Industrial Health
Online ISSN : 1880-8026
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ISSN-L : 0019-8366
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Volume 48, Issue 5
Displaying 1-22 of 22 articles from this issue
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Editorial
Original Articles
  • Relative Contribution of Translational and Rotational Vibration to Discomfort
    Ykä MARJANEN, Neil J. MANSFIELD
    Article type: Original Article
    2010 Volume 48 Issue 5 Pages 519-529
    Published: 2010
    Released on J-STAGE: October 13, 2010
    DOIhttps://doi.org/10.2486/indhealth.MSWBVI-19
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    Understanding how vibration affects discomfort is an important factor for improving work and travelling experience. Methods of evaluating health effects from whole-body vibration are closely linked to those for evaluating discomfort in ISO 2631-1. The standard includes a method to evaluate discomfort using twelve axes of vibration with a similar approach to that for evaluating health effects; thus using all twelve axes gives a possibility to evaluate both health and discomfort. The full 12-axis method has not been widely used in practice or validated in a multi-axis environment. The standard guidance is not explicit, thus different interpretations are possible especially when determining the method of comparing or combining vibration in different axes. Furthermore there are not enough studies conducted in multi-axis environments to suggest the optimal combination of axes. In this study ISO 2631-1 method was tested and optimised using a multi-axis test bench at Loughborough University, UK. Subjects were exposed to stimuli which represented vibration characteristics from field measurements. Each stimulus, lasting 15 s, was judged using a continuous judgement, cross-modal matching method. The seat translational and rotational and the backrest translational axes were used in the analyses. There was no vibration at the floor, in order to constrain the number of independent variables. Results showed that correlation for discomfort improved with more complex analysis procedures. However a good correlation was also achieved using just seat translational axes with optimised multiplying factors. The results showed that frequency weightings and r.m.s. averaging improved correlation between vibration and subjective ratings of discomfort. Multiplying factors specified in ISO 2631-1 degraded the correlation between objective and subjective measures of discomfort, therefore an improved set of factors were determined. The new factors showed improvement by placing more emphasis on seat fore-and-aft and lateral axes.
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  • Time Domain Detection of Shocks and Impacts in Whole-body Vibration
    Anthony J. BRAMMER, George RODDAN, James B. MORRISON
    Article type: Original Article
    2010 Volume 48 Issue 5 Pages 530-537
    Published: 2010
    Released on J-STAGE: October 13, 2010
    DOIhttps://doi.org/10.2486/indhealth.MSWBVI-01
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    A method for detecting shocks and impacts in whole-body vibration time histories has been developed that is suitable for implementation as a computer algorithm. The procedure consists of comparing the magnitudes of a higher-order mean value and the impulsiveness calculated for successive time segments of the acceleration-time history. The indicators were the ratio of the 12th-order root mean value to the root mean square RMT/RMS, and the impulsiveness corresponding to a cumulative probability value of 0.97, I(0.97) (i.e., the magnitude of the positive and negative excursions exceeded 3% of the time divided by 2RMS). Both indicators have a value of 2.16 for random vibration with a Gaussian amplitude distribution, and deviate from this value when the motion possesses other characteristics. For seat motion in the Z-direction analyzed using frequency weighting Wb, and time segments of ~20 s, shocks and impacts could be identified when RMT/RMS ≥ 2.5, and I(0.97) ≤ 2.6. A subjective visual classification of 160 exposures to vibration recorded in a range of military vehicles operating under different conditions was performed by a jury of two observers. The subjective classification agreed with computer identification of shocks and impacts in 94% of the cases.
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  • Frequency Weightings for Fore-and-aft Vibration at the Back: Effect of Contact Location, Contact Area, and Body Posture
    Miyuki MORIOKA, Michael J. GRIFFIN
    Article type: Original Article
    2010 Volume 48 Issue 5 Pages 538-549
    Published: 2010
    Released on J-STAGE: October 13, 2010
    DOIhttps://doi.org/10.2486/indhealth.MSWBVI-05
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    Fore-and-aft vibration of a backrest can influence discomfort and the risk of injury associated with whole-body vibration. Relevant standards (BS 6841:1987 and ISO2631-1:1997) recommend the Wc frequency weighting for evaluating fore-and-aft vibration of backrests, but do not specify the precise location for measuring vibration. This study determined equivalent comfort contours for fore-and-aft vibration of the backs of seated persons from 2 to 80 Hz using the method of magnitude estimation, examining the effect of input location, contact area, and body posture. The equivalent comfort contours indicate decreased sensitivity to vibration acceleration at frequencies greater than 8 Hz. Equivalent comfort contours with a full backrest were similar to those with contact at only the highest location on the back. The derived frequency weightings are broadly consistent with frequency weighting Wc but suggest somewhat greater sensitivity at frequencies greater than 30 Hz and vary in shape with changes in vibration magnitude. It is concluded that with low and moderate magnitudes of vibration the severity of fore-and-aft vibration of a backrest can be assessed from the frequency-weighted fore-and-aft acceleration measured at the highest point of contact between the backrest and the body if the frequency weighting Wc is employed in the evaluation.
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  • High Cycle Fatigue Behaviour of Functional Spinal Units
    Gerd HUBER, Daniel M. SKRZYPIEC, Anke KLEIN, Klaus PÜSCHEL, Micha ...
    Article type: Original Article
    2010 Volume 48 Issue 5 Pages 550-556
    Published: 2010
    Released on J-STAGE: October 13, 2010
    DOIhttps://doi.org/10.2486/indhealth.MSWBVI-11
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    Vibrations have been shown to be an important risk factor for spinal pathologies. The underlying mechanisms are poorly understood and in vivo data scarce and difficult to obtain. Consequently numerical models are used to estimate spinal loading; requiring fatigue strength information, which was obtained in this study for spinal specimens from young and old male donors of working age in vitro. Bone mineral density (BMD) and endplate area were determined using CT scans. Three groups were investigated: young specimens in neutral posture, young in flexed posture, and old in neutral posture. The loading consisted of 300,000 sinusoidal compression cycles of 2 kN, inducing a nucleus pressure peek of approximately 1.4 MPa. No failure of the young specimens in neutral posture was observed, but four specimens from older donors with low BMD failed. The product between endplate area and BMD was shown to be useful to predict fatigue strength for old donors and should therefore be considered with regard to whole body vibration injuries. In flexed posture, two specimens from young donors failed. One failure can be attributed to low BMD following the trend for the old specimens; the other failure could not be explained, leaving the influence of flexion yet unclear.
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  • Biodynamic Response and Spinal Load Estimation of Seated Body in Vibration Using Finite Element Modeling
    Wenping WANG, Babak BAZRGARI, Aboulfazl SHIRAZI-ADL, Subhash RAKHEJA, ...
    Article type: Original Article
    2010 Volume 48 Issue 5 Pages 557-564
    Published: 2010
    Released on J-STAGE: October 13, 2010
    DOIhttps://doi.org/10.2486/indhealth.MSWBVI-34
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    Trunk biomechanical models play an indispensable role in predicting muscle forces and spinal loads under whole-body vibration (WBV) exposures. Earlier measurements on the force-motion biodynamic response (impedance, apparent mass) at the body-seat interface and vibration transmissibility (seat to head) have led to the development of different mechanical models. Such models could simulate the overall passive response and serve as an important tool for vehicle seat design. They cannot, however, evaluate physiological parameters of interest under the WBV. On the contrary, anatomical models simulating human's physiological characteristics can predict activities in muscles and their dynamic effects on the spine. In this study, a kinematics-driven nonlinear finite element model of the spine, in which the kinematics data are prescribed, is used to analyse the trunk response in seated WBV. Predictions of the active model (i.e., with varying muscle forces) as compared with the passive model (i.e., with no muscle forces) compared satisfactorily with measurements on vertical apparent mass and seat-to-head transmissibility biodynamic responses. Results demonstrated the crucial role of muscle forces in the dynamic response of the trunk. Muscle forces, while maintaining trunk equilibrium, substantially increased the compression and shear forces on the spine and, hence, the risk of tissue injury.
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  • Seat-to-head Transfer Function of Seated Men ―Determination with Single and Three Axis Excitations at Different Magnitudes
    Barbara HINZ, Gerhard MENZEL, Ralph BLÜTHNER, Helmut SEIDEL
    Article type: Original Article
    2010 Volume 48 Issue 5 Pages 565-583
    Published: 2010
    Released on J-STAGE: October 13, 2010
    DOIhttps://doi.org/10.2486/indhealth.MSWBVI-03
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    Most research has investigated the seat-to-head transmissibility during single-axis excitations. Associations between head accelerations and discomfort or effects on vision were reported. Possible differences between the seat-to-head transmissibility determined during different vibration magnitudes with a variable number of excitation axes have not been systematically examined. An experimental study was performed with 8 male subjects sitting on a rigid seat with hands on a support. They were exposed to random whole-body vibration (E1=0.45 ms-2, E2=0.90 ms-2, and E3=1.80 ms-2) to single- and three-axis vibration. All translational and rotational seat-to-head transmissibilities were calculated. The effects of the factors vibration magnitude and number of axes on the peak modulus and frequency of the seat-to-head transmissibilities were tested. In general the head motions follow constant pattern. These pattern of head motions comprise a combination of rotational and translational shares of transmissions, i.e. the curves show a dependence on the factors ‘vibration magnitude’ and ‘number of vibration axes’. Mechanical properties of the soft tissue, relative motions of body parts, and muscle reactions were supposed to cause the nonlinearities of the head. Future research should consider effects of multi-axis vibration, if conclusions shall be drawn for the evaluation of possible health effects and model validations.
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  • A Longitudinal Study of Low Back Pain and Daily Vibration Exposure in Professional Drivers
    Massimo BOVENZI
    Article type: Original Article
    2010 Volume 48 Issue 5 Pages 584-595
    Published: 2010
    Released on J-STAGE: October 13, 2010
    DOIhttps://doi.org/10.2486/indhealth.MSWBVI-02
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    The aim of this study was to investigate the relation between low back pain (LBP) outcomes and measures of daily exposure to whole-body vibration (WBV) in professional drivers. In a study population of 202 male drivers, who were not affected with LBP at the initial survey, LBP in terms of duration, intensity, and disability was investigated over a two-year follow-up period. Vibration measurements were made on representative samples of machines and vehicles. The following measures of daily WBV exposure were obtained: (i) 8-h energy-equivalent frequency-weighted acceleration (highest axis), A(8)max in ms-2 r.m.s.; (ii) A(8)sum (root-sum-of-squares) in ms-2 r.m.s.; (iii) Vibration Dose Value (highest axis), VDVmax in ms-1.75; (iv) VDVsum (root-sum-of-quads) in ms-1.75. The cumulative incidence of LBP over the follow-up period was 38.6%. The incidence of high pain intensity and severe disability was 16.8 and 14.4%, respectively. After adjustment for several confounders, VDVmax or VDVsum gave better predictions of LBP outcomes over time than A(8)max or A(8)sum, respectively. Poor predictions were obtained with A(8)max, which is the currently preferred measure of daily WBV exposure in European countries. In multivariate data analysis, physical work load was a significant predictor of LBP outcomes over the follow-up period. Perceived psychosocial work environment was not associated with LBP.
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  • An Evaluation of the Methods for Deriving Representative Frequency Response Functions of the Human Whole-body System
    Ren G. DONG, Thomas W. MCDOWELL, Daniel E. WELCOME, John Z. WU
    Article type: Original Article
    2010 Volume 48 Issue 5 Pages 596-605
    Published: 2010
    Released on J-STAGE: October 13, 2010
    DOIhttps://doi.org/10.2486/indhealth.MSWBVI-07
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    The biodynamic response functions of the human whole-body system measured with subjects participating in an experiment are commonly arithmetically averaged and used to represent their mean response functions in many studies. The reported means were further averaged to form the reference means for standardization and various applications. The objectives of this study are to clarify whether this response-based averaging process could significantly misrepresent the characteristics of the original functions, and to explore appropriate methods for deriving representative functions. A set of reported mechanical-equivalent models for 12 subjects was used to derive the vertical and fore-and-aft cross-axis response functions expressed in apparent mass. The response-based average was directly compared with the response derived from a property-based derivation method. This study found that the response-based average could differ from the property-based mean response by more than 30%, especially in the fore-and-aft cross-axis response functions. This study also theoretically demonstrated that the discrepancies result from the non-linear relationship between the apparent mass and the properties of a dynamic system. Therefore, the discrepancies depend on the variability of the subjects' dynamic properties. Practically, the discrepancies in the vertical response could be reduced to an acceptable level (e.g., <10%) if a sufficient number of subjects with similar body weights are selected or grouped in the measurement. However, it is very difficult to reduce the discrepancies in the fore-and-aft cross-axis to such a level. While more demanding than the response-based method, the property-based method is theoretically more reliable for deriving the representative response functions for each axis.
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  • Long Term WBV Measurements on Vehicles Travelling on Urban Paths
    Giovanni MOSCHIONI, Bortolino SAGGIN, Marco TARABINI
    Article type: Original Article
    2010 Volume 48 Issue 5 Pages 606-614
    Published: 2010
    Released on J-STAGE: October 13, 2010
    DOIhttps://doi.org/10.2486/indhealth.MSWBVI-31
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    This paper describes the results of a long-term whole-body-vibration monitoring campaign performed on different cars with different drivers. The weighted and the un-weighted root-mean-square acceleration, the MTVV and the VDV have been monitored on five different cars in regular usage for over one hundred hours of measurements on urban roads and highways. The variability of the above parameters has been statistically analyzed in order to assess the time requested for the convergence of standard indexes to their average values. The aim is to supply a general reliability evaluation so as to minimize the on-field tests and to provide a scientific support to the design of such experiments. A comparison between different vehicles is presented and discussed; the correlation with speed measured by a GPS system is analyzed with probabilistic assessments. Results showed that the minimum time for reliable measurement was approximately 30 min for each driving condition (urban, carriage road, highway). The MTVV/aw ratio was usually larger than 1.5 (even on short measurement periods), thus indicating the unsuitability of the basic ISO 2631 criterion. The 8-h based VDV provided indications compatible with the av criterion.
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  • Biodynamic Responses of the Seated Human Body to Single-axis and Dual-axis Vibration
    Yi QIU, Michael J. GRIFFIN
    Article type: Original Article
    2010 Volume 48 Issue 5 Pages 615-627
    Published: 2010
    Released on J-STAGE: October 13, 2010
    DOIhttps://doi.org/10.2486/indhealth.MSWBVI-26
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    Occupational exposures to vibration always involve multi-axis vibration. Since human responses to vibration are highly nonlinear and cross-coupled, it is to be expected that excitation in one axis will alter response to vibration in another axis. The purpose of this study was to investigate nonlinearity in the apparent masses of subjects seated without a backrest and exposed to single-axis and dual-axis vertical and fore-and-aft excitation. The driving point apparent masses and cross-axis apparent masses in the two translational directions were measured with twelve subjects exposed to random vibration (0.2 to 20 Hz) in all 15 possible combinations of four vibration magnitudes (0, 0.25, 0.5, or 1.0 ms-2 r.m.s.) in the fore-and-aft and vertical directions. With single-axis excitation (either fore-and-aft or vertical), the median in-line apparent mass exhibited a nonlinear characteristic in which the body softened with increasing magnitude of vibration. With dual-axis excitation, at all magnitudes of vertical excitation the resonance frequency in the vertical apparent mass reduced as the magnitude of fore-and-aft vibration increased, and at all except the greatest magnitude of fore-and-aft excitation the resonance frequency in the fore-and-aft apparent mass reduced as the magnitude of vertical vibration increased. The coherency between the fore-and-aft acceleration and the fore-and-aft force was lowered by the addition of vertical excitation, and the coherency between the vertical acceleration and the vertical force was lowered by the addition of fore-and-aft excitation. The nonlinearity evident in both in-line apparent masses was also evident in the cross-axis apparent masses. It is concluded that with dual-axis excitation the fore-and-aft and vertical response of the seated human body is nonlinear, with resonance frequencies decreasing with increasing magnitude of vibration. Consequently, vibration in one axis (either fore-and-aft or vertical) affects the apparent mass of the body measured in the other axis (either vertical or fore-and-aft).
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  • Assessment and Prediction of Whole-body Vibration Exposure in Transport Truck Drivers
    Rocco NITTI, Paolo DE SANTIS
    Article type: Original Article
    2010 Volume 48 Issue 5 Pages 628-637
    Published: 2010
    Released on J-STAGE: October 13, 2010
    DOIhttps://doi.org/10.2486/indhealth.MSWBVI-24
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    The European Directive 2002/44/EC on the minimum Health and Safety prescriptions regarding the exposure of workers to vibrations, was implemented in Italy through the Legislative Decree 187/2005, recently amended by the Legislative Decree 81/2008. The Decrees contain legal obligations and minimum requirements for the evaluation by direct measurement, which is the reference method, although not always appropriate or necessary, and by means of vibration data banks or information provided by equipment manufacturers. The values assessed must be representative of the actual working environment: in order to adapt assessed values to real working conditions it may be useful to adopt some statistical models. Statistically significant relationships were observed by means of a multiple linear regression on a limited set of measures on different models of trucks, in different operating conditions and settings: the relative influence of predictor variables was then assessed. Finally a short digression about the evolution of the suspension fitting has been made in order to briefly describe the historical context of WBV exposure level reduction and the state of the art of industrial vehicle comfort improvement technologies.
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  • Assessing Combined Exposures of Whole-body Vibration and Awkward Posture― Further Results from Application of a Simultaneous Field Measurement Methodology
    Nastaran RAFFLER, Ingo HERMANNS, Detlef SAYN, Benno GÖRES, Rolf E ...
    Article type: Original Article
    2010 Volume 48 Issue 5 Pages 638-644
    Published: 2010
    Released on J-STAGE: October 13, 2010
    DOIhttps://doi.org/10.2486/indhealth.MSWBVI-27
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    The drivers of ten vehicles (tram, helicopter, saloon car, van, forklift, two mobile excavators, wheel loader, tractor, elevating platform truck) were studied with regard to the combined exposures of whole-body vibration and awkward posture during occupational tasks. Seven degrees of freedom (DOFs), or body angles, were recorded as a function of time by means of the CUELA measuring system (Computer-assisted registration and long-term analysis of musculoskeletal workloads) for the purpose of posture assessment. The vibrational exposure is expressed as the vector sum of the frequency-weighted accelerations in the three Cartesian coordinates; these were recorded simultaneously with the posture measurement. Based upon the percentage of working time spent under different workloads, a scheme is proposed for classification of the two exposures into three categories. In addition, a risk of adverse health effects classified as low, possible or high can be assigned to the combination of the two exposures. With regard to posture, the most severe exposure was measured for the drivers of the wheel loader and for the tractor driver, whereas the lowest exposure was measured for the helicopter pilots and van drivers. With regard to the combination of whole-body and posture exposures, the tractor driver and the elevating platform truck driver exhibited the highest workloads.
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  • A Quasi-static Discomfort Measure in Whole-body Vibration
    Salam RAHMATALLA, Rosalind SMITH, John MEUSCH, Ting XIA, Tim MARLER, M ...
    Article type: Original Article
    2010 Volume 48 Issue 5 Pages 645-653
    Published: 2010
    Released on J-STAGE: October 13, 2010
    DOIhttps://doi.org/10.2486/indhealth.MSWBVI-28
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    A new methodology for objective evaluation of discomfort in whole-body vibration (WBV) is introduced in this work. The proposed objective discomfort characterizes discomfort based on the relative motion between adjacent segments of the human body from neutral positions. It peaks when the joints reach their limits. The objective discomfort has been tested on five subjects in the fore-aft direction using discrete sinusoidal frequencies of 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 7, 8, 9, 10, 12, 14, and 16 Hz. Each frequency file runs for 15 s with a 3 s resting period as a reference for discomfort comparison. All files run at a constant acceleration of 0.7 m/s2. The subjects were tested with back support and without back support, and their subjective discomfort was reported based on the Borg CR-10 scale. The proposed objective discomfort has shown significant correlation with the subjective discomfort. The objective discomfort has also been tested on five subjects under multiple-axis random WBV with three common industrial seating configurations (seat-mounted control, floor-mounted control, and steering wheel), and has shown promising results.
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  • A Variable Parameter Single Degree-of-freedom Model for Predicting the Effects of Sitting Posture and Vibration Magnitude on the Vertical Apparent Mass of the Human Body
    Martin G.R. TOWARD, Michael J. GRIFFIN
    Article type: Original Article
    2010 Volume 48 Issue 5 Pages 654-662
    Published: 2010
    Released on J-STAGE: October 13, 2010
    DOIhttps://doi.org/10.2486/indhealth.MSWBVI-33
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    Models of the vertical apparent mass of the human body are mostly restricted to a sitting posture unsupported by a backrest and ignore the variations in apparent mass associated with changes in posture and changes in the magnitude of vibration. Using findings from experimental research, this study fitted a single degree-of-freedom lumped parameter model to the measured vertical apparent mass of the body measured with a range of sitting postures and vibration magnitudes. The resulting model reflects the effects of reclining a rigid backrest or reclining a foam backrest (from 0 to 30 degrees), the effects of moving the hands from the lap to a steering wheel, the effects of moving the horizontal position of the feet, and the effects of vibration magnitude (from 0.125 to 1.6 ms-2 r.m.s.). The error between the modelled and the measured apparent mass was minimised, for both the apparent masses of individual subjects and the median apparent masses of groups of 12 subjects, for each sitting posture and each vibration magnitude. Trends in model parameters, the damping ratios, and the damped natural frequencies were identified as a function of the model variables and show the effects of posture and vibration magnitude on body dynamics. For example, contact with a rigid backrest increased the derived damped natural frequency of the principal resonance as a result of reduced moving mass and increased stiffness. When the rigid backrest was reclined from 0 to 30º, the damping decreased and the resonance frequency increased as a result of reduced moving mass. It is concluded that, by appropriate variations in model parameters, a single degree-of-freedom model can provide a useful fit to the vertical apparent mass of the human body over a wide range of postures and vibration magnitudes. When measuring or modelling seat transmissibility, it may be difficult to justify an apparent mass model with more than a single degree-of-freedom if it does not reflect the large influences of vibration magnitude, body posture, and individual variability.
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  • Whole-body Vibratory Response Study Using a Nonlinear Multi-body Model of Seat-occupant System with Viscoelastic Flexible Polyurethane Foam
    Gauri JOSHI, Anil K. BAJAJ, Patricia DAVIES
    Article type: Original Article
    2010 Volume 48 Issue 5 Pages 663-674
    Published: 2010
    Released on J-STAGE: October 13, 2010
    DOIhttps://doi.org/10.2486/indhealth.MSWBVI-13
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    Vehicle occupants are exposed to low frequency vibrations with possible harmful effects such as mild discomfort, lower back pain, and even injury to the spine. Occupational drivers and operators of heavy machinery are exposed to significantly longer duration and higher levels of vibration. Thus, the modeling and prediction of biodynamic response of seated occupants to such vibrations is very important. Since the properties of seating foam affect the response of the occupant, there is need for good models of seat-occupant systems through which the effects of foam properties on the dynamic response can be directly evaluated. A nonlinear planar seat-occupant model which incorporates the nonlinear viscoelastic behavior of seating foam has been developed. This model is used to study response of the occupant to harmonic excitation applied at the seat base, in terms of the frequency response in vertical and fore-and-aft directions, the deflection shapes at resonance, as well as the seat-to-head-transmissibility. In addition, to better understand the role of flexible polyurethane foam in characterizing the system behavior, the response of a single-degree-of-freedom foam-block system is also studied. The effects of different masses riding on the foam block and undergoing vertical vibrations at different acceleration levels are also investigated.
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  • Influence of Mono-axis Random Vibration on Reading Activity
    M.K. BHIWAPURKAR, V.H. SARAN, S.P. HARSHA, V.K. GOEL, Mats BERG
    Article type: Original Article
    2010 Volume 48 Issue 5 Pages 675-681
    Published: 2010
    Released on J-STAGE: October 13, 2010
    DOIhttps://doi.org/10.2486/indhealth.MSWBVI-09
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    Recent studies on train passengers' activities found that many passengers were engaged in some form of work, e.g., reading and writing, while traveling by train. A majority of the passengers reported that their activities were disturbed by vibrations or motions during traveling. A laboratory study was therefore set up to study how low-frequency random vibrations influence the difficulty to read. The study involved 18 healthy male subjects of 23 to 32 yr of age group. Random vibrations were applied in the frequency range (1-10 Hz) at 0.5, 1.0 and 1.5 m/s2 rms amplitude along three directions (longitudinal, lateral and vertical). The effect of vibration on reading activity was investigated by giving a word chain in two different font types (Times New Roman and Arial) and three different sizes (10, 12 and 14 points) of font for each type. Subjects performed reading tasks under two sitting positions (with backrest support and leaning over a table). The judgments of perceived difficulty to read were rated using 7-point discomfort judging scale. The result shows that reading difficulty increases with increasing vibration magnitudes and found to be maximum in longitudinal direction, but with leaning over a table position. In comparison with Times New Roman type and sizes of font, subjects perceived less difficulty with Arial type for all font sizes under all vibration magnitude.
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  • Influence of Support Conditions on Vertical Whole-body Vibration of the Seated Human Body
    Anand M-PRANESH, Subhash RAKHEJA, Richard DEMONT
    Article type: Original Article
    2010 Volume 48 Issue 5 Pages 682-697
    Published: 2010
    Released on J-STAGE: October 13, 2010
    DOIhttps://doi.org/10.2486/indhealth.MSWBVI-25
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    The vibration transmission to the lumbar and thoracic segments of seated human subjects exposed to whole body vibration of a vehicular nature have been mostly characterised without the back and hand supports, which is not representative of general driving conditions. This non-invasive experimental study investigated the transmission of vertical seat vibration to selected vertebrae and the head along the vertical and fore-aft axes of twelve male human subjects seated on a rigid seat and exposed to random vertical excitation in the 0.5-20 Hz range. The measurements were performed under four different sitting postures involving combinations of back support conditions and hands positions, and three difference magnitudes of vertical vibration (0.25, 0.5 and 1.0 m/s2 rms acceleration). The results showed significant errors induced by sensor misalignment and skin effects, which required appropriate correction methodologies. The averaged corrected responses revealed that the back support attenuates vibration in the vertical axis to all the body locations while increasing the fore-aft transmissibility at the C7 and T5. The hands position generally has a relatively smaller effect, showing some influences on the C7 and L5 vibration. Sitting without a back support resulted in very low magnitude fore-aft vibration at T5, which was substantially higher with a back support, suggestive of a probable change in the body's vibration mode. The effect of back support was observed to be very small on the horizontal vibration of the lower thoracic and lumbar regions. The results suggest that distinctly different target body-segment biodynamic functions need to be defined for different support conditions in order to represent the unique contribution of the specific support condition. These datasets may then be useful for the development of biodynamic models.
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  • Apparent Mass and Seat-to-head Transmissibility Responses of Seated Occupants under Single and Dual Axis Horizontal Vibration
    Santosh MANDAPURAM, Subhash RAKHEJA, Paul-Émile BOILEAU, Setsuo M ...
    Article type: Original Article
    2010 Volume 48 Issue 5 Pages 698-714
    Published: 2010
    Released on J-STAGE: October 13, 2010
    DOIhttps://doi.org/10.2486/indhealth.MSWBVI-15
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    The apparent mass and seat-to-head-transmissibility response functions of the seated human body are investigated under exposures to fore-aft (x), lateral (y), and combined fore-aft and lateral (x and y) axis whole-body vibration. The experiments were performed to study the effects of hands support, back support and vibration magnitude on the body interactions with the seat pan and the backrest, characterised in terms of fore-aft and lateral apparent masses and the vibration transmitted to the head under single and dual-axis horizontal vibration. The data were acquired with 9 subjects exposed to two different magnitudes of vibration applied along the individual x- and y- axis (0.25 and 0.4 m/s2 rms), and along both the-axis (0.28 and 0.4 m/s2 rms) in the 0.5 to 20 Hz frequency range, and analyzed to derive the biodynamic responses. A method was further derived to obtain total seated body apparent mass response from those measured at the backrest and the seatpan. The results revealed coupled effects of hands and back support conditions on the responses, while the vibration magnitude effect was relatively small. For a given postural condition, the biodynamic responses to dual-axis vibration could be estimated from the direct- and cross-axis responses to single-axis vibration, suggesting weakly nonlinear behaviour.
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  • Design and Evaluation of a Suspension Seat to Reduce Vibration Exposure of Subway Operators: A Case Study
    Pierre MARCOTTE, Sylvie BEAUGRAND, Jérôme BOUTIN, Christian ...
    Article type: Original Article
    2010 Volume 48 Issue 5 Pages 715-724
    Published: 2010
    Released on J-STAGE: October 13, 2010
    DOIhttps://doi.org/10.2486/indhealth.MSWBVI-17
    JOURNAL FREE ACCESS
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    Subway operators have complained about discomfort caused by whole-body vibration. To address this problem, a suspension seat with extensive ergonomic features has been adapted to the confined space of the subway operator cab. The suspension was modified from an existing suspension in order to reduce the dominant frequency of the subway vertical vibration (2.4 Hz). The suspension seat has been extensively tested on a vertical hydraulic shaker. These tests have shown that the SEAT value was lower for a higher vibration level, for higher subject weight, and for the suspension adjusted at median height. The seat also produces a lower SEAT value when there was a predominance of the 6 Hz vibration component. The horizontal seat adjustments had no influence on the suspension SEAT value. Removing the suspension damper also decreases the SEAT value for all the tested configurations. The final version of the suspension seat prototype was validated during normal subway operation with 19 different operators having weight in the 5th, 50th and 95th percentile of the operator population. Accelerations were measured with triaxial accelerometers at the seat cushion, above the suspension and on the floor. In addition to the vibration measurements, each operator was asked about his perceived discomfort from vibration exposure. Globally, the suspension seat attenuated the vertical vibration (SEAT values from 0.86 to 0.99), but discomfort due to amplification of the 2.4 Hz component occurred when the suspension height was adjusted at the minimum, even when the global weighted acceleration was lower (SEAT value < 1). These results suggest that in order to reduce the discomfort caused by whole-body vibration, the transmissibility of the seat should also be considered, in particular when there is a dominant frequency in the vibration spectra.
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  • Examination of the Frequency-weighting Curve for Accelerations Measured on the Seat and at the Surface Supporting the Feet during Horizontal Whole-body Vibrations in x- and y-Directions
    Marianne SCHUST, Alexander KREISEL, Helmut SEIDEL, Ralph BLÜTHNER
    Article type: Original Article
    2010 Volume 48 Issue 5 Pages 725-742
    Published: 2010
    Released on J-STAGE: October 13, 2010
    DOIhttps://doi.org/10.2486/indhealth.MSWBVI-06
    JOURNAL FREE ACCESS
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    In a laboratory experiment, six male subjects were exposed to sinusoidal (0.8, 1.6, 3.15, 6.3 and 12.5 Hz) or random octave band-width white noise (mid-frequencies identical to those of the sinusoidal vibrations) whole-body vibration in x- or y-directions, at six levels of magnitude (0.4, 0.8 and 1.6 m/s2 r.m.s. non- and frequency-weighted) with two repetitions. In order to examine time effects, additional reference stimuli were used. Each subject was exposed to these 304 exposure conditions with a duration of about one minute on four different days (76 exposures per day). The subject's sensations of vibration intensity and vibration comfort were obtained by cross modality matching (length of a line). The subjects sat with an upright posture on a hard seat without backrest, hands on the thighs. The derived equivalent sensation contours suggest an underestimation of the sensation varying in extent from 2 dB to 8 dB at 1.6, 3.15, 6.3 and 12.5 Hz in comparison with the reference frequency 0.8 Hz for both types and directions of signals by the current evaluation methods according to ISO 2631-1 with the most pronounced effects revealed at the frequencies 3.15 and 6.3 Hz and at lower intensities (overall vibration total value aov around 0.48 m/s2 to 0.8 m/s2 at the reference frequency 0.8 Hz).
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