Annals of Occupational Hygiene Advance Access originally published online on December 9, 2004
Annals of Occupational Hygiene 2005 49(3):267-275; doi:10.1093/annhyg/meh077
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© 2004 British Occupational Hygiene Society Published by Oxford University Press;
Original Article |
Whole-Body Vibration Exposure and Non-neutral Neck Postures During Occupational Use of All-terrain Vehicles
1 Occupational Medicine, Department of Public Health and Clinical Medicine, Umeå University, SE-901 87 Umeå, Sweden; 2 Department of Occupational and Environmental Medicine, SE-851 86 Sundsvall, Sweden; 3 University Hospital, Biomedical Engineering and Informatics, SE-901 85 Umeå, Sweden
* Author to whom correspondence should be addressed. Tel: +46 70 3543045; fax: +46 90 7869110; e-mail: borje.rehn{at}physiother.umu.se
ABSTRACT
Objectives: The purpose of this study was to characterize whole-body vibration (WBV) exposure from various all-terrain vehicles (ATVs) like snowgroomers, snowmobiles and forwarders, and to investigate how frequently the drivers' cervical spine is positioned in a non-neutral rotational position during operation.
Methods: Field measurements of WBV were measured according to the international standard ISO 2631-1 in 19 ATVs. Simultaneous recordings of frequency and duration of rotational neck movements exceeding 15° were achieved through an observational method, PEOflex®.
Results: The sum of the vectors of frequency-weighted r.m.s. acceleration varied between 0.5 and 3.5 m s–2, which meant that for most vehicles they exceeded the action value stated by the European Union (
). In general, snowmobiles achieved the highest vibration total value. The dominant vibration direction for the snowmobile was the x-axis but the z-axis also had relatively high vibration dose values and maximal transient vibration values. The z-axis was the dominant vibration direction for the snowgroomer and the y-axis for the forwarder. Frequency and duration of non-neutral rotational neck postures were relatively low for all driver categories.
Conclusions: Vibration magnitudes in ATVs are considerably high than the EU's action value and the health guidance caution zones in ISO 2631-1. The dominant vibration direction varies depending on the machine type. Duration and frequency of non-neutral rotational positions do not seem to constitute single ergonomic risk factors for musculoskeletal symptoms in the neck among professional drivers of ATVs. However, synergistic effects with other factors are conceivable.
Keywords: neck posture • shock • vehicle • whole-body vibration
INTRODUCTION
There is an association between jobs with exposure to whole-body vibration (WBV) and low back pain (Bernard, 1997
). Occupational drivers of various land-based vehicles, such as trucks (Kelsey and Hardy, 1975; Gruber, 1976; Kelsey et al., 1984; Bovenzi et al., 2002), buses (Barbaso, 1958; Backman and Jarvinen, 1983; Bovenzi, 1992; Bovenzi and Zadani, 1992), motor vehicles (Heliövaara, 1987) and heavy trucks (Schmidt, 1969), have been reported to have increased risks for back problems from epidemiological studies. Despite this association, there is not enough evidence to outline an exposure–response relationship between exposure to seated WBV and lower back disorders (Bovenzi and Hulshof, 1998; Lings and Leboeuf-Yde, 2000).
To date, no firm conclusions can be drawn about the relationship between exposure to seated WBV and neck disorders (Viikari-Juntura et al., 1994; Bernard, 1997; Ariëns et al., 2000). However, occupational drivers frequently report problems from this region of the spine as well (Magnusson et al., 1996; Krause et al., 1997; Hammar, 1983; Jonsson et al., 1983). Awkward body postures and constrained positions due to mechanical workload are examples of potential confounding risk factors that are inherently associated with prolonged seated WBV exposure (Johanning, 1991), which makes it difficult to identify independent effects on the musculoskeletal system.
A cross-sectional study by Rehn et al. (2002) showed that occupational drivers of all-terrain vehicles (ATVs), such as forest machines, snowmobiles and snowgroomers, exhibited significantly increased risks for musculoskeletal symptoms primarily from the neck and shoulder region but surprisingly not from the lower back. From that study it was suggested that exposure characteristics in ATVs would be unlike other types of vehicles in terms of WBV magnitude, direction, frequency content and prevalence of shock. Thus, the effects of WBV on the musculoskeletal system may also differ. A special feature for drivers of ATVs would be exposure to intermittent and powerful shocks and jolts due to reckless driving on irregular and uneven terrain. Since awkward head postures have been proposed as a possible risk factor for neck problems among machine drivers (Eklund, 1994) and since the ATV drivers in Rehn et al.'s study (2002) predominately reported symptoms from the neck region, it would also be valuable to investigate how often the drivers of ATVs position their necks in non-neutral attitudes during operation. Explicit description of the characteristics and the role of each risk factor may contribute to a better understanding of the possible synergistic effects from WBV and awkward neck postures.
The aim of the present study was to characterize WBV and shock from forest machines, snowmobiles and snowgroomers. A further objective was to simultaneously record non-neutral rotational positions of the neck used by the drivers during operation.
MATERIALS AND METHODS
Field measurements
Measurements were performed during ordinary occupational use on a sample of 19 different ATVs, each belonging to the category of snowgroomers (n = 7), snowmobiles (n = 6) or forest machines (n = 6). Measurements for snowgroomers were carried out during operation on ski slopes, downhill and uphill, with various inclinations and surface types. One of the snowgroomers was measured during preparation of ski tracks. The riders of the snowmobiles in this study performed different jobs. Three drivers performed line-work for energy companies; two drivers transported themselves for various duties on ski slopes or nearby ski tracks and one policeman routinely inspected an area. Exposure measurements for forest machines concerned only the forwarder type during loading, unloading and transportation with and without cargo. All vehicles were measured during operation in various terrain conditions in the northern part of Sweden. WBV for snowgroomers and snowmobiles were measured during the winter season whereas forwarders were measured during other seasons. The registration periods varied from 5 to 35 min to complete at least one typical working cycle. Two of the forwarders did not accomplish a complete working cycle.
Drivers and ATVs
The drivers in this study had participated in an earlier cross-sectional study on musculoskeletal symptoms (Rehn et al., 2002). All subjects in that study were asked for permission to be contacted again for further investigations. Those who had given their consent and had business nearby were contacted by phone to ask if they wanted to participate in the exposure assessment and an agreement was reached for a suitable time. None of the subjects contacted refused further participation and all of them were experienced drivers of the vehicle in question (holding their current job for at least 3 years).
The number of vehicles in each group of ATVs was considered as representative for this explorative study. The snowgroomers included in this study were from three different manufacturers with the year of production ranging from 1986 to 1992. There were three manufacturers of the snowmobiles and two manufacturers of the forwarders with the year of production ranging from 1989 to 2001 and 1988 to 2001, respectively.
Exposure assessment
Vibration measurements were made, in accordance with ISO 2631-1 (ISO 2631-1, 1997), on the interface between the seat and the pelvis. The measurement plate held three mutual orthogonal accelerometers (Brüel & Kjaer 4322®) for the x- (back to front), y- (right to left), and z-axes (foot to head). The signals were bandpass-filtered (0.1–1000 Hz) and amplified by a charge amplifier (Brüel & Kjaer 5974®) and then recorded on an eight-channel DAT-recorder (Sony PC 208Ax®). Calibration signals from an accelerometer calibrator (Brüel & Kjaer 4294®) were recorded before each field measurement.
A web camera (Logitech, QuickCam®Pro3000), attached to a technical suction device, originally designed for carrying windows, was fastened onto various places of the front window or the bonnet of the snowmobiles depending on the different space possibilities. The suction device could be easily attached using a plain handgrip. It was possible to adjust the direction of the camera lens. Before a ride, it was checked that the camera image covered the whole area from the top of the head to the shoulders of the driver. The web camera was connected to a portable computer. There were four drivers who were not filmed during operation. All the remaining drivers were however filmed simultaneously with the vibration measurement, i.e. synchronized. None of the drivers stated presented substantial problems from the neck, shoulders or back region.
Data analyses
Frequency-weighted r.m.s. magnitudes, aw [equation (1)], were used to describe the vibration conditions. The r.m.s. magnitudes are based on the second power of the acceleration time history in either x-, y- or z-directions (ISO 2631-1, 1997).
 | (1) |
The vibration total value (av), is the sum of vectors of the frequency weighted acceleration magnitudes for all three directions, i.e. x, y and z [equation (2)]. This measure is, according to ISO 2631-1, primarily recommended for the assessment of discomfort (ISO 2631-1, 1997). When using the formula for health risk evaluation in seated persons, the x- and y-axes are adjusted with a multiplying factor of 1.4. Then the dominant axis is determined.
 | (2) |
When the crest factor is over nine it is recommended that additional methods such as maximum transient vibration value (MTVV) and vibration dose value (VDV) are used [ISO 2631-1, 1997]. MTVV [equation (3)], is the highest magnitude of aw(t0) read during the measurement period. The running r.m.s. evaluation method was used to assess MTVV and the time integration constant (
) was set to 1 s.
 | (3) |
 |
).
 | (4) |
), crest factors were formed by the frequency-weighted peak magnitudes in relation to the r.m.s. magnitudes determined over the complete duration of measurement. The r.m.s. magnitudes, MTTV, VDV and crest factors were calculated using LabView®, Version 6.1 (National Instruments). The frequency component analysis of weighted r.m.s. magnitudes, using one-third octaves (between 0.1–100 Hz), was determined by use of Pulse Labshop®, Version 5.1 (Brüel & Kjaer).
It is possible to estimate the weighted r.m.s. acceleration for an 8 h working day from the exposure data sampled for less than 8 h by assuming that human response is related to energy and that the exposure is kept constant during the working day (ISO 2631-1, 1997). The formula in equation (5), was used for estimating the recommended exposure time, according to the directive on physical agents stated by the European Union (EU), with regard to the exposure action r.m.s. value 0.5 m s–2 (9.1 m s–1.75 for VDV) and the exposure limit r.m.s. value 1.15 m s–2 (21 m s–1.75 for VDV) for daily exposure (Council of the European Union, 2002).
 | (5) |
).
The frequency (number of events) and duration of voluntary neck rotations to the left and to the right were analysed using the Portable Ergonomic Observation method, PEOflex® (Fransson-Hall et al., 1996). During the analysis, the observer continuously registered neck rotations, the neck relative to torso (i.e. with the sternum as the reference line and the chin and nose as the measurement line), from video recordings simulated in real time by hitting predefined keys on the computer keyboard. A more detailed description of the method is given by Fransson-Hall et al., (1996). A non-neutral neck position was set at a rotation in the transverse plane exceeding 15°. Before conducting the analysis, this position was memorized by the observer after having two people holding this position and measuring it by more exact methods. The limit of 15° was a proposal from a Swedish expert group that developed a model to help in the process of evaluating occupational diseases for the Swedish national occupational injury insurance (Tegner et al., 1983). The group suggested a detrimental effect if the neck sustained a rotational posture between 15 and 45° for more than 75% of the working time. In the present study, the same observer made all PEOflex registrations and the reliability (i.e. intraobserver reliability) was estimated as a test–retest correlation.
Statistics
Vibration magnitudes, r.m.s. and VDV, were compared between axes and groups using a generalized linear model to decide one-way ANOVA. This was followed by post-hoc multiple comparisons using Tukey's honestly significant difference test for cases with equal variances and Tamhane's T2 test for cases with unequal variances. Homogeneity of variance was analysed using the Levene test. Checks for normality with regard to vibration magnitudes for vehicles and vibration directions were performed by analysing standardized residuals and using the Kolmogorov–Smirnov test. Two outliers were allowed. A P-value <0.05 was considered statistically significant. The agreement between two recordings from PEOflex was tested for all subjects by calculating the proportion of intrarater agreement (Ps) using an equation [equation (6)] presented by Fleiss (1981). By applying this equation, it was possible to test the reliability for both duration and frequency. The range of Ps is 0.00 (total disagreement) to 1.00 (total agreement). Registration of the start and stop times was made to the nearest second.
 | (6) |
RESULTS
Frequency-weighted r.m.s. acceleration
When comparing frequency-weighted r.m.s. acceleration between directions, after adjustment with a sensitivity factor of 1.4 for the x- and y-directions, it was found that snowgroomers had their highest vibration magnitudes in the z-direction (P < 0.05) compared with the other directions (Table 1). In contrast, the dominant direction for snowmobiles was found to be the x-axis, except for two vehicles (see Table 1). The magnitude of vibration along the z-axis for snowmobiles was in general lower compared with the other axes. Forwarders had the most dominant vibration direction in the y-axis with significantly higher mean magnitudes compared with the other vibration directions.
|
Between vehicles, snowmobiles had higher vibration magnitudes in the x-axis compared with snowgroomers (P < 0.05). Forwarders had the highest vibration magnitudes in the y-axis and compared with snowgroomers there were a significant difference (P < 0.05). Snowgroomers had higher magnitudes in the z-direction compared with forwarders (n.s.), but not when compared with snowmobiles. The highest vibration magnitudes, as defined by the vibration total value (av), were found in the group of snowmobiles.
VDV and MTVV
In general, snowgroomers had their highest VDV in the z-direction, which was significantly higher when compared with the other directions. MTVV was dominant in the x-axis. The dominant magnitudes for snowmobiles, VDV and MTVV, were found in the x- and z-axes. Forwarders had their highest VDV in the y-direction (n.s.) and the highest MTVVs were found in the z-axis. Statistically significant differences in VDVs between vehicles were found between snowmobiles and snowgroomers in the x-axis, and also between snowmobiles and snowgroomers and between forwarders and snowgroomers concerning the y-axis (Table 1).
Frequency analysis
The frequency analysis showed that snowgroomers had a more widespread and smooth frequency span in the x-axis, from 
0.2 to 50 Hz. Snowmobiles had their highest vibration magnitudes in the span from 0.2 to 2.5 Hz in the x-axis (Fig. 1). Forwarders had their highest magnitudes in the x-axis, varying between 0.8 and 5.0 Hz.
|
When comparing vibration along the y-axis, it was found that forwarders had their highest magnitudes in the frequency span from 1.0 to 2.0 Hz with a distinct peak in the 1.25 Hz one–third octave band. Snowmobiles had their highest vibration magnitudes in the y-direction ranging between 0.2 and 10 Hz. The frequency span for snowgroomers in the y-axis was more widely spread when compared with the other vehicle types. For the z-direction it seemed that all vehicles have their peaks between 2.0 and 8.0 Hz. Snowmobiles had a more widespread frequency span in the z-direction compared with the other vehicle types (Fig. 1.).
WBV risk evaluation
Table 2 shows that, when evaluating the most dominant direction by using the r.m.s. magnitudes (x- and y-axis adjusted for sensitivity by a factor of 1.4), there were two vehicles in this study that had vibration magnitudes below the action value for an 8 h working day, which is stated in the directive on physical agents from the EU. There were three snowmobiles and one forwarder that had vibration magnitudes exceeding the limit value. All the other vehicles had vibration magnitudes below the limit value. When analysing VDVs, one snowmobile, one snowgroomer and one forwarder showed magnitudes exceeding the action value. No vehicle had a VDV exceeding the limit value.
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In general, shorter exposure times were achieved using eqns (B.1) and (B.2) from ISO 2631-1 and most vehicles had vibration magnitudes that restricted recommended daily exposure times below 8 h.
Awkward neck positions
The ergonomic analysis showed that the frequency of rotational positions exceeding 15° have a median of two rotations per minute for all driver categories. The duration spent in this position for drivers of snowgroomers and snowmobiles varies from 4 to 7% (mean) of the total registration period, whereas for forwarder drivers the amount of time spent in a non-neutral neck position varies from 10 to 19%. Most events were short, often no longer than 2 s. The longest duration for which the neck rotated by more than 15° was 22 s, which was achieved by one forwarder driver (Table 3).
|
Reliability of observational method
The intraobserver-reliability (Ps) for neck movements in drivers of snowgroomers varied between 0.67 and 0.89 (median 0.79) for frequency and 0.49–0.74 (median 0.67) for duration. The corresponding results for drivers of snowmobiles was 0.68–0.82 (median 0.78) for frequency and 0.51–0.80 (median 0.79) for duration. The reliability for drivers of forwarders varied between 0.82 and 0.89 (median 0.87) for frequency and between 0.72 and 0.86 (median 0.78) for duration.
DISCUSSION
On a group basis, it can be concluded that during occupational use, snowgroomers have their dominant vibration magnitudes in the z-axis, snowmobiles have higher vibration magnitudes in the x-axis and the dominant direction for forwarders is the y-axis. The various dominant directions are probably due to the design of the respective vehicle type and reflect their particular function. High vibration total values for ATVs are produced by relatively high magnitudes in all defined orthogonal directions and compared with the directive on physical agents from the EU and ISO 2631-1, the vibration magnitudes must be regarded as considerably high. Both the frequency and duration of rotational neck positions exceeding 15°, and thereby maintaining extreme neck positions, were found to be low in this group of drivers and thoroughly under the limits suggested by the Swedish expert group (Tegner et al., 1983).
WBV and health effects
Studies (Johanning, 1991; Boshuizen et al.. 1992; Bovenzi and Zadani, 1992; Bovenzi and Betta, 1994; Malchaire et al., 1996) that have demonstrated a positive relationship between lower back disorders and WBV exposure characteristics in various groups of drivers are listed in Table 4. From these studies, it is deduced that the most critical direction for low back pain seems to be the z-axis. Vibration magnitudes obtained in the z-axes for ATVs are comparable with those for other vehicle types. The rationale is that ATV drivers also have an increased risk of low back problems. The dominant frequency bands for ATVs are similar to the other vehicle types and the peaks in the frequency span for the vertical direction also includes resonance frequencies for the cervical spine (1–4 Hz) where the highest physiological strain possibly occurs (Dupuis, 1989). Compared with studies on other vehicle types, the most distinct feature for WBV characteristics in ATVs is the strong influence of lateral vibration. This may account for the pattern of musculoskeletal symptoms observed for drivers of ATVs as reported by Rehn et al. (2002) i.e. musculoskeletal symptoms primarily in the neck region. There seems to be no epidemiological study in the international literature that describes VDV in relation to musculoskeletal health problems, so no comparison can be made. Boshuizen et al. (1992), have however reported crest factors exceeding six, indicating that vibration in fork-lift trucks and freight-container tractors contain significant jerks. When calculating allowed exposure times, using the limits suggested by the EU, there were more restrictions for r.m.s. magnitudes than for VDV. The interpretation of this finding is that vibration magnitudes are high but relatively stable and that the frequency and magnitude of transients is not exaggerated, although there are some transients of high magnitude in all directions.
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Postural load and synergistic effects
Apart from studies reporting measurements of WBV, there are relatively few attempts in the literature to objectively assess other ergonomic risk factors for drivers in their real working environment. Studies that involve ergonomic measurements of drivers have often made use of self-reported questionnaires to assess the postural load (Riihimäki et al., 1989
; Bovenzi, 1992; Bovenzi and Betta, 1994; Krause et al., 1997; Bovenzi et al., 2002). All these studies show postural load to be an important factor for low back pain. Krause et al. (1997) showed a positive dose–response relationship between physical workload and neck pain among urban transit operators.
Although there is limited evidence, postural load such as a maintained extreme position has the potential of being the principal risk factor for the development of musculoskeletal disorders in the neck region (Hagberg et al., 1995), and the detrimental effects in a vibrating environment may be even worse. We found only short periods with rotational neck postures exceeding 15°. It could be discussed whether these short periods may have negative effects on their own or in combination with vibration exposure for professional ATV drivers. A non-neutral neck position, occurring simultaneously with exposure to excessive shock would, on the other hand, be undesirable. For the lumbar region, it has been postulated that an awkward posture, which places the spinal joints in extreme positions, especially in a rotational attitude, would be particularly hazardous during simultaneous exposure to WBV and shock (Wikström et al., 1994). This suggestion for synergistic effects could be applied for the neck region as well.
There are other types of postural load affecting the neck that are involved in the operation of these types of vehicles. Prospective research has found that mere sitting at work for more than 95% of the working time would be a risk factor for neck pain (Ariëns et al., 2001). The same study showed a positive trend for the relationship between neck flexion and neck pain. Static contraction of the neck and shoulder muscles, leading to muscle overload, is often assumed to be a causative factor (Magnusson and Pope, 1998). Counteracting for the weight of the head when the cervical spine is flexed forward (Hagberg, 1984) and excessive exposure times with the arms in flexed or abducted positions (Ohlsson et al., 1995) are associated with static muscle activity in the neck and shoulder area. These factors may all be existent for ATV drivers and must be evaluated in a similar manner for a full understanding of how body posture and WBV might interact, and what effects they might have on the musculoskeletal system of the neck region.
Methodological considerations
The vibration characteristics for ATVs are primarily attributed to the uneven terrain, which is typical of the working environment that the drivers are restricted to. It is however unclear, what influence other determinants like driving speed, driving style and vehicle characteristics (seat, tyres, chassis suspension) have.
This is also the reason why it is not possible to strictly compare specific vehicles. The vibration magnitudes in this study are considered a test sample and direct generalization over the whole working day is not feasible and so also is the evaluation of awkward neck positions. Few measurements containing only some working cycles is a simplification and may introduce bias in the evaluation of adverse health effects caused by driving an ATV.
Due to restricted space and geometry, the camera was placed in various positions in front of the driver. This procedure created a two-dimensional picture in the approximate frontal plane, which can be used for evaluation of movements around a frontal axis (Eklund et al., 1994). Important aspects of three-dimensional movements of the neck cannot be estimated. The intraobserver reliability in this study was quite low when compared with the intrarater agreement in the study by Fransson-Hall et al. (1996), which reflects the difficulties in estimating the rotational neck position using the procedure described. The main purpose of using PEOflex was not to get a precise measurement, but merely to find out if excessive head rotations are present when manoeuvring ATVs. Since there were only few subjects in the present study, the generalization can be questioned. The results do however support the idea that it is possible to operate these types of vehicles without frequent and long-lasting excessive head rotations.
CONCLUSIONS
Analysis of vibration characteristics during occupational use of ATVs, such as snowmobiles, snowgroomers and forwarders, shows that the dominant vibration direction for translational movements varies, depending on the ATV type. The vibration magnitudes for ATVs are high when compared with the action value stated in the EU's directive on physical agents (vibration) and with ISO2631-1. Non-neutral rotational positions of the neck are ergonomic risk factors that occur infrequently and with short durations for professional drivers of ATVs. Further evaluations of both WBV exposure and other ergonomic risk factors are necessary to reach a better understanding of the association between vehicle driving and musculoskeletal disorders in the neck.
ACKNOWLEDGEMENTS
The National Institute for Working Life (NIWL), Technical Risk Factors, Sweden, provided financial support and measurement equipment for this work. We wish to thank Patrik Holmlund at NIWL for technical support with LabView. We would also like to thank Gunnevi Sundelin, Ingvar Bergdahl, Christina Ahlgren, Carin From and Bengt Järvholm at Occupational Medicine, Department of Public Health and Clinical Medicine, Umeå University, Sweden, for valuable contributions throughout the study process.
Received September 25, 2003; in final form September 1, 2004
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