The influence of seat backrest angle on perceived discomfort during exposure to vertical whole-body vibration

National and International Standards (e.g. BS 6841 and ISO 2631-1) provide methodologies for the measurement and assessment of whole-body vibration in terms of comfort and health. The EU Physical Agents (Vibration) Directive (PAVD) provides criteria by which vibration magnitudes can be assessed. However, these standards only consider upright seated (90°) and recumbent (0°) backrest angles, and do not provide guidance for semi-recumbent postures. This article reports an experimental programme that investigated the effects of backrest angle on comfort during vertical whole-body vibration. The series of experiments showed that a relationship exists between seat backrest angle, whole-body vibration frequency and perceived levels of discomfort. The recumbent position (0°) was the most uncomfortable and the semi-recumbent positions of 67.5° and 45° were the least uncomfortable. A new set of frequency weighting curves are proposed which use the same topology as the existing BS and ISO standards. These curves could be applied to those exposed to whole-body vibration in semi-recumbent postures to augment the existing standardised methods. PRACTITIONER SUMMARY: Current vibration standards provide guidance for assessing exposures for seated, standing and recumbent positions, but not for semi-recumbent postures. This article reports new experimental data systematically investigating the effect of backrest angle on discomfort experienced. It demonstrates that most discomfort is caused in a recumbent posture and that least was caused in a semi-recumbent posture.     

The vibration of inclined backrests: perception and discomfort of vibration applied parallel to the back in the z-axis of the body

This study determined how backrest inclination and the frequency of vibration influence the perception and discomfort of vibration applied parallel to the back (vertical vibration when sitting upright, horizontal vibration when recumbent). Subjects experienced backrest vibration at frequencies in the range 2.5 to 25 Hz at vibration magnitudes up to 24 dB above threshold. Absolute thresholds, equivalent comfort contours, and the principal locations for feeling vibration were determined with four backrest inclinations: 0° (upright), 30°, 60° and 90° (recumbent). With all backrest inclinations, acceleration thresholds and equivalent comfort contours were similar and increased with increasing frequency at 6 dB per octave (i.e. velocity constant). It is concluded that backrest inclination has little effect on the frequency dependence of thresholds and equivalent comfort contours for vibration applied along the back, and that the W (d) frequency weighting in current standards is appropriate for evaluating z-axis vibration of the back at all backrest inclinations. STATEMENT OF RELEVANCE: To minimise the vibration discomfort of seated people, it is necessary to understand how discomfort varies with backrest inclination. It is concluded that the vibration on backrests can be measured using a pad between the backrest and the back, so that it reclines with the backrest, and the measured vibration evaluated without correcting for the backrest inclination.     

Evaluation of reaction time performance and subjective workload during whole-body vibration exposure while seated in upright and twisted postures with and without armrests

There is little knowledge on performance during vibration exposure combined with occupational hazards such as bent or twisted postures. In addition, little information is available on the effective use of armrests during performance-related tasks. This paper investigates the influence of sitting in different working postures on the reaction time and perceived workload of subjects exposed to whole-body vibration. Twenty-one subjects were exposed to 1–20 Hz random vibration in the vertical and fore-and-aft directions. A choice reaction time task was completed while seated in four posture conditions: upright or twisted, with and without armrests. Following the task, participants completed the NASA TLX workload assessment. Posture combined with whole-body vibration exposure had a significant influence on the ability to perform the task. The combined environmental stressors significantly degraded the performance; not only did their reaction times become compromised, the participants’ workload demand also increased. The most severe decrement in performance and workload was experienced while seated in a twisted posture with no armrest support. The inclusion of armrests significantly improved the participants’ ability to complete the task with a lower workload demand.

Relevance to industry

Twisted postures have been observed in a variety of machine operations and it is important to determine their influence on operator workload. Many off-road machines have suspension seats fitted with armrests; this paper demonstrates that armrest support provides additional benefits for off-road machine operators under combined environmental stressors.     

Equivalent comfort contours for vertical seat vibration: effect of vibration magnitude and backrest inclination

This study determined how backrest inclination and the frequency and magnitude of vertical seat vibration influence vibration discomfort. Subjects experienced vertical seat vibration at frequencies in the range 2.5-25 Hz at vibration magnitudes in the range 0.016-2.0 ms(-2) r.m.s. Equivalent comfort contours were determined with five backrest conditions: no backrest, and with a stationary backrest inclined at 0° (upright), 30°, 60° and 90°. Within all conditions, the frequency of greatest sensitivity to acceleration decreased with increasing vibration magnitude. Compared to an upright backrest, around the main resonance of the body, the vibration magnitudes required to cause similar discomfort were 100% greater with 60° and 90° backrest inclinations and 50% greater with a 30° backrest inclination. It is concluded that no single frequency weighting provides an accurate prediction of the discomfort caused by vertical seat vibration at all magnitudes and with all backrest conditions. PRACTITIONER SUMMARY: Vertical seat vibration is a main cause of vibration discomfort for drivers and passengers of road vehicles. A frequency weighting has been standardised for the evaluation of vertical seat vibration when sitting upright but it was not known whether this weighting is suitable for the reclined sitting postures often adopted during travel.     

Apparent mass of small children: experimental measurements

A test facility and protocol were developed for measuring the seated, vertical, whole-body vibration response of small children of less than 18 kg in mass over the frequency range from 1 to 45 Hz. The facility and protocol adhered to the human vibration testing guidelines of BS7085 and to current codes of ethics for research involving children. Additional procedures were also developed which are not currently defined in the guidelines, including the integral involvement of the parents and steps taken to maximize child happiness. Eight children were tested at amplitudes of 0.8 and 1.2 m/s(2) using band-limited, Gaussian, white noise acceleration signals defined over the frequency interval from 1 to 50 Hz. Driving point apparent mass modulus and phase curves were determined for all eight children at both test amplitudes. All results presented a single, principal, anti-resonance, and were similar to data reported for primates and for adult humans seated in an automotive posture which provided backrest support. The mean frequency of the apparent mass peak was 6.25 Hz for the small children, as compared to values between 6.5 - 8.5 Hz for small primates and values between 6.5 - 8.6 Hz for adults seated with backrest support. The peak value of the mean, normalized, apparent mass was 1.54 for the children, which compares to values from 1.19 to 1.45 reported in the literature for small primates and 1.28 for adults seated with backrest support. ISO standard 5982, which specifies a mean, normalized, apparent mass modulus peak of 1.50 at a frequency of 4.0 Hz for adults seated without backrest support, provides significant differences.     

The effect of seat back inclination on spine height changes

The effect of backrest inclination on spinal height changes was tested during static sitting and seated whole-body vibrations. The vibration input was sinusoidal with a frequency of 5 Hz and an acceleration of 0.1 g rms. The backrest inclinations tested were 110 degrees and 120 degrees . The 110 degrees backrest caused less shrinkage than did the 120 degrees during static sitting, whereas the opposite was true when vibration was present, although the differences between the backrests were not statistically significant. Only when the results were compared with results from exposure to unsupported sitting were the differences statistically significant for both static sitting and seated vibrations when the 110 degrees backrest was used and for vibration with the 120 degrees backrest. Thus we conclude that an inclined backrest reduces the effects of vibration. More importantly, emphasis should be placed upon seats and seat materials that can attenuate vibration.     

The vibration of inclined backrests: perception and discomfort of vibration applied parallel to the back in the z-axis of the body

This study determined how backrest inclination and the frequency of vibration influence the perception and discomfort of vibration applied parallel to the back (vertical vibration when sitting upright, horizontal vibration when recumbent). Subjects experienced backrest vibration at frequencies in the range 2.5 to 25 Hz at vibration magnitudes up to 24 dB above threshold. Absolute thresholds, equivalent comfort contours, and the principal locations for feeling vibration were determined with four backrest inclinations: 0° (upright), 30°, 60° and 90° (recumbent). With all backrest inclinations, acceleration thresholds and equivalent comfort contours were similar and increased with increasing frequency at 6 dB per octave (i.e. velocity constant). It is concluded that backrest inclination has little effect on the frequency dependence of thresholds and equivalent comfort contours for vibration applied along the back, and that the W _d frequency weighting in current standards is appropriate for evaluating z-axis vibration of the back at all backrest inclinations.

Statement of Relevance: To minimise the vibration discomfort of seated people, it is necessary to understand how discomfort varies with backrest inclination. It is concluded that the vibration on backrests can be measured using a pad between the backrest and the back, so that it reclines with the backrest, and the measured vibration evaluated without correcting for the backrest inclination.     

Biomechanical response of the musculoskeletal system to whole body vibration using a seated driver model

ObjectiveThis study aimed to assess the effects of backrest inclination and vibration frequency on muscle activity in a dynamic environment using a musculoskeletal model.MethodThe muscle activity modeling method was used to analyze a full body musculoskeletal system of a seated person with a public domain rigid body model in an adjustable car seat. This model was established using AnyBody Modeling System, based on the inverse dynamic approach. And the min/max criterion in dealing with the muscle redundancy problem. Ten healthy subjects were exposed to whole body vibration (WBV) with five frequencies (3, 4.5, 6, 7, and 8 Hz) in the vertical direction in a randomized order on three separate days. The displacement of the seat-pan and head was measured using a hybrid Polaris spectra system to obtain the seat-to-head (STH) transmissibility. Muscle oxygenation was measured using near-infrared spectroscopy. The validity of the model was tested using STH transmissibility and muscle oxygenation.ResultsIncreased vibration frequency caused high muscle activities of the abdomen and the right leg with a backrest forward inclination angle. The muscle activities of the left leg decreased at a backrest backward inclination except at inclination angles of 15° and 30°. Muscle activity of the lumbar suddenly increased at a backrest inclination angle of 5° and vibration frequency of 5 Hz. Muscle activities were higher under vibration than that without vibration.ConclusionVibration frequency significantly affected the muscle activity of the lumbar area. Likewise, the inclination degree of the backrest significantly affected the muscle activities of the right leg and the abdomen. The combination of vibration and forward inclination of the backrest can be used to maximize the muscle activity of the leg, similar to the abdomen and lumbar muscles.Relevance to the industryThe musculoskeletal model established in the present study provides a method that can be used to investigate the biomechanical response of seated drivers to WBV. This model helps protect drivers from occupational injury.     

Apparent mass of small children: experimental measurements

A test facility and protocol were developed for measuring the seated, vertical, whole-body vibration response of small children of less than 18 kg in mass over the frequency range from 1 to 45 Hz. The facility and protocol adhered to the human vibration testing guidelines of BS7085 and to current codes of ethics for research involving children. Additional procedures were also developed which are not currently defined in the guidelines, including the integral involvement of the parents and steps taken to maximize child happiness. Eight children were tested at amplitudes of 0.8 and 1.2 m/s² using band-limited, Gaussian, white noise acceleration signals defined over the frequency interval from 1 to 50 Hz. Driving point apparent mass modulus and phase curves were determined for all eight children at both test amplitudes. All results presented a single, principal, anti-resonance, and were similar to data reported for primates and for adult humans seated in an automotive posture which provided backrest support. The mean frequency of the apparent mass peak was 6.25 Hz for the small children, as compared to values between 6.5 – 8.5 Hz for small primates and values between 6.5 – 8.6 Hz for adults seated with backrest support. The peak value of the mean, normalized, apparent mass was 1.54 for the children, which compares to values from 1.19 to 1.45 reported in the literature for small primates and 1.28 for adults seated with backrest support. ISO standard 5982, which specifies a mean, normalized, apparent mass modulus peak of 1.50 at a frequency of 4.0 Hz for adults seated without backrest support, provides significant differences.     

Estimating reduced oxygenation levels in the erector spinae lumbar muscle region during seated whole-body vibration

Previous research has demonstrated deficiency in blood supply to lumbar muscles in the form of decrease in oxygenation and blood volume during short duration of exposure to seated whole-body vibration (WBV). However, it is not clear if these WBV-induced lumbar muscle responses are comparable, for example, to that of an endurance exercise-induced oxygenation and blood volume responses?On a separate day, eight healthy participants performed a seated arm cranking exercise until volitional exhaustion. On three separate days, participants were exposed to 3, 4.5, and 6 Hz on a vibration simulator for a period of 16 min. During the fifth minute of WBV ‘with’ and ‘without’ backrest support, participants performed rhythmic handgrip contractions for 1 min. Oxygenation and blood volume responses from the lumbar region were measured utilizing Near-infrared spectroscopy.A percent change in oxygenation and blood volume responses during WBV was expressed as a function of spectroscopy-derived minimum (at the exhaustion) and maximum (during recovery from WBV) responses obtained from the arm cranking exercise. Highest decrease in spectroscopy-derived responses (represented in mean values) was observed: at 4.5 Hz; sitting ‘without’ backrest support; and handgrip contractions during exposure to WBV.Spectroscopy-derived hemodynamic responses obtained during the endurance exercise were significantly lower than the corresponding values measured at different WBV conditions, implying that although the spinal resonance frequency of 4.5 Hz decreases oxygen saturation considerably, progress of oxygen depletion is further evidenced during an endurance exercise.Relevance to industryEstablishing fully oxidized and reduced physiologic states for the lumbar muscle by occluding arterial blood flow is difficult. However, by utilizing an aerobic protocol until volitional exhaustion, lumbar oxygenation and blood volume responses for a variety of WBV-related exposures can be compared. It was concluded that WBV-induced lumbar hemodynamic responses fall well within the reduced and oxidized conditions established through the endurance arm cranking exercise.     

Whole-body vertical biodynamic response characteristics of the seated vehicle driver: Measurement and model development

The vertical driving-point mechanical impedance characteristics applicable to seated vehicle drivers are measured in the 0.625–10 Hz frequency range with excitation amplitudes ranging from 1.0 to 2.0 m s⁻² using a whole-body vehicular vibration simulator. The measurements are performed for seated subjects with feet supported and hands held in a driving position. Variations in the seated posture, backrest angle, and nature and amplitude of the vibration excitation are introduced within a prescribed range of likely conditions to illustrate their influence on the driving-point mechanical impedance of seated vehicle drivers. Within the 0.75–10 Hz frequency range and for excitation amplitudes maintained below 4 m s⁻², a four-degree-of-freedom linear driver model is proposed for which the parameters are estimated to satisfy both the measured driving-point mechanical impedance and the seat-to-head transmissibility characteristics defined from a synthesis of published data for subjects seated erect without backrest support. The parameter identification technique involves the solution of a multivariable optimization function comprising the sum of squared magnitude and phase errors associated with both the mechanical impedance and seat-to-head transmissibility target values, subject to limit constraints identified from the anthropometric and biomechanical data. The model response, however, is found to provide a closer agreement with the mechanical impedance target values than that with the seat-to-head transmissibility. From the model, the main body resonant frequencies computed on the basis of both biodynamic response functions are found to be within close bounds to that expected for the human body.

Relevance to industry

The development of an appropriate analytical seated vehicle driver model should provide means of estimating the forces and motions being transmitted within the body under specific vehicular vibration environments. Furthermore, its use in conjunction with a corresponding model for the vehicle seat should allow the prediction of the driver's vibration exposure levels and the seat's ability to attenuate the vibration in particular vehicles.     

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.     

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.     

Mechanisms of the effects of vibration frequency,level and duration on continuous manual control performance

This paper describes three experiments, performed to determine the effects of vibration frequency, level and duration on a zero order, pursuit tracking task, and to discover the mechanisms responsible for these effects. The first experiment investigated the effect on tracking performance of vertical, sinusoidal vibration of the control stick in the frequency range 4 to 64 Hz. Control dynamics were either isotonic (displacement), isometric (force) or spring-centred. The second experiment investigated the effect of level of vertical, 4 Hz and 16 Hz whole-body and control vibration on performance with the isotonic and isometric controls. Experiment 3 investigated the effects of duration of continuous, vertical, whole-body vibration at 4 Hz, for durations up to 1 h, on performance with the isotonic and isometric controls. Performance measures included closed-loop transfer functions of the human operator and components of mean-square tracking error correlated with the forcing functions and vibration, and those due to operator-generated noise or remnant.

The results indicated that the primary effects of vibration on the tracking task were increases in remnant and vibration-correlated error. Perceptual and motor sources are suggested for the increased remnant. The effects were largest with 4 Hz vibration and were found to be effectively constant throughout 1 h exposures to continuous 4 Hz whole-body vibration, but after relatively short periods the effect on overall tracking performance was effectively masked by large increases in response lags and suppression of coherent responses, which occurred in both static and vibration conditions as a consequence of diminished levels of arousal.     

Vibration and comfort HI. Translational vibration of the feet and back

The effects on discomfort of the frequency and direction of the translational vibration of a footrest and flat firm backrest have been studied in two experiments. At frequencies in the range 2.5-63 Hz, the first experiment determined the levels of fore-and-aft, lateral and vertical vibration of the feet of seated subjects which caused them discomfort equivalent to that from 0.8 m/s² r.m.s. 10 Hz vertical vibration of a firm flat seat. The levels of fore-and-aft, lateral and vertical vibration at the back of a seat which were equivalent to 0.8 m/s² r.m.s. 10 Hz vertical seat vibration were determined in the second experiment. The vibration of the feet or back occurred without simultaneous vibration at the seat.

Individual and group equivalent comfort contours are presented. It is concluded that the data provide a useful initial indication of the relative contribution of foot and back vibration to discomfort. Equivalent comfort contours for foot vibration were similar for all three directions of vibration. The contours for vibration of the back show a high sensitivity to fore-and-aft vibration. The results obtained from two additional studies show that vibration from a backrest and other variations in seating conditions can influence subject comfort.     

Transmission of roll and pitch seat vibration to the head

A series of experiments has investigated the transmission of roll and pitch seat vibration to the heads of seated subjects. Head motion was measured in all six axes using a light-weight bite-bar while seated subjects were exposed to random motion at frequencies of up to 5 Hz at 1.0 rad.s ^?2 r.m.s. Subjects sat on a rigid flat seat in two body postures: ‘back-on’ (back in contact with backrest) and ‘back-off’ (no backrest contact). The influence of the position of the centre of rotation was also investigated.

Motion at the head occurred mostly in the lateral, roll and yaw axes during exposure to roll seat vibration and in the fore-and-aft, vertical and pitch axes during exposure to pitch seat vibration. A reduction in the magnitude of head motion occurred when the subjects sat in a 'back-off' posture compared with a 'back-on' posture. Varying the position of the centre of rotation along the lateral axis during roll seat vibration affected vertical and pitch head motion: least head motion occurred when the centre of rotation was in line with the subject's mid-sagittal plane. Varying the position of the centre of rotation along the vertical axis during roll seat vibration affected head motion in the mid-coronal plane: roll head motion decreased as the position of the centre of rotation was raised from below the seat surface to above the seat surface. Varying the centre of rotation (along the fore-and-aft and vertical axes) during pitch seat vibration altered head motion in the mid-sagittal plane. Head motion increased with increasing distance of the centre of rotation in front or behind the subject's ischial tuberosities and increased as the seat was raised from below the centre of rotation to above the centre of rotation.     

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.     

Whole-body vibration and visual performance: an examination of spatial filtering and time-dependency

Two experiments have examined the effects of whole-body vibration on visual performance. The first experiment concerned alphanumeric reading performance and contrast thresholds for gratings subtending 7-5, 10 and 12-5 cycles per degree (c deg)^?1. Seated subjects were exposed to vertical sinusoidal whole-body vibration (4 Hz, 2-5 ms^?2 r.m.s.). Greatest reading errors occurred with characters exhibiting a high spatial complexity in their vertical axis. Reductions in contrast sensitivity due to vibration increased with increasing spatial frequency, the greatest loss occurring with horizontally orientated gratings.

In the second experiment, contrast thresholds for horizontally orientated gratings subtending 1-5 and 12-5cdeg^?1 were obtained from ten subjects at five-minute intervals during a 60-minute whole-body vibration exposure (20 Hz I -7 m s ^?2 r.m.s.), a 20-minute pre-exposure and a 60-minute post-exposure period. There were no significant changes in contrast thresholds for gratings subtending 1-5cdeg^minus;1 during or after vibration exposure. A large variation was found in the effect of vibration upon performance with the higher spatial frequency grating both during and after vibration exposure. Significant correlations between vertical head motion and contrast sensitivity were obtained with five of the ten subjects, suggesting that time-dependent changes in seat-to-head transmissibility were partly responsible for the results. Other time-dependent changes were found with the high spatial frequency grating. Possible explanations are discussed.     

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.     

On human response to prolonged repeated whole-body vibration

The experiment was aimed at investigating human response to different doses of whole-body vibration (WBV), at checking adaptation to repeated exposures, at further evaluating the frequency weighting, and at examining the effect of a distinct interruption of prolonged exposure. Eight male seated subjects were exposed for 3 h to sinusoidal WBV in the z-axis with the frequencies 4 Hz and 8 Hz, at a constant acceleration level of 1·0ms^-2 rms,each frequency being repeated 4 times on consecutive days. Transmissibility, impedance, bioelectrical activity of trunk muscles, postural sway, performance in vigilance tasks, and the subjectively assessed psychological state, efforts, and stress experienced in performing the tasks were investigated. The transmissibility decreased during exposure time at 4 Hz and increased at 8 Hz when a controlled posture was maintained. The power-spectral density distribution and amplitude of postural sway were affected by WBV, depending on both duration and frequency. Performance data and rating data exhibited decrements and adverse effects, being greater beyond the ‘fatigue-decreased proficiency’ boundary (FDPB); adaptation and habituation were more pronounced at the FDPB dose. Generally, there were no cumulative effects. A pause for 20min did not essentially affect the reactions investigated.