Ralph Blüthner

Isolated and combined effects of prolonged exposures to noise and whole-body vibration on hearing, vision and strain

SummaryThis study was carried out in order: (1) to examine the effects of isolated and combined prolonged exposures to noise and whole-body vibration on hearing, vision and subjectively experienced strain, and (2) to check the combined effects with repeated exposures. Six male subjects were exposed twice to noise (N) at 92 dBA, whole-body vibration (V) in the Z-axis at 4 Hz and 1.0 ms−2 rms, and noise and vibration (NV) for 90 min with each condition. Temporary threshold shifts of hearing (TTS) and their integrals (ITTS) were measured at 4, 6, 10, and 12 kHz. Visual acuity was examined by means of a very sensitive test. Cross-modality matching (CMM) of the handgrip force was used to judge the subjectively experienced strain. NV induced a clear tendency of higher TTS and ITTS than N, with several significant differences most pronounced at 10 kHz. With repeated exposures, the effect of NV decreased, while the reactions to N and V remained unchanged. The individual reactions to NV differed. The influence of the duration of exposures on vision depended on the condition; N caused time-dependent changes, whereas V did not. CMM-data increased with the duration of the exposure during V and NV. N was generally judged to be more straining than V; NV caused higher strain than V during the first 30 min of exposure only. Correlations between different effects suggest certain links between them. Additionally, less motivation — daily obtained by a questionnaire — often correlated with higher ITTS during N and NV. The results also illustrate the combined effects on the individual susceptibility, repetition of exposure, the kind of response, and, possibly, the actual psychic state.

The relationship between the electromyogram-amplitude and isometric extension torques of neck muscles at different positions of the cervical spine

A group of 12 healthy men volunteered for the experiment. Electromyograms (EMG) were obtained from semispinalis capitis, splenius capitis, levator scapulae, and trapezius muscles. The flexion angle of the cervical spine was precisely adjusted to 0°, 10°, 20°, and 30° relative to the horizontal, with a constant angle of the atlanto-occipital joint. The subjects made eight short (about 2 s) vertical extension forces (6%, 12%,18%, 24%, 30%, 36%, 42%, and 48% of maximal voluntary peak contraction force). For each position, the centre of pressure under the head was determine as the basis for the calculation of the external lever arm. The presence of motor endplate regions was ascertained by multiple surface electrodes. The slopes of individual linear regression lines for the root mean square (rms)-values were dependent on the existence of endplates in the area of the electrodes — endplates caused smaller rms values per Newton metres of external torque. Significant intersubject differences between regression equations could not be eliminated by the normalization of EMG-parameters and/or torques. The elimination of gravity, the continuous monitoring of positions, and the consideration of localization of motor endplate regions were essential prerequisites for the acquisition of reliable relationships between EMG of different neck muscles and external torques. Two important conclusions were derived for the prediction of torques from EMG measurements: firstly, individual regression equations which take into account the position of the head and neck should be used; secondly, normalization procedures do not justify the application of average regressions to a group of subjects.

Application of finite-element models to predict forces acting on the lumbar spine during whole-body vibration.

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.

Control of positioning the cervical spine and its application to measuring extensor strength

The great variability of the flexion of the cervical spine renders an exact description of the control of various positions difficult. A method was developed enabling a precise control of positioning the cervical spine and head in the sagittal plane. In three repeated measurements the mean values of the position of external anatomical landmarks and distances between them exhibited a good reproducibility. Any variable effect of gravity on the activity of the neck muscles at different positions of the cervical spine was eliminated by the passive compensation of gravity. The significance of methodical details is illustrated by the results of an applied study. The maximum strength of neck extensors was examined in 12 male subjects in a supine position at four different flexion angles from 0 to 30 degrees of the cervical spine. The vertical force component was measured. The maximum voluntary moments of forces about the bilateral motion axis of the C7T1 motion segment exhibited a tendency to decrease with increasing flexion.

Examination of the myoelectric activity of back muscles during random vibration – methodical approach and first results

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. RELEVANCE 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.

Bidimensional accelerations of lumbar vertebrae and estimation of internal spinal load during sinusoidal vertical whole-body vibration: a pilot study.

Accelerations, a, in z- and x-directions were measured on the skin over spinous processes L3 and L4 in three subjects during sinusoidal whole-body vibration (WBV) at 4·5 and 8 Hz and 1·5 and 3·0 ms(-2) r.m.s. A method for the prediction of bone accelerations was applied using measurements on the skin. Relative accelerations were calculated by subtracting aL4 from aL3. The phase relations between relative accelerations in the z-direction indicating compression and the absolute maximum az of L4 exhibited marked between-subject variability. One subject was selected for a detailed analysis in the time domain of head, shoulder and upper trunk accelerations, and for comparison with an invasive study. Bidimensional acceleration data confirmed the suggestion that relative motions in the z-direction are combined with angular motions. The results indicate complex internal loads with coupled bending, compression and shear forces.

Examination of spinal column vibrations: a non-invasive approach

SummaryAccelerations of vertebrae during whole-body vibration (WBV) are used in occupational biomechanics for the prediction of internal stress. To avoid invasive techniques, a method for the calculation of bone accelerations was developed using measurements on the skin. The soft tissue between spinous processes L3 and T5 and miniature accelerometers stuck to the skin over them was modelled by a simple Kelvin element, whose parameters i.e. angular natural frequencyωn4 and critical dampingζ describe an approximate transfer function between the bone (input) and the skin surface (output). The parameters were determined from free damped oscillations of the accelerometer-skin complex in the Z-axis, and depended significantly on the factors “subject” and “point of measurement”. In one subject, the time courses of bone accelerations during sinusoidal WBV (4.5 and 8 Hz; 1.5 m·s−2 RMS) were calculated using separate transfer functions for each of 11 different spinal levels. Since the output signals on the skin were non-sinusoidal, the skin accelerations had to be treated with an inverse transfer function in the frequency domain. A comparison of accelerations measured on the skin and predicted for the bone mainly indicates that absolute peak values of bone accelerations are smaller and occur earlier. Both kinds of acceleration hint at differences in WBV-induced internal stress within the spine.

Transfer functions as a basis for the verification of models – variability and restraints

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. RELEVANCE The results illustrate the loss of information by averaging individual transfer functions and the consequence of a limited validity and applicability in occupational health, ergonomics, and design.

Back muscle response to transient whole-body vibration

Abstract This study was performed in order to clarify, (1) if trunk muscles react immediately to a transient whole-body vibration (WBV), (2) to which extent the timing of EMG depends on the direction of transient WBV and/or on the muscle group, and (3) to which degree after-effects of transient WBVs have to be considered. Five healthy males were exposed to transient displacements (nearly sinusoidal or half-sinusoidal waveforms with peak accelerations of about ±2.7 ms −2 ). Four EMGs (m. erector spinae at 3 levels and m. trapezius) were averaged and normalized. The alternating activation-inactivation of the EMG-responses nearly exhibited a mirror symmetry when the direction of displacements changed. Responses occurred earlier at the shoulder than at the lumbar level. An increased health risk was predicted for (1) the initial phase of a sudden upward displacement without motion in the history preceding the transient WBV and (2) a downward displacement with a dominating frequency near 6–8 Hz. The immediate muscular reactions suggest the necessity to include muscle forces in calculations of the spinal load under transient WBV, except for the first 50 to 100 ms of an event without motion in its preceding time history. The direction and preceding history of a transient WBV should be considered in future evaluation procedures as a characteristic of WBV-exposure.

Estimation of disc compression during transient whole-body vibration

This study was performed to examine health effects of transient whole-body vibrations on the lumbar spine. The aim was to detect extremes in the time course of compressive load acting on the disc L3-4 in order to estimate the health risk which depends on the amplitude of peak values of compression. Five healthy males were repeatedly exposed to various transient displacements with nearly sinusoidal or half-sinusoidal waveforms, different durations, and peak accelerations between about 1.4 and 4.1 ms(-2). Accelerations in the z direction were measured on the skin over the spinous processes of L3-4 in five subjects and averaged individually. Complete time series of dynamic compressive forces were calculated by means of a biomechanical model using the calculated effective mass of the human body above the disc L3-4 and relative accelerations between the vertebrae L3-4 for the first time. The amplitudes of the absolute peak values of the compressive forces were influenced only by the interaction between the initial direction and the duration of the waveform. Direct comparisons with the results of other authors are impossible due to methodical differences and missing data in the time domain. The nearly constant peak compressive forces with a shorter duration of transients connected with a higher-frequency content support the proposal to put more weight on vibrations above 8 Hz in a revised International Standard ISO 2631. The comparison of the calculated internal forces with results of in-vitro studies indicates a possible health risk for persons with a low vertebral strength during repetitive exposures to moderate transient whole-body vibrations.