Variability in human response to whole-body vibration The effects of instructions

Two studies are described in this paper with the aim of assessing the degree to which the instructions given to a subject during an experiment designed to investigate human reaction to vibration, affect the vibration equal sensation contour which is produced.

In the first study, 100 subjects produced equal sensation contours by equating pairs of vibration stimuli. After each pair, subjects were required to record the basis on which they had made their judgements. The results demonstrated that subjects differ in the concepts which they use to equate vibration stimuli, although the majority equate in terms of the degree to which parts of the body are shaken.

In the second study, 48 subjects were required to produce equal sensation contours using the terms of either ‘comfort’ or ‘discomfort’ or ‘body shake’ or ‘sensation’. The overall contour shapes produced by the four instruction groups were not significantly different, although the frequency ranges of maximum vibration sensitivity were shown to be significantly different.

Implications of these findings are discussed.     

Analysis of non-linear response of the human body to vertical whole-body vibration

The human response to vibration is typically studied using linear estimators of the frequency response function, although different literature works evidenced the presence of non-linear effects in whole-body vibration response. This paper analyses the apparent mass of standing subjects using the conditioned response techniques in order to understand the causes of the non-linear behaviour. The conditioned apparent masses were derived considering models of increasing complexity. The multiple coherence function was used as a figure of merit for the comparison between the linear and the non-linear models. The apparent mass of eight male subjects was studied in six configurations (combinations of three vibration magnitudes and two postures). The contribution of the non-linear terms was negligible and was endorsed to the change of modal parameters during the test. Since the effect of the inter-subject variability was larger than that due to the increase in vibration magnitude, the biodynamic response should be more meaningfully modelled using a linear estimator with uncertainty rather than looking for a non-linear modelling.     

Subjective response to seated fore-and-aft direction whole-body vibration

Subjective response to seated, fore-and-aft direction, whole-body vibration of the type experienced in automobiles was investigated. Fore-and-aft acceleration was measured at the seat guide of a small automobile when driving over two representative road surfaces, and was replicated in a laboratory setting using a whole-body vibration test rig and rigid seat. A single 15 s section of each of the two acceleration time histories was band-pass filtered to the frequency interval from 0.5 to 50.5 Hz, and was used as a base stimulus. Thirteen test stimuli were then constructed for each base stimulus by rescaling to BS 6841 W_d frequency-weighted r.m.s. amplitudes from 0.01 to 0.86 m/s². Two groups of 16 participants (8 male and 8 female in each case) rated the discomfort of the test stimuli. The first group was asked to use the psychophysical method of magnitude estimation while the second used a Borg CR-10 scale. The order of presentation of the test stimuli was fully randomised and each was repeated three times. For each group of participants, regression analysis was used to determine both the individual and the group mean Stevens’ Power Law exponent describing the relationship between stimulus amplitude and subjective response. All mean power exponents were found to be less than unity, with the CR-10 scale having produced smaller exponents than magnitude estimation. The power exponents ranged from 0.66 to 0.91, corroborating the value of 0.84 obtainable from the guidelines of standard BS 6841.The results suggest that the numerical response scale provided in the BS 6841 guidelines is appropriate for use in the case of automobile fore-and-aft vibration, but that the semantic labels under-represent the actual human subjective response in this direction. Psychophysical test method, vibration stimulus range and test participant gender were all found to affect the Stevens’ Power Law exponent achieved from subjective testing. Each factor may therefore require control when attempting to compare human responses to vibration originating from different automobiles.     

Subjective response to whole-body vibration The effects of posture

In an investigation of the effects of posture on subjective responses to whole-body vibration, 20 undergraduate subjects produced equal sensation contours adopting three postures each on different occasions. The postures adopted were standing, sitting upright and sitting slouched.

The results indicated significant differences in the contour shapes from the three postures, and the level set in the sitting postures were significantly lower than in the standing posture. No difference was obtained between the two sitting postures.

Implications of these findings are discussed regarding the role of transmissibility in subjective response to vibration, and the necessity to produce different standards for different postures.     

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.     

Response of the seated human body to whole-body vertical vibration: discomfort caused by sinusoidal vibration

Frequency weightings for predicting vibration discomfort assume the same frequency-dependence at all magnitudes of vibration, whereas biodynamic studies show that the frequency-dependence of the human body depends on the magnitude of vibration. This study investigated how the frequency-dependence of vibration discomfort depends on the acceleration and the force at the subject–seat interface. Using magnitude estimation, 20 males and 20 females judged their discomfort caused by sinusoidal vertical acceleration at 13 frequencies (1–16 Hz) at magnitudes from 0.1 to 4.0 ms^? 2 r.m.s. The frequency-dependence of their equivalent comfort contours depended on the magnitude of vibration, but was less dependent on the magnitude of dynamic force than the magnitude of acceleration, consistent with the biodynamic non-linearity of the body causing some of the magnitude-dependence of equivalent comfort contours. There were significant associations between the biodynamic responses and subjective responses at all frequencies in the range 1–16 Hz.

Practitioner Summary: Vertical seat vibration causes discomfort in many forms of transport. This study provides the frequency-dependence of vibration discomfort over a range of vibration magnitudes and shows how the frequency weightings in the current standards can be improved.     

Response of the seated human body to whole-body vertical vibration: biodynamic responses to sinusoidal and random vibration

The dependence of biodynamic responses of the seated human body on the frequency, magnitude and waveform of vertical vibration has been studied in 20 males and 20 females. With sinusoidal vibration (13 frequencies from 1 to 16 Hz) at five magnitudes (0.1–1.6 ms^? 2 r.m.s.) and with random vibration (1–16 Hz) at the same magnitudes, the apparent mass of the body was similar with random and sinusoidal vibration of the same overall magnitude. With increasing magnitude of vibration, the stiffness and damping of a model fitted to the apparent mass reduced and the resonance frequency decreased (from 6.5 to 4.5 Hz). Male and female subjects had similar apparent mass (after adjusting for subject weight) and a similar principal resonance frequency with both random and sinusoidal vibration. The change in biodynamic response with increasing vibration magnitude depends on the frequency of the vibration excitation, but is similar with sinusoidal and random excitation.     

Evaluation of responses to broad-band whole-body vibration

Abstract

The experiment was aimed at investigating the human response to different modes, frequencies and intensities of whole-body vibration (WBV), in order to check the evaluation procedures currently recommended. Six male seated subjects were exposed to sinusoidal (SIN) and octave-band-wide vibration (OWV) in the z axis with the frequencies or centre frequencies, respectively, of 2,4, 8 and 16 Hz at two intensity levels (except for 2 Hz), in accordance with the frequency weighting of ISO 2631 (ISO 1978 a). The 14 exposure conditions were compared by means of a slightly modified, complete paired comparison, the total number of exposures amounting to 1044. Subjective judgements of the severity of WBV, annoyance and the ability to control a constant sitting posture were obtained along with the bioelectrical activity of trunk muscles, transmissibility and impedance. An integral assessment of the exposures was rendered possible by the complex evaluation of different human responses. OWV and SIN with identical a_zw r.m.s. values (ISO 1978 a) produced almost identical effects. The results clearly speak in favour of the weighting procedure. This procedure was also supported by an additional pilot study with two-octave-band-wide vibration. The superiority of the weighting procedure suggests lower limits for broad-band vibration than those recommended at present (ISO 1978 a). Human response to WBV in the range near 4 Hz was more pronounced than that of equivalent exposures with other frequencies. Generally, higher intensities induced stronger effects. The biomechanical data exhibited a non-linearity for the WBV levels of intensity investigated. The patterns of myoelectric and biomechanical reactions depended on both anatomical and exposure conditions. The individual responses in discriminating the exposure conditions significantly agreed, but the extent of agreement between the individual responses varied for the effects investigated.     

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.     

A quantitative meta-analytic examination of whole-body vibration effects on human performance

Whole-body vibration exerts a substantive influence in many work environments. The primary objective for this work was to quantify such effects by identifying those moderating variables that influence the degree to which performance is affected. To achieve this, a comprehensive meta-analysis was conducted, which synthesized the existing research evidence. A total of 224 papers and reports were identified and, from these 115 effect sizes were derived from 13 experiments that survived the screening procedure. Results indicate that vibration acts to degrade the majority of goal-related activities, especially those with high demands on visual perception and fine motor control. Gaps in the current research literature are identified and suggestions offered with regard to a more theoretically-driven approach to testing vibration effects on human performance.     

A quantitative meta-analytic examination of whole-body vibration effects on human performance

Whole-body vibration exerts a substantive influence in many work environments. The primary objective for this work was to quantify such effects by identifying those moderating variables that influencethe degree to which performance is affected. To achieve this, a comprehensive meta-analysis was conducted, which synthesized the existing research evidence, A total of 224 papers and reports were identified and, from these 115 effect sizes were derived from 13 experiments that survived the screening procedure. Results indicate that vibration acts to degrade the majority of goal-related activities, especially those with high demands on visual perception and fine motor control. Gaps in the current research literature are dentified and suggestions offered with regard to a more theoretically-driven approach to testing vibration effects on human performance.     

The influence of seat backrest angle on human performance during whole-body vibration

This study investigated the effects of reclined backrest angles on cognitive and psycho-motor tasks during exposure to vertical whole-body vibration. Twenty participants were each exposed to three test stimuli of vertical vibration: 2-8 Hz; 8-14 Hz and 14-20 Hz, plus a stationary control condition whilst seated on a vibration platform at five backrest angles: 0° (recumbent, supine) to 90° (upright). The vibration magnitude was 2.0 ms(-2) root-mean-square. The participants were seated at one of the backrest angles and exposed to each of the three vibration stimuli while performing a tracking and choice reaction time tasks; then they completed the NASA-TLX workload scales. Apart from 22.5° seat backrest angle for the tracking task, backrest angle did not adversely affect the performance during vibration. However, participants required increased effort to maintain performance during vibration relative to the stationary condition. These results suggest that undertaking tasks in an environment with vibration could increase workload and risk earlier onset of fatigue. PRACTITIONER SUMMARY: Current vibration standards provide guidance for assessing exposures for seated, standing and recumbent positions, but not for semi-recumbent postures. This paper reports new experimental data systematically investigating the effect of backrest angle on human performance. It demonstrates how workload is elevated with whole-body vibration, without getting affected by backrest angle.     

Postural stability after whole-body vibration exposure

Fall injuries to operators of heavy mobile equipment are more frequent during egress rather than while climbing onto the equipment. One possible contributing factor is a loss of postural stability during egress. Because many of these operators are exposed to prolonged periods of whole-body vibration (WBV) while controlling the equipment, a study was conducted to determine if postural stability is impaired by WBV exposure. Subjects were tested for a difference in standing postural sway before and after seated WBV exposure. The WBV consisted of 40 minutes of vertical vibration generated by a motion platform, which emulated actual vibration levels of an operating under-ground shuttle car. Two independent variables, each at two levels, consisted of vibration and vision. The measured dependent variables were postural sway amplitude and velocity of sway. No significant difference between the vibration and no vibration conditions were found. Based on the results of this study, it cannot be concluded that WBV at the exposed frequencies influences postural stability. The results suggest that other factors may be the primary contributors to fall injuries while exiting the vehicle, such as problems with foot placement accuracy or egress system design.     

Effects of display vibration and whole-body vibration on visual performance

Fifteen subjects performed a numeral reading task during (a) vibration of the display, (b) vibration of the subject, (c) simultaneous vibration of both subject and display. Sinusoidal motion at eleven frequencies (0·5 to 5·0 Hz) was presented at five acceleration magnitudes (1·0 to 2·5ms^?2 r.m.s.). Measures of reading time and reading error showed that for all except the highest frequencies, vibration of the display resulted in the poorest performance. Simultaneous whole-body-and-display vibration produced least performance decrement. The effects of both the viewing conditions and the vibration frequency are discussed in relation to known characteristics of the visual system.     

Inter-individual postural variability in seated drivers exposed to whole-body vibration

Long-term occupational exposure to whole-body vibration (WBV) is a cause of low back pain for seated drivers. Poor and long-term seated postures are considered as a cofactor in the risk. It depends on the vehicle's ergonomics and tasks. Differences in posture may also be observed between operators doing identical tasks. An experiment has been performed in order to simultaneously measure posture and WBV for 12 drivers in 3 vehicles (loader, dumper and excavator) during controlled tasks. The inter-individual postural variability has been evaluated. The positions and movements of the body were measured with the CUELA system (computer-assisted recording and long-term analysis of musculoskeletal loads). Significant differences were observed between the three vehicles in the WBV, positions and movements of the body. Significant postural differences were observed between drivers (EN 1005-4 2005). Individual strategies for performing a task were also identified.