The Comparisons of Whole Body Vibration ...

Proceedings of the Human Factors and Ergonomics Society 2016 Annual Meeting

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The Comparisons of Whole Body Vibration Exposures and Supporting Musculature Loading between Single- and Multi-axial Suspension Seats during Agricultural Tractor Operation Jeong Ho Kim1, Jack T Dennerlein2, Peter W Johnson3 1

School of Biological and Population Health Sciences, Oregon State University, Corvallis, OR Department of Physical Therapy, Movement, and Rehabilitation Science, Northeastern University, Boston, MA 3 Department of Environmental and Occupational Health Sciences, University of Washington, Seattle, WA

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Copyright 2016 by Human Factors and Ergonomics Society. DOI 10.1177/1541931213601212

Due to rough terrain, agricultural tractor drivers are likely exposed to a high level of whole body vibration, especially impulsive shocks. These WBV exposures are often predominant in the foreaft (x) or lateral (y) axis. However, the current industry standard seats are designed to reduce mainly vertical (z) axis WBV exposures, and therefore, may be less effective in reducing tractor drivers’ exposure to WBV. Therefore, in a repeated-measures design with 11 subjects, this study evaluated efficacy of a multi-axial (vertical + lateral) suspension seat in reducing WBV exposure and low back (erector spinae) muscle activity relative to an industry standard single-axial suspension seat. The results showed that while there was no difference in fore-aft (x) and vertical (z) axis WBV exposures between the seats, the multi-axial suspension seat had lower A(8) lateral (y) WBV exposures [median (interquartile range): 0.7 (0.41, 0.83) m/s2] and VDV(8) [13.5 (7.4, 16.4) m/s1.75] WBV exposures than the single-axial suspension seats [ A(8): 0.81 (0.48 0.93) m/s2; VDV(8): 13.5 (8.7, 18.5) m/s1.75] (p = 0.02 and 0.04, respectively). Low back muscle activity was also lower on the multi-axial suspension seats, however this difference was not significantly significant. These results indicate that mu the multi-axial suspension may have potential to reduce the WBV exposures and muscular loading on low back among agricultural vehicle operators.

INTRODUCTION Whole-body vibration (WBV) exposures in professional vehicle operators have been associated with musculoskeletal disorders including low back pain (LBP). LBP is the most common cause for lost productivity and disability in the workplace (Punnet et al., 2005). Punnett and her colleagues (2005) estimated that the work-related LBP may result in approximately 818,000 disability-adjusted lost life years annually. Due to predominantly working on off-road terrain, agricultural tractor drivers are likely exposed to a high level of WBV exposure, especially impulsive shocks. These transient shock exposures are known to contribute to the degeneration of lumbar spine more than the continuous steady state component (Mayton et al., 2008). In general, as agricultural tractors are operated on rough terrain, the tractor drivers are likely exposed to high levels of WBV, especially impulsive shocks while operating the tractors on rough terrain (Mayton et al, 2008).

The European Union (EU) Vibration Directives (Directive 2002/44/EC) suggests that the WBV exposure measures are calculated based on the predominant axis among fore-aft (x), lateral (y), and vertical (z) axes. Moreover, the current industrial-standard seats for agricultural tractors are equipped with a single-axial (vertical) suspension. However, in off-road vehicles such as agricultural vehicles, the predominant exposure axis is not necessarily vertical (z-axis) but either fore-aft (x-axis) or lateral (y-axis); and therefore, the current industry-standard agricultural tractor seats may be less effective in reducing lateral components of WBV exposures compared to a multi-axial (vertical + lateral) suspension seat. Due to the greater potential for lateral translations in off-road vehicles, it is thought that muscle loads in the low back may be substantial to counterbalance the inertia of the torso and neck caused by lateral WBV exposures. Agricultural vehicle operator’s long driving hours can also result in the


overuse and damage to the low back and neck muscles, which is a known precursor of musculoskeletal injuries. Therefore, the aim of this study was to determine whether there were differences between a single-axial (vertical) and multi-axial (vertical + lateral) suspension seat in WBV exposures and muscle activity in the low back regions.

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seats and an identical tri-axial accelerometer (Model 352C33; PCB Piezotronics; Depew, NY) magnetically mounted to the floor according to International Organization for Standardization (ISO) 2631-1 whole body vibration standards (Figure 1).

METHODS Subjects In a repeated-measures design, a total of 11 professional truck or agricultural tractor drivers (9 males and 2 females) were recruited in this laboratory experimental study. All the subjects were experienced truck or tractor drivers with no pre-existing musculoskeletal disorders in the upper extremities and low back. The subjects’ average (± SD) age and driving experience was 47.5 (± 10.9) and 24.9 (± 13.0) years, respectively. Their average (± SD) height, weight, and body mass index were 180.1 (± 7.8) cm, 105.8 (± 28.0) kg, and 32.5 (± 8.1). The experimental protocol was approved by the University’s Human Subjects Committee and all subjects provided their written consent prior to their participation in the study. Whole Body Vibration Simulation In this laboratory experiment, actual fieldmeasured vibration profiles collected from an agricultural tractor were played into and simulated on a six-degree-of-freedom (6-DOF) motion platform (MBE-6DPF, Moog Inc., East Aurora, NY). The 6-DOF motion platform consisted of 6 electric linear servo actuators which replicated the same WBV exposure measured in the field. The field-measured vibration profiles used for this WBV simulation were collected from six road segments ranging including smooth paved roads, gravel road, farm fields, and extreme off-road terrain. The simulated vibration profile was a total of 24 minutes; the order and duration of each road segments were chosen in order to reflect the WBV exposures commonly experienced by agricultural tractor drivers. Each subject tried both a single-axial (vertical) suspension (MSG 97; Grammer; Amberg, Germany) and multi-axial (vertical + lateral) suspension seat (MSG 97; Grammer; Amberg, Germany). These two seats were identical expect for the presence of the lateral suspension in the multi-axial seat. Whole Body Vibration exposure measures The WBV data from the motion platform were collected using tri-axial seat-pad accelerometers (Model 356B40; PCB Piezotronics; Depew, NY) mounted on the

Figure 1. Tri-axial accelerometer placements on floor and seat. Raw un-weighted acceleration data were simultaneously collected on floor and seat at 1,280 Hz using an eight-channel data recorder (Model DA-40; Rion Co. LTD; Tokyo, Japan) during the WBV simulation. A custom-built LabVIEW program (v2012; National Instruments; Austin, TX) was used to calculate the WBV exposure parameters as follows: • Root mean square (r.m.s) weighted average acceleration (Aw) calculated at the seat pan, floor, and head (m/s2): =

: Frequency-weighted acceleration at time, t; : Measurement duration in seconds • Vibration dose value (VDV), which is more sensitive to impulsive vibration and reflects the total, as opposed to average vibration, over the measurement period at the seat pan and floor of the motion platform (m/s1.75):


= To enable comparisons across all measurements, both parameters ( and ) were normalized to reflect 8 hours of tractor operation (e.g. 8 and 8 ). Then, these values were compared to the EU daily action limits (0.5 m/s2 and 9.1 m/s1.75 respectively). Lastly, based on the A(8), VDV(8) WBV exposures, the vehicle operating time to reach EU daily action limits was calculated. Electromyography measures Electromyography (EMG) was bilaterally collected from erector spinae (ES) muscle using a data logger (Mega ME6000; Mega Electronics; Kupio, Finland) and Ag/AgCl surface electrodes (Blue Sensor N; Ambu; Ballerup, Denmark) at a sampling rate of 1000 Hz during the entire experiment session. The skin preparation, muscle identification and electrode placement were performed according to the European Recommendation for Surface Electromyography (Hermens et al., 1999). After collecting the raw EMG data, a band pass filter of 20-400 Hz was applied. A high pass filter cutoff of 20Hz was selected to minimize vibration artifacts in the EMG data. The filtered EMG data were normalized as a percentage of the sub-maximal Reference Voluntary Contraction (%RVC) for ES muscle. The RVC was chosen to reduce a risk of injuries as the low back is susceptible to injuries. ES RVCs were obtained during 30 degree truck forward flexion. Subjects were asked to practice their MVCs before the actual measurement. Each contraction time lasted for three seconds (Soderberg & Knutson, 2000) with a 2minute break between contractions. Three MVCs were collected from each muscle; the maximum of the highest RMS signal over a one-second period was identified and used to normalize the EMG data. Then, the 10th %tile (static), 50th %tile (median) and 90th %tile (peak) amplitude probability density function (APDF) muscle activities were calculated (Jonsson, 1982). Statistical data analysis As a result of a small sample and non-normality of the data, Wilcoxon signed-rank tests (JMP Ver. 11 Pro, SAS Institute; Cary, SC) were used to determine the differences in WBV exposures and low back muscle activity between a single- and multi-axial suspension seats. Per statistical guidelines for health science journals (Altman et al., 1983), non-normal data were summarized with median and interquartile ranges.

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Statistical significance was noted when p-values were less than 0.05.

RESULTS The results showed that the y (lateral) axis was predominant axis for the A(8) and VDV(8) WBV exposures measured from the suite of agricultural tractor operation activities we tested (Table 1). Both predominant-axis (y) A(8) and VDV(8) measures were above the EU daily action limits (0.5 m/s2 and 9.1 m/s1.75, respectively). Although all the floor-measured WBV exposure did not differ between the single- and multi-axial suspension seats, differences were observed at the seat level. While there was no difference in the fore-aft (xaxis) and vertical (z-axis) WBV exposures between the seats, the lateral (y-axis) A(8) and VDV(8) measures on the multi-axial suspension seat were lower compared to the single-axial suspension seat (p = 0.02 and 0.04, respectively). Table 1. Median [25th, 75th percentile] A(8) and VDV(8) exposures measured at the seats [n = 11]. Suspension Single-axial Multi-axial P-value* 0.57 0.54 1.4X 0.78 [0.49, 0.76] [0.49, 0.76] 0.81 0.70 A(8) 1.4Y 0.02* m/s2 [0.48 0.93] [0.41, 0.83] 0.35 0.35 Z 0.54 [0.32, 0.38] [0.31, 0.37] 10.8 10.3 1.4X 0.72 [8.9, 14.8] [8.9, 14.7] 15.5 13.5 VDV(8) 1.4Y 0.04* m/s1.75 [8.7, 18.5] [7.4, 16.4] 6.5 6.3 Z 0.13 [5.9, 7.1] [5.7, 6.9] *P-values were calculated from Wilcoxon signed-rank tests. Statistical differences are denoted by asterisks. Bold numbers indicate “above action limits”: A(8) > 0.5 m/s2 and VDV(8) > 9.1 m/s1.75.

The vehicle operating time to reach the EU daily action limits based on A(8) WBV exposures showed that tractor operators with the multi-axial suspension seat (4.1 hours) could operate their tractors approximately 1 hour longer than those with the single-axial suspension seat (3 hours). The time to reach action limits based on VDV(8) showed that drivers with the multi-axial suspension seat (1.7 hours) could operate their tractors


approximately 42 minutes longer than those with the single-axial suspension seat ( 1.0 hours). As shown in Table 2, the low back muscle activity was lower on the multi-axial suspension seat when compared to the single-axial suspension seat. However, these differences were not statistically significant. Table 2. Median [25th, 75th percentile] normalized low back muscle activity (%RVC) exposures measured at the seats [n = 11]. Suspension Single-axial Multi-axial P-value* 126.5 47.4 10th 0.19 [25.2, 192.6] [36.6, 81.3] 162.30 107.3 50th 0.65 [41.4, 285.1] [68.1, 460.8] 180.8 119.6 90th 0.67 [60.7, 333.3] [73.8, 475.5] *P-values were calculated from Wilcoxon signed-rank tests.

DISCUSSION The study evaluated the efficacy of a multi-axial (vertical + lateral) suspension seat for reducing whole body vibration exposures and muscle loading on the low back in agricultural applications. The results showed that the multi-axial suspension may have potential to reduce the WBV exposures and muscular loading on low back among agricultural vehicle operators. The results showed that both A(8) and VDV(8) WBV exposures were above the EU daily action limits (0.5 m/s2 and 9.1 m/s1.75, respectively). These values indicated that agricultural tractor operators may experience high levels of exposure to WBV. Furthermore, the vehicle operating time to reach the daily action limits indicates that the impulsive VDV(8) exposures were a more limiting factor than the weighted average vibration (A(8)) due to the reduced tractor operation times to reach daily action limits. These results support previous findings that agricultural tractor drivers are likely exposed to high levels of WBV, especially impulsive shocks while operating the tractors on rough terrain (Mayton et al, 008). The WBV exposure measures [A(8) and VDV(8)] showed that the predominant axis was the y (lateral) axis. This is in line with previous findings that WBV exposures in agricultural tractors are predominantly experienced in the y axis (Scarlett et al., 2007). The higher x- and y-axis WBV exposures relative to z-axis exposures indicates that agricultural tractor seats should be designed to also address the lateral component of the WBV exposures.

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Indeed, the low back EMG indicated potential benefit of multi-axial suspension seats in further reducing muscular loading during agricultural tractor operation. A multi-axial (vertical + lateral) suspension seat had lower muscular loading on the low back as compared to the industry-standard single-axial suspension seats. However, due to a small sample, these differences were not statistically significant. In conclusion, our simulated laboratory study findings imply that agricultural tractor drivers experience moderate to high exposures to WBV, especially with high impulsive exposures on fore-and-aft and lateral axes. The WBV exposure and EMG data suggest that a lateral suspension, in addition to the vertical suspension, may be more effective in further reducing the WBV exposures and muscular loadings on the low back during agricultural tractor operation. ACKNOWLEDGEMENTS This research was supported by a research grant from Bose Corporation. Authors would like to thank Dr. Michael Lin for his assistance in data collection. Authors also thank all the participants in this study.

REFERENCES Altman, D. G., Gore, S. M., Gardner, M. J., & Pocock, S. J. (1983) Statistical guidelines for contributors to medical journals. British Medical Journal, 286(6376), 1489-1493. European Union. (2002). Directive 2002/44/EC of the European Parliament and of the Council of 25 June 2002 on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents (vibration) (sixteenth individual Directive within the meaning of Article 16(1) of Directive 89/391/EEC). Official Journal of the European Communities L 177, 13-19. International Organization for Standardization. ISO 2631-1(1997): Mechanical vibration and shock Evaluation of human exposure to whole-body vibration - Part 1: general requirements. Geneva, Switzerland: International Organization for Standardization; 1997. Mayton, A. G., Kittusamy, N. K., Ambrose, D. H., Jobes, C. C., & Legault, M. L. (2008). Jarring/jolting exposure and musculoskeletal symptoms among farm equipment operators. International Journal of Industrial Ergonomics, 38(9-10), 758-766. Punnett, L., Pruss-Ustun, A., Nelson, D. I., Fingerhut, M. A., Leigh, J., Tak, S., & Phillips, S. (2005).


Estimating the global burden of low back pain attributable to combined occupational exposures. American Journal of Industrial Medicine, 48(6), 459469. Scarlett, A. J., Price, J. S., & Stayner, R. M. (2007). Whole-body vibration: Evaluation of emission and exposure levels arising from agricultural tractors. Journal of Terramechanics, 44(1), 65-73. Soderberg, G. L. & Knutson, L. M. (2000). A guide for use and interpretation of kinesiologic electromyographic data. Physical Theraphy, 80(5), 485-498.

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