and observational assessments of physical work demands in total time, we retrospectively defined ‘never’ as 0 minutes, ‘seldom’ as 0-10 minutes, and
‘often’ as time in excess of ten minutes.
Agreement between PalmTrac data and data from the self-administered questionnaire To test the agreement between the total time revealed by the PalmTrac observation and the estimated total time (based on the self-administered questionnaire) the total times were compared. The estimation of the total time for the physical work demands in the questionnaire is based on the chosen response item. Underestimation was assumed when the total time for the physical work demands revealed by PalmTrac (for instance; 12 minutes) was smaller than the time from the chosen response item in the questionnaire (for instance 15-30 minutes).
Agreement was assumed when the total time for the physical work demands revealed by a(8) = T0
1 �awv² Ti
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Chapter 2
________________________________________________________________________ 24 PalmTrac (for instance; 24 minutes) overlaps with the time from the chosen response item in the questionnaire (for instance 15-30 minutes). Overestimation was assumed when the total time for the physical work demands revealed by PalmTrac (for instance; 45 minutes) was greater than the time from the chosen response item in the questionnaire (for instance 15-30 minutes).
Results Drivers
Subject characteristics for the ten drivers (all male) are shown in Table 1. The average age of the group of drivers was 45 years (±11yrs), the average height 179 centimetre (± 8cm) and the average weight 93 kilograms (± 12kg). The workers drove on their vehicles on average for 16.6 years (± 11.3yrs).
Table 1: Age, height, weight and type of vehicle (specifics) for the ten randomly selected drivers Subject Age (yrs) Height (cm) Weight (kg) Type of Vehicle Specifics
12 30
34 185
190 105
105 Lorry
Wheeled loader (Ginaf M3232S EVS) (Ahlmann AZ 150)
3 37 192 100 Lawn-mowing
machine (Toro Groundmaster 40000D)
4 38 182 92 Wheeled loader (Ahlmann AZ 150)
WBV measurements
Table 2 presents the vibration magnitude (m/s2) for the ten selected drivers in x-, y-andz direction. The vector sum (av) of the WBV magnitude for the different kinds of vehicles ranged from 0.20 m/s2for the boat to 1.53 m/s2 for a wheeled loader. Seven out of the ten drivers are exceeding the action value of 0.5 m/s2 in the dominant axis assuming that the emission value during the actual measurement is maintained all work day long (8 hours).The dominant axis appeared to be dependent of the vehicle driven; the x-axis is dominant for drivers 4, 9 and 10, the y-axis is dominant for drivers 2, 3, and 5-8, and the z-axis is dominant for driver 1.
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Chapter 2
________________________________________________________________________ 25 Table 2: The measured WBV magnitude (m/s2) in x, y, and z-direction and the vector sum for 10 randomly selected drivers.
Illustration of a simultaneously measurement
Figure 2 shows an example of the added value of the simultaneous assessment of physical work demands and a WBV measurement under actual working circumstances for one specific driver (subject 6). For this three-minute period, the WBV measurement on the seat of the wheeled loader revealed frequency-weighted r.m.s acceleration values of ax= 0.57 m/s2, ay= 0.79 m/s2,az= 0.37m/s2and an acceleration total value (vector sum) of (av)= 1.31 m/s2. Examining the example that is illustrated in Figure 2 more closely, the dominant axis exceeds the action value of 0.5 m/s2 for ‘sitting twisted/ leaning back’, ‘sitting’ and ‘sitting while driving with and without load’ all during the task of ‘leveling off’.
Driver ax
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Comparison of physical work demands between PalmTrac and self-administered questionnaire
Results of the comparison in total time between the physical work demands assessed by the self-administered questionnaire and by the PalmTrac system are presented in Figure 3- 7. Each of the physical work demands is presented in a figure. Comparisons are made based on a total duration of a normal eight-hour working day.
Figure 3 shows that for ‘walking + standing’, twenty percent (N=2) of the drivers reported durations that were comparable to the PalmTrac assessments; sixty percent (N=6) underestimated the total time, and twenty percent (N=2) overestimated the total time in which
m/s2 Physical work
Demand ax ay az
Sitting
Leveling-off 0.27 0.42 0.15 Sitting 0.88 1.07 0.54 Sitting twisted/
leaning back 0.96 1.53 0.60 Sitting 0.22 0.36 0.80
Sitting (driving
without load) 0.67 0.97 0.44
Sitting 0.61 0.53 0.37 Transport 0.29 0.38 0.28 Sitting 0.14 0.24 0.28 Sitting (Driving
with load) 0.35 0.47 0.24
Leveling off 0.34 0.18 0.16 Sitting twisted/
leaning back 0.41 0.34 0.21 Sitting (Driving
with load)
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Total time PalmTrac (hrs) 1
2 3
Total time questionnaire, 1= 0 hrs, 2=<1hr, 3=1-3 hrs, 4=>3 hrs (annotation = number of driver)
they were walking and standing during a normal working day. Figure 4 shows that for
‘bending over (straight or twisted)’, eighty percent (N=8) of the drivers reported that they did not perform this activity during a normal working day. PalmTrac, however, showed that half (N=5) of these drivers actually did bend over. Workers apparently tended to underestimate the time that they spent in a bent-over (straight or twisted) posture. The twenty percent (N=2) of
‘bending over (straight or twisted)’, eighty percent (N=8) of the drivers reported that they did not perform this activity during a normal working day. PalmTrac, however, showed that half (N=5) of these drivers actually did bend over. Workers apparently tended to underestimate the time that they spent in a bent-over (straight or twisted) posture. The twenty percent (N=2) of