Seat-to-head transfer function of seated men--determination with single and three axis excitations at different magnitudes.

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.

Examination of the frequency-weighting curve for accelerations measured on the seat and at the surface supporting the feet during horizontal whole-body vibrations in x- and y-directions.

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/s(2) 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 a(ov) around 0.48 m/s(2) to 0.8 m/s(2) at the reference frequency 0.8 Hz).

Loads on a spinal implant measured in vivo during whole-body vibration.

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.

Measurement and modelling of x-direction apparent mass of the seated human body-cushioned seat system.

For modelling purposes and for evaluation of driver's seat performance in the vertical direction various mechano-mathematical models of the seated human body have been developed and standardized by the ISO. No such models exist hitherto for human body sitting in an upright position in a cushioned seat upper part, used in industrial environment, where the fore-and-aft vibrations play an important role. The interaction with the steering wheel has to be taken into consideration, as well as, the position of the human body upper torso with respect to the cushioned seat back as observed in real driving conditions. This complex problem has to be simplified first to arrive at manageable simpler models, which still reflect the main problem features. In a laboratory study accelerations and forces in x-direction were measured at the seat base during whole-body vibration in the fore-and-aft direction (random signal in the frequency range between 0.3 and 30 Hz, vibration magnitudes 0.28, 0.96, and 2.03 ms(-2) unweighted rms). Thirteen male subjects with body masses between 62.2 and 103.6 kg were chosen for the tests. They sat on a cushioned driver seat with hands on a support and backrest contact in the lumbar region only. Based on these laboratory measurements a linear model of the system-seated human body and cushioned seat in the fore-and-aft direction has been developed. The model accounts for the reaction from the steering wheel. Model parameters have been identified for each subject-measured apparent mass values (modulus and phase). The developed model structure and the averaged parameters can be used for further bio-dynamical research in this field.