Barbara Hinz

Intradiscal pressure together with anthropometric data – a data set for the validation of models

OBJECTIVE To provide a database of intradiscal pressure measurements together with anthropometric data as basis for the validation of models that predict spinal loads. DESIGN Intradiscal pressure was measured in a non-degenerated L4-5 disc of a volunteer. The anthropometric characteristics of this subject were extensively determined. BACKGROUND Since it is usually impossible to quantify the load in the spine directly, it is predicted by various biomechanical models. However, they often cannot be validated because of the few in vivo data and missing anthropometric characteristics pertaining to them. METHODS A pressure transducer (diameter 1.5 mm) was implanted in the nucleus pulposus of a non-degenerated L4-5 disc of a volunteer. Pressure was determined during exercises while standing, lifting activities, sitting unsupported on a stool or an ergonomic sitting ball, sitting in different postures and others. The anthropometric characteristics were determined using different tools. RESULTS Pressure values: relaxed standing 0.5 MPa; standing flexed forward 1.1 MPa; standing extended backward 0.6 MPa; sitting unsupported 0.46 MPa; maximum values during lateral bending 0.6 MPa, during axial rotation 0.7 MPa, lifting a 20 kg weight with a round flexed back 2.3 MPa, with flexed knees 1.7 MPa, close to the body 1.1 MPa; sitting unsupported relaxed 0.45 MPa, actively straightening the back 0.55 MPa, with flexion 0.9 MPa; non-chalant sitting 0.3 MPa and others. Anthropometric characteristics with emphasis on data for the trunk are provided in tables.Conclusions. Intradiscal pressure depends on the kind of preceding activity, posture, external loads and muscle activity. RELEVANCE The data set can be used to verify a biomechanical model adjusted to the individual characteristics by a comparison of measured and predicted intradiscal pressures.

Effects of sinusoidal whole-body vibration on the lumbar spine: the stress-strain relationship

SummaryThe aim of this experimental study was to estimate the strain in the lumbar spine due to whole-body vibration (WBV). Four male subjects were exposed to vertical sinusoidal WBV with frequencies ranging from 1 to 15 Hz at two intensities (I1 = 1.5 ms−2 rms; I2 = 3.0 ms−2 rms). The compressive forces acting on the disc L3-4 during the extreme values of acceleration were predicted on the basis of anthropometric data, EMG of back muscles and the acceleration of the upper trunk, using a simple biomechanical model. The estimated mechanical activity of back muscles was not able to protect the spine under many exposure conditions. The highest compressive forces were predicted for WBV with 7.5, 8 and 4.5 Hz. The results suggest the possibility of fatigue failures at the endplates of lumbar vertebrae after intense long-term exposure to WBV.

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.

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.

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.

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.