Contrast thresholds and fixation disparity during 5-Hz sinusoidal single- and dual-axis (vertical and lateral) whole-body vibration

The present study assumed that whole-body vibration, transmitted through the seat, impairs spatial retinal resolution and oculomotor alignment parallel to the vibration axis. More specifically, that the decrement increases gradually from single-axis lateral via single-axis vertical and dual-axis linear to dual-axis circular motion. Twenty participants (19–26 years of age) with good vision volunteered for the experiment where in three sessions one of the following three conditions, contrast threshold, nonius bias or fixation disparity, for vertically and horizontally oriented test patterns was determined during five experimental conditions. The latter comprised a control (a _z= a _y= 0) and four conditions where 5-Hz sinusoidal motion of 1.2 ms^?2 rms were applied separately, either in the vertical or in the lateral direction, or simultaneously in both directions, once without and once with a phase shift of 90°, thus causing dual-axis linear or circular motion. Contrast thresholds for horizontal gratings and the variability of vertical fixation disparity increased significantly whenever the participants were exposed to vertical motion (alone or combined with lateral motion). These effects may result in an increased difficulty in properly recognizing characters and graphic patterns containing horizontal lines and in the development of asthenopic complaints.     

Equivalent comfort contours for fore-and-aft,lateral, and vertical whole-body vibration in the frequency range 1.0 to 10 Hz

Abstract

Standards assume vibration discomfort depends on the frequency and direction of whole-body vibration, with the same weightings for frequency and direction at all magnitudes. This study determined equivalent comfort contours from 1.0 to 10?Hz in each of three directions (fore-and-aft, lateral, vertical) at magnitudes in the range 0.1 to 3.5?ms^?2?r.m.s. Twenty-four subjects sat on a rigid flat seat with and without a beanbag, altering the pressure distribution on the seat but not the transmission of vibration. The rate of growth of vibration discomfort with increasing magnitude of vibration differed between the directions of vibration and varied with the frequency of vibration. The frequency-dependence and direction-dependence of discomfort, therefore, depended on the magnitude of vibration. The beanbag did not affect the frequency-dependence or direction-dependence of vibration discomfort. It is concluded that different weightings for the frequency and direction of vibration are required for low and high magnitude vibration.

Practitioner summary: When evaluating whole-body vibration to predict vibration discomfort, the weightings appropriate to different frequencies and different directions of vibration should depend on the magnitude of vibration. This is overlooked in all current methods of evaluating the severity of whole-body vibration.     

Mechanisms of vibration-induced interference with manual control performance

Abstract

An experiment is described in which eight subjects performed three simple tasks (A, B and C) in static conditions and during exposure to whole-body vertical (z-axis) vibration at 0-5 and 40 Hz, at an acceleration magnitude of 2-1 ms^-2 r.m.s. All subjects performed all conditions with and without an arm support. The objective was to explore the mechanisms that may cause disruption of manual control performance during vibration exposure. With task A subjects simply held a control with no visual feedback of activity at the control. With task B, subjects used the control to hold a controlled element stationary on a display. Task C was the same as task B, except that subjects had improved visual feedback of movement of the controlled element. Results showed that both 0-5 and 40 Hz vibration caused significant increases in control activity at frequencies of up to about 1 Hz compared with the condition without vibration. With visual feedback in task C, subjects were able to detect drifting of the controlled element on the display and introduced compensatory control activity at frequencies above about 0 2 Hz. The arm support reduced the magnitude of vibration transmitted to the control at 4-0 Hz, but did not otherwise change the results.     

Contrast thresholds and fixation disparity during 5-Hz sinusoidal single- and dual-axis (vertical and lateral) whole-body vibration

The present study assumed that whole-body vibration, transmitted through the seat, impairs spatial retinal resolution and oculomotor alignment parallel to the vibration axis. More specifically, that the decrement increases gradually from single-axis lateral via single-axis vertical and dual-axis linear to dual-axis circular motion. Twenty participants (19-26 years of age) with good vision volunteered for the experiment where in three sessions one of the following three conditions, contrast threshold, nonius bias or fixation disparity, for vertically and horizontally oriented test patterns was determined during five experimental conditions. The latter comprised a control (a(z) = a(y) = 0) and four conditions where 5-Hz sinusoidal motion of 1.2 ms(-2) rms were applied separately, either in the vertical or in the lateral direction, or simultaneously in both directions, once without and once with a phase shift of 90 degrees, thus causing dual-axis linear or circular motion. Contrast thresholds for horizontal gratings and the variability of vertical fixation disparity increased significantly whenever the participants were exposed to vertical motion (alone or combined with lateral motion). These effects may result in an increased difficulty in properly recognizing characters and graphic patterns containing horizontal lines and in the development of asthenopic complaints.     

In-vehicle vibration study of child safety seats

This paper reports experimental measurements of the in-vehicle vibrational behaviour of stage 0&1 child safety seats. Road tests were performed for eight combinations of child, child seat and automobile. Four accelerometers were installed in the vehicles and orientated to measure as closely as possible in the vertical direction; two were attached to the floor and two located at the human interfaces. An SAE pad was placed under the ischial tuberosities of the driver at the seat cushion and a child pad, designed for the purpose of this study, was placed under the child. Four test runs were made over a pave’ (cobblestone) surface for the driver's seat and four for the child seat at both 20?km?h^?1 and 40?km?h^?1. Power spectral densities were determined for all measurement points and acceleration transmissibility functions (ATFs) were estimated from the floor of the vehicle to the human interfaces. The system composed of automobile seat, child seat and child was found to transmit greater vibration than the system composed of automobile seat and driver. The ensemble mean transmissibility in the frequency range from 1 to 60 Hz was found to be 77% for the child seats systems as opposed to 61% for the driver's seats. The acceleration transmissibility for the child seat system was found to be higher than that of the driver's seat at most frequencies above 10 Hz for all eight systems tested. The measured ATFs suggest that the principal whole-body vibration resonance of the children occurred at a mean frequency of 8.5, rather than the 3.5 to 5.0 Hz typically found in the case of seated adults. It can be concluded that current belt-fastened child seats are less effective than the vehicle primary seating systems in attenuating vibrational disturbances. The results also suggest the potential inability of evaluating child comfort by means of existing whole-body vibration standards.     

Variations in response to whole-body vibration Intensity dependent effects

When equal sensation contours were obtained from 24 subjects starting at four different intensities (0.5, 1.0, 2.0 and 3.5 m s^?2r.m.s.), significant differences in the shapes of the average contours were obtained. As the overall intensity range increased (from 0.5 to 3.5 m s^?2 r.m.s.), the contours became significantly less linear in shape. Implications of these results for the International Standard on human exposure to whole-body vibration are discussed.     

Brain potential correlates of supraliminal contrast functions and defocus

In six subjects with normal corrected vision, perceived suprathreshold contrast of a sine‐wave grating pattern was assessed by an intermodal matching technique in combination with the method of ratio estimation. Vertical gratings were produced on an oscilloscope screen at spatial frequencies of 1.0, 2.0, 3.5, 5.0, 9.0 and 15.0 cycles per degree (c/deg) and at contrast levels between 0.03 and 0.55. Steady‐state visual evoked potentials (VEP) were recorded from the occiput to phase‐reversing gratings of the same spatial frequency and contrast range. A linear relation was found in logarithmic coordinates between perceived and physical contrast and between VEP amplitude and physical contrast. The slopes varied with spatial frequency and were steeper at low and high spatial frequencies than in the intermediate range for both subjective contrast and VEP.

In a second group of six subjects, the effect of retinal image defocus (myopia) on the psychophysical power functions was determined by placing spherical plus lenses in front of the eyes. A marked change in slope was obtained with blurred stimuli, and this effect was more pronounced with an increase in spatial frequency. Power transformations can thus be induced by neural and optical factors. Growth rate of supraliminal response functions seems to be inversely related to detection threshold of sensory systems.     

Perception of fore-and-aft whole-body vibration intensity measured by two methods

This experimental study investigated the perception of fore-and-aft whole-body vibration intensity using cross-modality matching (CM) and magnitude estimation (ME) methods. Thirteen subjects were seated on a rigid seat without a backrest and exposed to sinusoidal stimuli from 0.8 to 12.5 Hz and 0.4 to 1.6 ms^? 2 r.m.s. The Stevens exponents did not significantly depend on vibration frequency or the measurement method. The ME frequency weightings depended significantly on vibration frequency, but the CM weightings did not. Using the CM and ME weightings would result in higher weighted exposures than those calculated using the ISO (2631-1, 1997) W_d. Compared with ISO W_k, the CM and ME-weighted exposures would be greater at 1.6 Hz and lesser above that frequency. The CM and ME frequency weightings based on the median ratings for the reference vibration condition did not differ significantly. The lack of a method effect for weightings and for Stevens exponents suggests that the findings from the two methods are comparable.     

Seated body apparent mass response to vertical whole body vibration: Gender and anthropometric effects

The gender and anthropometric effects on apparent mass characteristics of the seated body exposed to vertical vibration are investigated through laboratory measurements. The study was conducted on 31 male and 27 female subjects, exposed to three levels of vertical vibration (0.25, 0.50 and 0.75 m/s² rms acceleration) in the 0.50 to 20 frequency range, while seated without a back support and against a vertical back support. The apparent mass responses were analyzed by grouping datasets in three ranges of mass-, build- and stature-related parameters for the male and female subjects. Comparisons of responses of male and female subjects with comparable anthropometric properties showed distinctly different biodynamic responses of the two genders. The primary resonance frequency of male subjects was significantly (p < 0.001) higher than the female subjects of comparable body mass but the peak magnitude was comparable for both the gender groups. The male subjects showed greater softening with increasing excitation magnitude compared to the female subjects, irrespective of the sitting condition. The male subjects showed significantly higher peak magnitude response than those of the female subjects for the same anthropometric properties, except for the total and lean body mass. The peak magnitude was linearly correlated with the body mass, body mass index, body fat and hip circumference (r² > 0.7), irrespective of the back support and excitation conditions for both the genders.Relevance to the industryThe apparent mass responses of the human body exposed to whole-body vibration form an essential basis for an understanding of mechanical-equivalent properties of the body, developments in frequency-weightings for assessment of exposure risks and anthropodynamic manikins for assessment of seats. The effects of gender and anthropometric parameters on the AM response are vital for seeking better seat designs, and anthropodynamic manikins for assessments of seating for male as well as female workers.     

Equivalent comfort contours for vertical vibration of steering wheels: effect of vibration magnitude, grip force, and hand position

Vehicle drivers receive tactile feedback from steering-wheel vibration that depends on the frequency and magnitude of the vibration. From an experiment with 12 subjects, equivalent comfort contours were determined for vertical vibration of the hands at two positions with three grip forces. The perceived intensity of the vibration was determined using the method of magnitude estimation over a range of frequencies (4-250 Hz) and magnitudes (0.1-1.58 ms⁻² r.m.s.). Absolute thresholds for vibration perception were also determined for the two hand positions over the same frequency range. The shapes of the comfort contours were strongly dependent on vibration magnitude and also influenced by grip force, indicating that the appropriate frequency weighting depends on vibration magnitude and grip force. There was only a small effect of hand position. The findings are explained by characteristics of the Pacinian and non-Pacinian tactile channels in the glabrous skin of the hand.     

Fore-and-aft and dual-axis vibration of the seated human body: Nonlinearity,cross-axis coupling,and associations between resonances in the transmissibility and apparent mass

This study examined how the apparent mass and transmissibility of the human body depend on the magnitude of fore-and-aft vibration excitation and the presence of vertical vibration. Fore-and-aft and vertical acceleration at five locations along the spine, and pitch acceleration at the pelvis, were measured in 12 seated male subjects during fore-and-aft random vibration excitation (0.25–20 Hz) at three vibration magnitudes (0.25, 0.5 and 1.0 ms⁻² r.m.s.). With the greatest magnitude of fore-and-aft excitation, vertical vibration was added at 0.25, 0.5, or 1.0 ms⁻² r.m.s. Forces in the fore-and-aft and vertical directions on the seat surface were measured to calculate apparent masses. Transmissibilities and apparent masses during fore-and-aft excitation showed a principal resonance around 1 Hz and a secondary resonance around 2–3 Hz. Increasing the magnitude of fore-and-aft excitation, or adding vertical excitation, decreased the magnitudes of the resonances. At the primary resonance frequency, the dominant mode induced by fore-and-aft excitation involved bending of the lumbar spine and the lower thoracic spine with shear deformation of tissues at the ischial tuberosities. The relative contributions to this mode from each body segment (especially the pelvis and the lower thoracic spine) varied with vibration magnitude. The nonlinearities in the apparent mass and transmissibility during dual-axis excitation indicate coupling between the principal mode of the seated human body excited by fore-and-aft excitation and the cross-axis influence of vertical excitation.Relevance to industryUnderstanding movements of the body during exposure to whole-body vibration can assist the optimisation of seating dynamics and help to control the effects of the vibration on human comfort, performance, and health. This study suggests cross-axis nonlinearity in biodynamic responses to vibration should be considered when optimising vibration environments.     

Difference thresholds for the perception of whole-body vertical vibration: dependence on the frequency and magnitude of vibration

When seeking to reduce vibration in transport it is useful to know how much reduction is needed for the improvement to be noticeable. This experimental study investigated whether relative difference thresholds for the perception of whole-body vertical vibration by seated persons depend on the frequency or magnitude of vibration. Relative difference thresholds for sinusoidal seat vibration were determined for 12 males at three vibration magnitudes and eight frequencies (2.5, 5, 10, 20, 40, 80, 160, 315 Hz) using the three-down-one-up method in conjunction with a two-interval-forced-choice procedure. The median relative difference thresholds were in the range 9.5% to 20.3%. There appeared to be a frequency-dependence at the lowest vibration magnitude, such that higher frequencies had higher difference thresholds. The relative difference thresholds depended on the vibration magnitude only at 2.5 and 315 Hz. The influence of both vibration frequency and vibration magnitude on the measured difference thresholds suggests that vision (at 2.5 Hz) and hearing (at 315 Hz) contributed to the perception of changes in vibration magnitude.     

Performance of a complex manual control task during exposure to vertical whole-body vibration between 0·5 and 5·0 Hz

An experiment is described in which seated subjects performed first-order pursuit tracking with a simultaneous discrete task; performance with the discrete task was dependent on performance of the continuous task. Vertical, z-axis, whole-body sinusoidal vibration was presented at frequencies from 0·5 to 5·0Hz at an acceleration magnitude of 2·0 ms^?2 r.m.s. in three separate sessions. In the first session, inter-subject and intra-subject variability masked any disruption caused by the vibration. After further training, all vibration frequencies disrupted performance of the continuous task. Disruption was independent of vibration frequency below 3·15Hz and increased at 4·0 and 5·0Hz. A visual mechanism was assumed to account for the increased disruption at these higher frequencies. Mechanisms which may have been responsible for the disruption below 3·15 Hz are discussed. Effects of vibration on the discrete task were attributable to disruption in performance of the continuous task. The results illustrate the importance of adequately training subjects prior to investigating vibration effects.     

Subjective response of standing persons exposed to fore-aft,lateral and vertical whole-body vibration

The aims of this study were to propose multiply scale factors for evaluation of discomfort of standing persons and to investigate whether there exist differences between multiplying factors used for evaluation of discomfort of standing persons and those of seated persons exposed to WBV. Twelve male subjects were exposed to twenty-seven stimuli that comprise three acceleration magnitudes (0.2, 0.4, and 0.8 m/s² r.m.s.) along fore-aft (x), lateral (y) or vertical (z) direction. The subjects with seated or standing posture on the platform of the vibration test rig rated the subjective discomfort for each stimulus that has frequency contents ranging from 1.0 Hz to 20 Hz with a constant power spectrum density. The order of presentation of the test stimuli was fully randomized and each stimulus was repeated three times. The subjective scale for discomfort was calculated by using the category judgment method. The best combinations of multiplying factors were determined by calculating correlation coefficients of regression curves in-between subjective ratings and vibration magnitudes. In all the directions, body posture significantly influenced on subjective discomfort scales. Particularly in the fore-aft and lateral direction, the upper limit of all the categories for the standing posture resulted in higher vibration acceleration magnitudes than those for the seated posture. In contrast, in the vertical direction, only the upper limit of category “1: Not uncomfortable” for standing posture was observed to be higher than that for seated posture. The best agreement for ISO-weighted vibration acceleration occurred at x factor of 1.8 and y factor of 1.8 in the standing posture and x factor of 2.8 and y factor of 1.8 in the seated posture. The results suggest that seated people respond more sensitively and severely in perception of discomfort to fore-aft and lateral vibration than standing people do while standing people respond more sensitively and severely to vertical vibration than seated people do. Thus the effects of body postures on multiplying factors should be considered in evaluation of discomfort caused by whole-body vibration.Relevance to industryThis study reports differences in subjective response of standing persons to fore-aft, lateral and vertical whole-body vibration. The results obtained in this study propose the fundamental data on the sensitivity to whole-body vibration exposed with standing posture.     

Influence of forest machine function on operator exposure to whole-body vibration in a cut-to-length timber harvester

The influence of machine function (tree felling and processing, and machine movement over the terrain) on operator exposure to whole-body vibration in a cut-to-length (CTL) timber harvester was evaluated. Vibrations were measured on the seat and the cabin chassis in three orthogonal (x, y, z) axes for the tree felling and processing, and during motion on a test track. It was found that the level of vibration transmitted to the operator during felling and processing was mainly affected by the tree size (diameter). For tree diameter at breast height (dbh) range of 0.25 – 0.35 m that was investigated, the vertical (z-axis) vibration component during processing increased by up to 300%, and increased by 50% during felling. However, the associated vibration levels were not sufficient to pose any serious health risks to the operator for an exposure limit of 8 h. Vibration at the operator seat and cabin chassis was predominant in the lateral (y-axis) and vertical (z-axis) respectively, during vehicle motion over the standard test track. Vibration peaks of approximately 0.20 and 0.17 ms^?2 occurred at 5 and 3.2 Hz respectively.     

The Subjective Magnitude of Whole-Body Vibration

Abstract

Two experiments have been performed to investigate the relation between the level of whole-body vertical a^z vibration and the degree of discomfort it produces. The first experiment, which employed botli magnitude estimation and magnitude production methods, suggested that the relation between discomfort, ψ and vibration level, φ, could be adequately expressed in the form ψ=kφⁿ. However, the value of n differed greatly between subjects and had o mean value of 113 when determined by the method of magnitude estimation and 1-75 when determined by magnitude production. It is suggested that while the convenient value of unity will sometimes be a sufficient approximation to the value of N the response of all individuals cannot be well approximated by a single value.

The second experiment required subjects to adjust the level of a whole-body vibration to correspond to phrases on a four point semantic scale of discomfort. It is shown that the variability in the data obtained by this semantic method is greater than that obtained by comparable numerical scaling methods such as magnitude production and intensity matching.     

Do whole-body vibrations affect spatial hearing?

To assist the human operator, modern auditory interfaces increasingly rely on sound spatialisation to display auditory information and warning signals. However, we often operate in environments that apply vibrations to the whole body, e.g. when driving a vehicle. Here, we report three experiments investigating the effect of sinusoidal vibrations along the vertical axis on spatial hearing. The first was a free-field, narrow-band noise localisation experiment with 5- Hz vibration at 0.88 ms^? 2. The other experiments used headphone-based sound lateralisation tasks. Experiment 2 investigated the effect of vibration frequency (4 vs. 8 Hz) at two different magnitudes (0.83 vs. 1.65 ms^? 2) on a left–right discrimination one-interval forced-choice task. Experiment 3 assessed the effect on a two-interval forced-choice location discrimination task with respect to the central and two peripheral reference locations. In spite of the broad range of methods, none of the experiments show a reliable effect of whole-body vibrations on localisation performance.     

Effects of whole-body vertical shock-type vibration on human ability for fine manual control

The effects of vertical (z-axis) whole-body shock-type vibration on the ability for fine manual control were examined. The amplitudes and frequency of the shocks was varied, but a constant frequency-weighted acceleration of 1·25 m/s² r. m. s. was maintained. The examination of the shock's effects was carried out using an experimental system that simulated the actual workplace of earth-moving machinery. Control was measured using a first-order pursuit tracking-test, in which a seated subject was asked to use both hands to direct a cursor on a monitor using a steering wheel. Although the magnitude of shocks (peak amplitude of 6-10 m/s²) and the number of shocks per unit time (shock cycle of 10-40 s) were varied, and two types of shock (symmetric and asymmetric) used, no shock effect could be found by calculating an integrated square of tracking error during the whole exposure time. The tracking error only increased significantly during the moments that the subjects were exposed to a symmetrically shaped shock that reached the highest peak value (for the experiment) of 10 m/s². The results suggested that shocks with peak amplitudes below defined value induce no evident effect on the steering of vehicles.     

Whole-body vibration experienced by haulage truck operators in surface mining operations: A comparison of various analysis methods utilized in the prediction of health risks

Whole body vibration (WBV) was measured on eight surface haulage trucks in three size classes (35, 100, 150 ton haul capacities). Vibration was measured at the seat/operator interface in accordance with the ISO 2631-1 standard during 1 h of normal operation. Highest acceleration readings were observed in the z-axis (vertical). Estimated equivalent daily exposure values in the range of 0.44–0.82 ms^?2 were observed using the frequency-weighted r.m.s method and 8.7–16.4 ms^?1.75 using the vibration dose value method. Assessment was carried out using ISO 2631-1 and 2631-5. Operators of surface haulage trucks are regularly exposed to WBV levels that exceed safety limits as dictated by the ISO 2631-1 standard. However, according to ISO 2631-5 the probability of an adverse health effect remains low. These findings confirm an apparent disagreement between the two analysis methods.     

Whole-body vibration exposure in sport: four relevant cases

This study investigates the whole-body vibration exposure in kite surfing, alpine skiing, snowboarding and cycling. The vibration exposure was experimentally evaluated following the ISO 2631 guidelines. Results evidenced that the most critical axis is the vertical one. The weighted vibration levels are always larger than 2.5 m/s² and the vibration dose values are larger than 25 m/s^1.75. The exposure limit values of the EU directive are reached after 8–37 min depending on the sport. The vibration magnitude is influenced by the athletes’ speed, by their skill level and sometimes by the equipment. The large vibration values suggest that the practice of sport activities may be a confounding factor in the aetiology of vibration-related diseases.