Gerhard Menzel

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.

A cohort study of sciatic pain and measures of internal spinal load in professional drivers

In a prospective cohort study of 537 male professional drivers, the occurrence of sciatic pain showed stronger associations with measures of internal lumbar load expressed in terms of daily compressive dose, Sed (MPa), and risk factor, R (non-dimensional), according to ISO/WD 2631-5 (2013), than with measures of daily vibration exposure calculated as either 8-h energy-equivalent frequency-weighted acceleration (ms− 2 r.m.s.) or vibration dose value (ms− 1.75) according to the EU Directive on mechanical vibration (2002). Herniated lumbar disc, previous lumbar trauma and physical work load were also powerful predictors of the occurrence of sciatic pain over time. Psychosocial work environment was poorly associated with sciatic pain. The boundary values of risk factor (R) for low and high probabilities of adverse health effects on the lumbar spine, as proposed by international standard ISO/WD 2631-5 (2013), tend to underestimate the health risk in professional drivers. Practitioner Summary: In a prospective cohort study of professional drivers, measures of internal spinal load were better predictors of the occurrence of sciatic pain than the measures of daily vibration exposure established by the EU Directive (2002). Herniated lumbar disc, lumbar trauma and physical work load were also associated with sciatic pain.

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.

Effect of whole-body vibration with different frequencies and intensities on auditory evoked potentials and heart rate in man

SummaryAuditory evoked brain potentials (AEP) and electrocardiogram (ECG) were recorded from 9 healthy male subjects during sinusoidal whole-body vibration exposure (WBV) in the longitudinal (±az) direction with four frequencies (1 Hz, 2 Hz, 4 Hz, and 8 Hz) and two intensities as well as under non-WBV conditions. The sequences of the different experimental conditions were arranged according to a 9×9 Latin Square design. The sound of the electrohydraulic vibrator was masked by a constant noise level. A subtraction technique was used to eliminate vibration-synchronous activity contaminating the electroencephalogram. The AEP amplitude N1-P2 revealed systematic effects of different WBV frequencies and intensities. The amplitude decreased along with an increase in intensity (16 dB) by about 10 per cent. It diminished increasingly with a monotonic trend in the order non-WBV, WBV 8 Hz, WBV 4 Hz, WBV 2 Hz, and WBV 1 Hz. The interbeat-interval histograms computed from the ECG exhibited the highest mean values at MBV of 1 Hz, high intensity, and the lowest ones at WBV of 4 Hz, high intensity. The AEPs are reaffirmed as an informative measure for studying the WBV effect on central nervous information processing, although the modes of action are not yet fully known. Efferent influences on the acoustic input, cross-modality interaction, sensory mismatch, and changes of central nervous activation level are discussed as potential mechanisms.

Effects of isolated and combined exposures to whole-body vibration and noise on auditory-event related brain potentials and psychophysical assessment.

SummaryAuditory event-related brain potentials (ERP) in response to two different tone stimuli (1.1 kHz or 1 kHz, 80 dB, 50 ms; given by headphones at a regular interstimulus interval of 5 s with a probability distribution of 70:30) were recorded from 12 healthy male subjects (Ss) during four different conditions with two repetitions: A - 60 dBA white noise (wN), no wholebody vibration (WBV); B - 60 dBA wN plus sinusoidal WBV in the az-direction with a frequency of 2.01 Hz and acceleration of 2 m ·s−2 root mean square; C - 80 dBA wN, no WBV; D - 80 dBA wN plus WBV. Each condition consisted of two runs of about 11 min interrupted by a break of 4 min. During the break with continuing exposure, but without auditory stimuli, Ss judged the difficulty of the tone-detection task and intensity of noise by means of cross-modality matching (CMM). Vibration-synchronous activity in the electrocardiogram was eliminated by a subtraction-technique. Noise caused an attenuation of the N1 and P2 amplitudes and prolongation of P3 latencies. The WBV did not cause systematic ERP effects. Condition B was associated with higher N1 and smaller P3 amplitudes. The factor “condition” had a significant effect on the peak latencies of P3 to target stimuli and the task difficulty judged by CMM. Both effects exhibited significant linear increases in the sequence of conditions A, B, C, D. For the evaluation of exposure conditions at work, it can be suggested that noise has a strong systematic effect which can be enhanced by WBV. The P3 latency is considered as an advantageous measure for the detection of objective effects of physical environmental factors, correlating with relevant subjective responses.

Measures of internal lumbar load in professional drivers – the use of a whole-body finite-element model for the evaluation of adverse health effects of multi-axis vibration

The present study aimed to (1) employ the method for evaluation of vibration containing multiple shocks according to ISO/CD 2631–5:2014 (Model 1) and DIN SPEC 45697:2012 in a cohort of 537 professional drivers, (2) deliver the results for a re-analysis of epidemiological data obtained in the VIBRISKS study, (3) clarify the extent to which vibration acceleration and individual variables influence risk values, such as the daily compressive dose Sed and the risk factor R, and (4) compare the results with in vivo measurements and those obtained in previous studies with similar models. The risk factor R was influenced by the acceleration, lifetime exposure duration, sitting posture, age at the start of exposure and body mass/body mass index in order of decreasing effect. Age and annual and daily exposure duration had only a marginal effect. The daily compressive dose Sed and the risk factor R showed weak linear association with the daily vibration exposure A(8) and the vibration dose value VDV. The study revealed high shear forces in the lumbar spine. Practitioner Summary: In a re-analysis of an epidemiological study of professional drivers, a software tool available with standards DIN SPEC 45697:2012 and ISO/CD 2631–5:2014 Model 1 was used to calculate the risk to the lumbar spine in terms of daily compressive dose Sed and risk factor R. The tool was found to be suitable for risk assessment in a large cohort.

The effect of low-frequency whole-body vibration under different visual conditions on auditory evoked potentials.

Auditory evoked brain potentials (AEP) were recorded from 9 healthy males during sinusoidal whole-body vibration (WBV) in the longitudinal (+/- az) direction with 0.6 Hz, 1.85 ms-2rms (F1), 1.01 Hz, 4.27 ms-2rms (F2) and without WBV (F3) under 3 visual conditions--homogeneous bright visual field (B), normal vision (N), and complete darkness (D). The sequences of the different experimental conditions were arranged according to a 9 X 9 Latin Square design. A subtraction technique was used to eliminate vibration-synchronous activity from the EEG. The N1 and N1P2 amplitudes decreased during F1 and F2, compared to F3. The latencies of N1 and P2 increased during F1 and F2. The effects of F1 and F2 did not differ. The visual conditions exhibited no systematic effect on the AEP. The results suggest (1) F1 and F2 to be equivalent exposure conditions and (2) the dominance of vestibular-auditory interactions, compared with visual-auditory ones.