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.

After spinal surgery, patients often want to know whether driving a car or using public transportation can be dangerous for their spine. In order to answer this question, a clinically proven vertebral body replacement (VBR) has been modified. Six load sensors and a telemetry unit were integrated into the inductively powered implant. The modified implant allows the measurement of six load components. Telemeterized devices were implanted in five patients; four of them agreed to exposure themselves to whole-body vibration. During the measurements, the patients sat on a driver seat fixed to a hexapod. They were exposed to random single-axis vibrations in X, Y, and Z directions as well as in multi-axis XYZ directions with frequencies between 0.3 and 30 Hz. Three intensity levels (unweighted root mean square values of 0.25, 0.5 and 1.0 m/s(2)) were applied. Three postures were studied: sitting freely, using a vertical backrest, and a backrest declined by an angle of 25 degrees . The patients held their hands on their thighs. As expected, the maximum force on the VBR increased with increasing intensity and the number of axes. For the highest intensity level and multi-axis vibration, the maximum forces increased by 89% compared to sitting relaxed. Leaning at the backrest as well as lower intensity levels markedly decreased the implant loads. Driving a car or using public transportation systems-when the patient leans towards the backrest-leads to lower implant loads than walking, and can therefore be allowed already shortly after surgery.

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).

For the evaluation of seats under comfort and health aspects by means of a Finite-Element (FE) simulation a detailed static and dynamic occupant model is required. The occupant model CASIMIR, which is presented here, in combination with seat model was used in different comfort simulation projects where a good correlation to measurements - seat pressure distribution and transfer function - was found.In order to increase the accuracy of results for internal quantities e.g. the load on the intervertebral disks, CASIMIR was enhanced to improve the force transmission from the seat to the human body. Due to the consideration of the different tissues by corresponding material parameters and the modelling of the filamentary force transmission via springs and dampers for muscles, the human anatomy is modelled more realistically.

The optimal design of driver seats with horizontal suspension requires knowledge of human response with respect to the perception of the vibration intensity and seat comfort or of the performance in motor tasks. In an experimental study, 12 male volunteers (body mass 59–97.3kg) were exposed to whole body vibrations in isolated x- or y-direction (three levels of magnitude) and biaxial xy-direction (combination of the x- and y-exposures on level two) sitting on a driver seat. The suspensions in x- and y-directions were randomly locked or unlocked. A brake and an accelerator foot pedal had to be pressed on demand as fast as possible. The perceptions of the vibration intensity, the seat comfort and the effort to carry out the motor task were judged by cross modality matching (modality: length of a line). The intensity judgements significantly increased with raising vibration magnitude. They were significantly higher for locked suspension. With only some exceptions, the judgements of the seat comfort decreased significantly with increasing magnitude, locked suspension and time. The effort judgements significantly increased with raising magnitude and time and revealed a tendency towards a lower effort with activated suspension. The reaction times showed no significant influences of vibration magnitude, suspension or time, but higher demands seemed to be compensated by enhanced effort. The wd-weighting did not adequately reflect the perceptions for the frequency spectra applied in this study in the x-axis. A modified ‘overall vibration total value’ determined from the non-weighted accelerations instead of the weighted ones (ISO 2631-1, Article 8.2.3) corresponded with the subjective judgements in case of exposure in x- and xy-directions. A clear definition of ‘comfort’ or ‘discomfort’ or the use of ‘intensity’ instead of these terms is recommendable.

Data of the impedance and/or the apparent mass of the sitting human body during the exposure to whole-body vibration in z-direction using rigid seats were standardized in the ISO 5982. These data are available as target functions for model developments. Models developed on this data basis should also apply to driver seats with a soft seat and backrest cushion, although the qualitative different contact conditions were neglected. Due to insufficient technical prerequisites, the determination of forces at the interface between subject and soft seat was impossible until very recently. Results of studies during static conditions showed clear differences in the pressure distributions between the rigid and the soft contact areas. In this experimental study pressure distributions on a seat cushion were measured during whole-body vibration in z-direction (random signal in the frequency range between 0.3 and 20Hz, vibration magnitudes 0.25, 0.8, and 1.6ms−2 unweighted root mean square measured at the seat base) with a sampling rate of 32ms. The apparent masses were calculated by the forces derived from the pressure distributions and accelerations measured at one point of the seat cushion near the buttocks. The moduli of the apparent masses derived for the soft seat were clearly lower than those determined for a rigid seat. These apparent masses showed a similar dependence on the vibration magnitude as the apparent mass functions derived in the usual way for rigid seats. Factors that could explain differences between the apparent mass functions derived for the soft and rigid seat were discussed and evaluated. The data of this study indicate the possibility and necessity to consider the contact conditions at the interface when deriving target functions for the model development. Recommendations for technical improvements and further experimental studies with driver seats were derived.

Apparent mass data describing the biodynamic responses of the human body during whole-body vibration in vertical direction using rigid seats has often been published and standardized. Such data has been used as a target function for developing models. At present there is no adequate database for the apparent mass during whole-body vibration in horizontal direction and in more than one vibration axis. Experiments and subsequent data analysis can be used to improve the database for modelling. An experimental study was performed with 13 male subjects sitting on a rigid seat without backrest, and with a hand resting on a support. They were exposed to random whole-body vibration with root-mean square (rms) values of about 0.25, 1.0 and 2.0ms−2 in vertical, fore-and-aft, and lateral direction, and additionally in two horizontal vibration axes and all three vibration axes simultaneously. The forces and accelerations were measured in x-, y-, and z-directions for each condition tested. With an increase of the vibration magnitude in the three directions measured, the apparent mass functions shifted to lower frequencies. At the same vibration magnitude, the apparent mass functions shifted to the lower frequencies with an increasing number of vibration axes. The effects of the factors vibration magnitude and number of axes on the apparent mass curves were tested. The method of the modal description was applied to all individual apparent mass functions in order to obtain representative target functions. Recommendations for the revision of ISO 5982 were summarized.

Seats with horizontal suspensions can help to reduce detrimental effects of whole-body vibration (WBV) on health, comfort and performance. Two seats were used to examine the effect of body mass and WBV-magnitude on the transmission of WBV from the seat base to the cushion. Both seats have suspension in the x-direction while Seat 2 has suspension also in the y-direction. Twelve subjects with a body mass ranging from 59.0 to 97.3kg volunteered for the study. A set of anthropometric characteristics was acquired. Three magnitudes of WBV were used with a truck-like signal (Seat 1, 0.3–0.59ms−2 wd-weighted rms values at the seat base, x-direction) and a tractor-like signal (Seat 2, 0.55–1.09ms−2 wd-weighted rms values at the seat base, x-direction, 0.52–1.07ms−2 wd-weighted rms values, y-direction). The magnitude of WBV had a significant effect on the transmissibility characterized by SEAT-values. A significant influence of the body mass on SEAT-values was found for the y-direction only. Other anthropometric characteristics proved to be more important for the prediction of SEAT values by multiple regressions. There was no significant correlation of SEAT-values, x-direction, with the body mass. Other anthropometric characteristics enabled a satisfactory prediction of SEAT values also for x-direction in several cases. Tests with only two subjects of extreme body mass are not suited to obtain comparable and representative results required for a comparison of different seats with a suspension in the x-direction. The effect of the WBV-magnitude on the WBV-transmissibility should be considered with the design, testing and application of suspended seats.

Auditory evoked brain potentials (AEP) were recorded from nine healthy male subjects during three types of condition: A — subject and visual field stationary; B — subject vibrated (z-axis, 0.6 Hz, 1.85 ms−2 rms), visual field stationary; C subject stationary, visual field vibrated (as for B). The visual surround was confined to a checkerboard pattern in front of the subject. Auditory stimuli (1000 Hz, 86 dB, inter-stimulus interval 7 s) were delivered via headphones to evoke AEP. Vibration-synchronous activity in the EEG was eliminated by a subtraction technique. In comparison with condition A, conditions B and C caused an attenuation of P2 and N1P2 components of AEP together with an increased latency of N1. Effects of conditions B and C did not differ. Direct vestibular stimulation and mechanisms specific for whole-body vibration were rejected as modes of action. The AEP-changes and the subjective evaluation of experimental conditions, arousal and performance, as well as symptoms of kinetosis (motion sickness) suggest a sensory mismatch, leading to a “latent kinetosis” with de-arousal, as the dominating mechanism by which the processing of information was affected. This suggestion was supported by an additional pilot study. Under real working conditions a similar effect can be expected during relative motion between the driver and his visual surround, i.e. even with perfect vibro-isolation of the driver's seat.

WBV-exposures are often linked with forced postures as prolonged sitting, bent forward sitting, or sitting without a backrest. No quantitative data are available to describe the exposure–effect relationships for different conditions of seating, posture, and the biological variability of workers. Experiments and subsequent predictions of forces acting within the spine during WBV can help to improve the assessment of the health risk. An experimental study was performed with 39 male subjects sitting on a suspension seat with or with no backrest contact. They were exposed to random whole-body vibration with a weighted r.m.s. value of 0·6 m/s2 at a relaxed or a forward bending posture. A two-dimensional finite element model was used for the calculation of the internal spinal load. The model simulates the human response on a suspension driver seat. Individual exposure conditions were considered by including the transfer functions between the seat cushion and the seat base as well as between the backrest and the seat base for the calculation of the vibration input to the buttocks and to the back respectively. The average peak seat transmissibility was higher for the seat with the backrest, but the peak seat-to-head transmissibility was higher for the seat without the backrest for both postures. The peak transmissibilities between the accelerations at the seat base and the compressive forces at L5/S1 were highest for the seat without the backrest during the bending posture. Various biological effects can result from identical exposures combined with different backrest contact and postures. The backrest contact and posture conditions should not be neglected in the assessment of health risk caused by whole-body vibration.

Back muscle forces contribute essentially to the whole-body vibration-induced spinal load. The electromyogram (EMG) can help to estimate these forces during whole-body vibration (WBV). Thirty-eight subjects were exposed to identical random low-frequency WBV (0.7, 1.0 and 1.4 m/s(-2) r.m.s. weighted acceleration) at a relaxed, erect and bent forward postures. The acceleration of the seat and the force between the seat and the buttocks were measured. Six EMGs were derived from the right side of the m. trapezius pars descendens, m. ileocostalis lumborum pars thoracis, m. ileocostalis lumborum pars lumborum; m. longissimus thoracis pars thoracis, in. longissimus thoracis pars lumborum and lumbar multifidus muscle. All data were filtered for anti-aliasing and sampled with 1000 Hz. Artefacts caused by the ECG in the EMG were identified and eliminated in the time domain using wavelets. The individually rectified and normalized EMGs were averaged across subjects. The EMGs without WBV exhibited characteristic patterns for the three postures examined. The coherence and transfer functions indicated characteristic myoelectric responses to random WBV with several effects of posture and WBV magnitude. A comprehensive set of transfer functions from the seat acceleration or the mean normalized input force to the mean processed EMG was presented.The results can be used for the development of more sophisticated models with a separate control of various back muscle groups. However; the EMG-force relationship under dynamic conditions needs to be examined in more detail before the results can be implemented. Since different reflex mechanisms depending on the frequency of WBV are linked with different types of active muscle fibres, various time delays between the EMG and muscle force may be necessary. (C) 2002 Elsevier Science Ltd. All rights reserved.

The knowledge of the extent and timing of the back muscle-response to random whole-body vibration is relevant for an improved evaluation of whole-body vibration with respect to health.

Objective. To elaborate methods for an elimination of artefacts and the analysis of the relationship between random whole-body vibration and electromyographic responses of back muscles.Design. A procedure involving wavelets and digital filtering has been used for the removal of artefacts from the electromyogram during whole-body vibration. Background. Back muscle forces contribute essentially to the whole-body vibration-induced spinal load. The electromyogram can help to estimate these forces during whole-body vibration.Methods. 38 subjects were exposed to identical random low-frequency whole-body vibration. Artefacts caused by the electrocardiogram in the electromyogram were identified by appropriate wavelets and eliminated in the time-domain. After averaging the individual high-pass filtered and rectified undistorted electromyograms across subjects, the transfer function from seat acceleration to the average electromyogram was determined and used for the prediction of the electromyogram.Results. A sufficient procedure involving wavelets and digital filtering has been elaborated for the removal of artefacts from the electromyogram of back muscles during whole-body vibration. A systematic relationship between random vibration and back muscle-response was obtained and described. The transfer function suggests two different reflex-mechanisms - one elicited below, the other above 4 Hz.Conclusions. The approach of analysing and predicting the muscle-response to random vibration by using the transfer function seems to be promising and could be a valuable tool for the future calculation of muscle forces as an input to active models.

OBJECTIVE:To predict forces acting on the spine during whole-body vibration for a variety of boundary conditions - body mass, height and posture.Design. Representative anthropometric data and models for an upright, relaxed and bent forward sitting posture were used to derive model families with 30 variants of a finite-element model. BACKGROUND:A given exposure to whole-body vibration can cause a variable health risk depending on the concomitant conditions. The latter could contribute to the considerable uncertainty of the current evaluation of whole-body vibration. METHODS:Plane symmetric linear finite-element models were used for the prediction of static and dynamic compression and shear forces acting on the lumbar discs during whole-body vibration. Transfer functions from seat acceleration to forces were determined. RESULTS:A bent forward posture augments essentially the compressive and shear stress, predicted for erect and relaxed sitting postures. The normal variation of body mass and height causes a considerable variation of static internal shear stress, but a minor variation of compressive pressure. The dynamic internal stress varies nearly proportionally to the body mass. The transfer functions from seat acceleration to compressive force depend significantly on the posture. CONCLUSIONS:The variability of the spinal loads for a given whole-body vibration and associated with a normal range of several biological factors suggests a ratio between the minimum and maximum internal loads of about 1:2. RELEVANCE:Finite-element models can be used to compare the health risk arising from different whole-body vibration exposures and individual conditions. These results help to prevent work-related disorders of the lumbar spine.

Objective. The seat-to-head transfer function of the human body reflects the biodynamic response. Based on measured data, biodynamic models have been proposed to reflect this response. They must satisfy usually the international published mean values of the seat-to-head transfer function. The question arises to what extent mean values reflect individual pattern of biodynamics.Methods. An experimental study was performed with 39 male subjects sitting on a hard seat without back rest and with supported feet. They were exposed to random whole-body vibration at three intensities with a relaxed and an erect posture. The accelerations in the z-direction were measured at the seat and head. The seat-to-head transfer functions with the associated coherence functions were calculated.Results. The biodynamic response characterised by the maximum of the seat-to-head transmissibility and the frequency of its occurrence is influenced by the posture of the subjects in a dominant way and shows an individual variability of considerable extent. The mean responses suggest a missing effect of vibration intensity, but individually different effects of the intensity were found. Repeated measurements confirmed this result.Conclusions. The application of a model validated by the comparison with mean values of the transmissibility could cause misleading conclusions, if it is used for the prediction of individual spinal loads. Models prepared for the calculation of individual loads should be validated by a mean individual transmissibility derived from repeated measurements.

To examine the response of back muscles to whole-body vibration (WBV) consisting of a sinusoidal “background” (4 Hz, r.m.s. acceleration 0.52 ms−2) and 10 interspersed periods with peak-to-peak amplitudes (“transients”) that varied in three steps (3.5, 7.2, and 11.1 ms−2), six surface electromyograms (EMGs) were obtained from different back muscles of 8 subjects at 3 different sitting postures (relaxed = R, bent forward = B and straight erect = E). The rectified undistorted EMGs and the force at the interface vibrator/subject of all subjects were averaged. The myoelectric activity at R was minimal, B and E were accompanied by a high basic EMG-activity and pronounced responses to the transients. The timing of the maximum EMG-responses varied. A nonlinear increase of the internal load was predicted with rising amplitudes of transients. The threshold for this prominent unfavourable biological effect might be located between 3.5 and 5 ms−2.

The guidance on the effects of vibration on health in standards for whole-body vibration (WBV) does not provide quantitative relationships between WBV and health risk. The paper aims at the elucidation of exposure–response relationships. An analysis of published data on the static and dynamic strength of vertebrae and bone, loaded with various frequencies under different conditions, provided the basis for a theoretical approach to evaluate repetitive loads on the lumbar spine (“internal loads”). The approach enabled the calculation of “equivalent”—with respect to cumulative fatigue failure—combinations of amplitudes and numbers of internal cyclic stress. In order to discover the relation between external peak accelerations at the seat and internal peak loads, biodynamic data of experiments (36 subjects, three somatotypes, two different postures—relaxed and bent forward; random WBV,aw, r.m.s. 1·4 ms−2, containing high transients) were used as input to a biomechanical model. Internal pressure changes were calculated using individual areas of vertebral endplates. The assessment of WBV was based on the quantitative relations between peak accelerations at the seat and pressures predicted for the disk L5/S1. For identical exposures clearly higher rates of pressure rise in the bent forward compared to the relaxed posture were predicted. The risk assessment for internal forces considered the combined internal static and dynamic loads, in relation to the predicted individual strength, and Miner's hypothesis. For exposure durations between 1 min and 8 h, energy equivalent vibration magnitudes (formula B.1, ISO 2631-1, 1997) and equivalent vibration magnitudes according to formula B.2 (time dependence over-energetic) were compared with equivalent combinations of upward peak accelerations and exposure durations according to predicted cumulative fatigue failures of lumbar vertebrae. Formula B.1 seems to underestimate the health risk caused by high magnitudes, formula B.2 is recommended for the evaluation of such conditions.

The operators of earth moving machinery are often exposed to a low frequency vibration environment caused by the movement of vehicles over uneven ground and the task carried out. The seat constitutes the least state of suspension before the driver. The efficiency of attenuation under consideration of the best design practice today is the basis for the revision of ISO 7096 for the testing seats for earth moving machinery. This standard requires the participation of two subjects with different body masses (52–55 kg; 98–103 kg). The aim of the study was to investigate (1) the extent and the influence of individual variability and posture change on the result of seat tests, and (2) the possibility of deducing representative results for the user population. 37 male subjects took part in the experimental investigations. They were exposed in three postures for 67 s to three acceleration signals in a vertical direction corresponding to the spectral classes (EM2, EM5, EM6) in ISO/DIS 7096 on two commercial suspension seats. The vertical accelerations were measured at the seat basis and at the interface between seat cushion and subject. The results of the analysis of variance show a significant influence of exposure, type of seat, and interactions exposure-by-posture, exposure-by-type of seat, and posture-by type of seat on the SEAT factor. Simple and multiple regression analyses were applied in order to test the predictability of the seat factor (SEAT) by anthropometric variables. The conclusions were drawn that the seat testing could be improved by (1) selecting subjects according to the 5th and 95th percentile masses of the population of vehicle or machinery users for which the seat is intended (ISO 10326), instead of fixed masses (ISO 7096), (2) considering other anthropometric parameters for the selection like the body height and body mass supported by the seat, and (3) the inclusion of several subjects near the 50th percentile in order to assess the variability of the SEAT factor.