Investigation of the transmission of fore and aft vibration through the human body

Understanding the behavior of human body under the influence of vibration is of great importance for the optimal motor vehicle system design. Therefore, great efforts are being done in order to discover as many information about the influence of vibration on human body as possible. So far the references show that the major scientific attention has been paid to vertical vibration, although intensive research has been performed lately on the other sorts of excitation. In this paper, the results of the investigation of behavior of human body, in seated position, under the influence of random fore and aft vibration are shown. The investigation is performed by the use of an electro-hydraulic simulator, on a group of 30 healthy male occupants.Experiments are performed in order to give results to improve human body modeling in driving conditions. Excitation amplitudes (1.75 and 2.25 m/s² rms) and seat backrest conditions (with and without inclination) were varied. Data results are analyzed by partial coherence and transfer functions. Analyses have been performed and results are given in detail.The results obtained have shown that the human body under the influence of random excitations behaves as a non-linear system and its response depends on spatial position.Obtained results give necessary data to define structure and parameters of human biodynamic model with respect to different excitation and seat backrest position.     

Transmission of roll and pitch seat vibration to the head

A series of experiments has investigated the transmission of roll and pitch seat vibration to the heads of seated subjects. Head motion was measured in all six axes using a light-weight bite-bar while seated subjects were exposed to random motion at frequencies of up to 5 Hz at 1.0 rad.s ^?2 r.m.s. Subjects sat on a rigid flat seat in two body postures: ‘back-on’ (back in contact with backrest) and ‘back-off’ (no backrest contact). The influence of the position of the centre of rotation was also investigated.

Motion at the head occurred mostly in the lateral, roll and yaw axes during exposure to roll seat vibration and in the fore-and-aft, vertical and pitch axes during exposure to pitch seat vibration. A reduction in the magnitude of head motion occurred when the subjects sat in a 'back-off' posture compared with a 'back-on' posture. Varying the position of the centre of rotation along the lateral axis during roll seat vibration affected vertical and pitch head motion: least head motion occurred when the centre of rotation was in line with the subject's mid-sagittal plane. Varying the position of the centre of rotation along the vertical axis during roll seat vibration affected head motion in the mid-coronal plane: roll head motion decreased as the position of the centre of rotation was raised from below the seat surface to above the seat surface. Varying the centre of rotation (along the fore-and-aft and vertical axes) during pitch seat vibration altered head motion in the mid-sagittal plane. Head motion increased with increasing distance of the centre of rotation in front or behind the subject's ischial tuberosities and increased as the seat was raised from below the centre of rotation to above the centre of rotation.     

Seat Reservation Allowing Seat Changes

We consider a new variant of the Seat Reservation Problem [4]in which seat changes are allowed. We analyze the problem using the competitive ratio and the competitive ratio on accommodating sequences [4]. A very promising algorithm defined in this paper is Min-Change, which will ask passengers to change seat, only if they would otherwise have been rejected. Min-Change belongs to a large class of conservative algorithms, for which we prove very high performance guarantees. For instance when assuming that all of the passengers could have been seated by an optimal off-line algorithm, at least of the passengers can be seated on-line with only one seat change and at least will be seated if two seat changes are allowed. This should be compared to the performance guarantee of for the best deterministic on-line algorithm when no seat changes are allowed [2].     

Effects of elastic seats on seated body apparent mass responses to vertical whole body vibration

Apparent mass (AM) responses of the body seated with and without a back support on three different elastic seats (flat and contoured polyurethane foam (PUF) and air cushion) and a rigid seat were measured under three levels of vertical vibration (overall rms acceleration: 0.25, 0.50 and 0.75 m/s²) in the 0.5 to 20 Hz range. A pressure-sensing system was used to capture biodynamic force at the occupant-seat interface. The results revealed strong effects of visco-elastic and vibration transmissibility characteristics of seats on AM. The response magnitudes with the relatively stiff air seat were generally higher than those with the PUF seats except at low frequencies. The peak magnitude decreased when sitting condition was changed from no back support to a vertical support; the reduction however was more pronounced with the air seat. Further, a relatively higher frequency shift was evident with soft seat compared with stiff elastic seat with increasing excitation.     

Assessment of the static fit of automobile lap-belt systems on front-seat passengers

While automobile seat belts are recognized as reducing injury in frontal collisions, the lap belt can cause injury to the abdominal contents during impact. A lap belt which is malpositioned, i.e., above the anterior superior iliac spines (ASIS) of the pelvis before impact, is suggested as a causative factor in many of these injuries. A questionnaire was completed by 198 adult passengers on lap-belt fit as well as vehicle, anthropometric and behavioural factors. All measures were self-reported. The fit of automobile seat belts was also investigated in the laboratory to determine some of the factors thought important in the pre-impact position of the lap belt. Seven factors, subject size, subject sitting posture, clothing thickness, vehicle configuration, vehicle seat position, seat back angle and the handling of the belt by the occupant, were assessed on 51 subjects sitting in six simulated vehicles. The sample purposely included a disproportionate number of tall, short, heavy and thin subjects.

The questionnaire responses indicated that a high proportion (49%) of lap belts were found to be malpositioned in a normal sitting posture. Moving around in the seat, especially slouching, greatly increased the proportion of malpositions. It was noteworthy that wearing a heavy winter coat did not cause belt malposition. A high incidence of malpositioned lap belts was also found in the laboratory study with 42% of the belts having their centre-lines above the ASIS and 89% having part of the belt overlying the ASIS in a normal upright seated position with the seat in the middle of its fore/aft movement. Slouching in the seat significantly increased this proportion. The malposition rate was adversely affected by a forward seat position in a significant manner. There was a large increase in the malposition rate when the occupant attached the belt compared to when the experimenter attached it, with malposition rates of 43% and 19% respectively in the rearmost seat position. There was no statistically significant effect of clothing thickness on belt fit. Although statistically significant correlations between belt inclination and malposition were found, no clear-cut belt-angle existed above which satisfactory fit existed.     

Evaluation of an intelligent seat system

This study was conducted to evaluate the effect of an intelligent seat system, a microprocessor-based interactive seat that automatically adjusts itself to fit a seated individual by making pressure-sensitive adjustments on its own. First, a standard American automobile seat ('baseline' seat) was assessed for comfort. Subjective ratings of comfort, pressure distribution and seated anthropometric measurements were recorded for 20 test subjects. These measurements were recorded while the subjects maintained a simulated driving position in a seat buck. The comfort scale was based on a rating of 1 to 10, with 1 corresponding to 'very poor' and 10 corresponding to 'very good.' Based on a nonlinear, multiple regression model that had been previously developed, the comfort rating of the seat was predicted based on the subjective ratings and the recorded values of 450 pressure measurements from 20 subjects. The predicted comfort value was 7.46 for the baseline seat. Following the baseline assessment, the intelligent seat system was installed into the standard American automobile seat. The objective and subjective assessments were then repeated for 17 subjects and the new predicted comfort rating was 8.06. A t-test performed on the subjective and objective measures indicated that this was a significant improvement in seat comfort. Overall, subjects felt the self-adjusting seat was more customized and more comfortable, providing a better fit.     

The impact of office chair features on lumbar lordosis,intervertebral joint and sacral tilt angles: a radiographic assessment

Abstract

Background: The purpose of this study was to determine which office chair feature is better at improving spine posture in sitting. Method: Participants (n = 28) were radiographed in standing, maximum flexion and seated in four chair conditions: control, lumbar support, seat pan tilt and backrest with scapular relief. Measures of lumbar lordosis, intervertebral joint angles and sacral tilt were compared between conditions and sex. Results: Sitting consisted of approximately 70% of maximum range of spine flexion. No differences in lumbar flexion were found between the chair features or control. Significantly more anterior pelvic rotation was found with the lumbar support (p = 0.0028) and seat pan tilt (p < 0.0001). Males had significantly more anterior pelvic rotation and extended intervertebral joint angles through L1–L3 in all conditions (p < 0.0001). Conclusion: No one feature was statistically superior with respect to minimising spine flexion, however, seat pan tilt resulted in significantly improved pelvic posture.

Practitioner Summary: Seat pan tilt, and to some extent lumbar supports, appear to improve seated postures. However, sitting, regardless of chair features used, still involves near end range flexion of the spine. This will increase stresses to the spine and could be a potential injury generator during prolonged seated exposures.     

Subjective response to seated fore-and-aft direction whole-body vibration

Subjective response to seated, fore-and-aft direction, whole-body vibration of the type experienced in automobiles was investigated. Fore-and-aft acceleration was measured at the seat guide of a small automobile when driving over two representative road surfaces, and was replicated in a laboratory setting using a whole-body vibration test rig and rigid seat. A single 15 s section of each of the two acceleration time histories was band-pass filtered to the frequency interval from 0.5 to 50.5 Hz, and was used as a base stimulus. Thirteen test stimuli were then constructed for each base stimulus by rescaling to BS 6841 W_d frequency-weighted r.m.s. amplitudes from 0.01 to 0.86 m/s². Two groups of 16 participants (8 male and 8 female in each case) rated the discomfort of the test stimuli. The first group was asked to use the psychophysical method of magnitude estimation while the second used a Borg CR-10 scale. The order of presentation of the test stimuli was fully randomised and each was repeated three times. For each group of participants, regression analysis was used to determine both the individual and the group mean Stevens’ Power Law exponent describing the relationship between stimulus amplitude and subjective response. All mean power exponents were found to be less than unity, with the CR-10 scale having produced smaller exponents than magnitude estimation. The power exponents ranged from 0.66 to 0.91, corroborating the value of 0.84 obtainable from the guidelines of standard BS 6841.The results suggest that the numerical response scale provided in the BS 6841 guidelines is appropriate for use in the case of automobile fore-and-aft vibration, but that the semantic labels under-represent the actual human subjective response in this direction. Psychophysical test method, vibration stimulus range and test participant gender were all found to affect the Stevens’ Power Law exponent achieved from subjective testing. Each factor may therefore require control when attempting to compare human responses to vibration originating from different automobiles.     

Comparisons of apparent mass responses of human subjects seated on rigid and elastic seats under vertical vibration

The apparent mass (AM) responses of human body seated on elastic seat, without and with a vertical back support, are measured using a seat pressure sensing mat under three levels of vertical vibration (0.25, 0.50 and 0.75 m/s² rms acceleration) in 0.50–20 Hz frequency range. The responses were also measured with a rigid seat using the pressure mat and a force plate in order to examine the validity of the pressure mat. The pressure mat resulted in considerably lower AM magnitudes compared to the force plate. A correction function was proposed and applied, which resulted in comparable AM from both measurement systems for the rigid seat. The correction function was subsequently applied to derive AM of subjects seated on elastic seat. The responses revealed lower peak magnitude and corresponding frequency compared to those measured with rigid seat, irrespective of back support and excitation considered.     

The effect of seat back inclination on spine height changes

The effect of backrest inclination on spinal height changes was tested during static sitting and seated whole-body vibrations. The vibration input was sinusoidal with a frequency of 5 Hz and an acceleration of 0.1 g rms. The backrest inclinations tested were 110 degrees and 120 degrees . The 110 degrees backrest caused less shrinkage than did the 120 degrees during static sitting, whereas the opposite was true when vibration was present, although the differences between the backrests were not statistically significant. Only when the results were compared with results from exposure to unsupported sitting were the differences statistically significant for both static sitting and seated vibrations when the 110 degrees backrest was used and for vibration with the 120 degrees backrest. Thus we conclude that an inclined backrest reduces the effects of vibration. More importantly, emphasis should be placed upon seats and seat materials that can attenuate vibration.     

The effects of lumbar massage on muscle fatigue, muscle oxygenation, low back discomfort, and driver performance during prolonged driving

An increasing dependence of society on automobiles for both work and leisure and the corresponding increase in time spent seated in the car has been correlated with a greater risk of low back pain and absence from work (Porter and Gyi 2002). This study examined the effects of three types of lumbar massage units on seating comfort, muscle fatigue, muscle oxygenation, muscle blood flow and driving performance during a 1 h simulated driving task. Electromyographic (EMG) signals were recorded from the right and left thoracic and lumbar erector spinae musculature. Average EMG (AEMG), mean power frequency (MPF), gaps and amplitude probability distribution function (APDF) parameters were analysed from the three massage seats and compared to a control seat. Near infrared spectroscopy (NIRS) and skin temperature from the right thoracic and lumbar erector spinae were used as an indication of muscle oxygenation and blood flow throughout the driving task. Ratings of perceived discomfort were used to assess driver discomfort, and driving performance was assessed by calculating mean lap times for the duration of each driving trial. The results showed statistically significant increases in skin temperature compared with the control seat after 60 min of driving. The NIRS results reflected these trends although the results were not statistically significant. AEMG and MPF measures showed no significant differences between the seats. MPF measures were found to increase over time, effects attributed to increases in muscle temperature. Gaps and APDF analyses revealed greater rest times and lower activation levels, respectively, with the control seat, which could result in increased loading of passive structures. This study demonstrated the beneficial effects of lumbar massage systems in increasing muscle blood flow and oxygenation. Although EMG parameters were not significantly different, the trends support the significant blood flow results. Future research should include longer driving times and adjustments in EMG measures to account for the effects of increasing muscle temperature on AEMG and MPF measures.     

The prediction of car driver size and position to enhance safety in crashes

The positions which car drivers adopt when driving will depend on their anthropometric characteristics, the range and type of adjustment available from the vehicle package and their preferred driving posture. The design and testing of systems to protect occupants in car crashes assumes that the size and position of the driver is ‘normal’ or ‘average’, although there is some accommodation for adjustability. If, however, the occupant protection system had information on the driver's chosen seat position, on whether the driver was particularly large or small and on whether the driver was sitting close to or further from the steering wheel, in a crash the system could tailor its performance and enhance the protection offered. This study investigated whether it was possible to predict the physical characteristics of the driver and the driver's position in relation to the steering wheel, from data that could be collected by sensors in the seat and seat mounting. In order to do this, anthropometric characteristics of drivers and their usual seated position in their own vehicle were measured and analyses were undertaken to identify whether there were any relationships between the driver-related and the vehicle-related measures. The results showed that it was possible to predict drivers' head and chest positions relative to injury-producing features of the vehicle such as the steering wheel (and hence the airbag) and to predict some physical dimensions of drivers.     

The effect of an adjustable sitting angle on the perceived discomfort from the back and neck-shoulder regions in building crane operators

In a previous working environment study of building crane operators, it has been found that approximately 70% experienced discomfort from the locomotor system. Comments by the interviewed crane operators indicated that it is, among other things, the forward flexed sitting position during lifts close to the crane that causes discomfort. This investigation sought to apply knowledge from the forestry industry concerning the beneficial effects of improved operator's seats to the work situation of crane operators.

On a construction site with three cranes, an operator's seat with adjustable sitting angle was installed in one of the cranes. Estimation of perceived strain-discomfort in the lumbar region of the back as well as in the neck-shoulder region was assessed according to Borg's scale. Data were collected from the crane operators seated in their ordinary operator's seat, seated in the test seat, and seated in another crane with an ordinary type of seat. The results showed that in 1/3 rd of all lifts, the crane operator was sitting bent-forward with little opportunity for relief via a backrest or armrests. The highest estimated discomfort values in the study were also obtained in an ordinary operator's seat on days with a high proportion of lifts close to the crane. When working in the test seat, none of the subjects gave an estimate higher than 0·5 (discomfort equivalent to very, very weak). An adjustable operator's seat could be a good alternative to a fixed seat, and more tests would be desirable.     

Using a pneumatic support to correct sitting posture for prolonged periods: A study using airline seats

Prolonged sitting with spine flexion has been linked to low back disorders. A variety of mechanisms account for this based on biomechanical and neurological variables. Airline seats typically cause pronounced lumbar flexion due to their hollowed seat back design. A pneumatic support, placed between the seat back and the lumbar spine, was tested to see if lumbar flexion was reduced. Results showed that when the seats were positioned in the upright position, 15 of 20 participants experienced reduced lumbar flexion (by 15° on average) with the support. The study was repeated on the five non-responders with the seatback set in the reclined position. This resulted in another four experiencing less lumbar flexion. Since seated flexion is associated with disc stress, reducing flexion with the support reduced lumbar stress. Spine flexion that results from prolonged sitting is associated with disc stress and pain. The pneumatic support tested here reduced spine flexion. While it is not known why airline seats are designed with no lumbar support, which causes excessive lumbar flexion while seated, the pneumatic support corrected this deficit. Reclining the seatback enhanced this effect.     

Application of a Damped Spring-Mass Human Vibration Simulator in Vibration Testing of Vehicle Seats

Abstract

There is a need for standardized methods for testing vehicle seats. Such methods would allow for the direct comparison of the merits of seats of diverse types and designs. At the present time standardized testing procedures are not possible because the dynamic characteristics of the human subjects occupying the seat during testing vary widely from man to man and affect the seat response. Testing with dead weight loading does not accurately portray the dynamic characteristics of the seat. A damped spring-mass system closely approximating to the dynamic characteristics of a seated man to vertical modes of vibration has been developed as the basis on which a standardized vehicle seat testing procedure can be built. Analysis of the problem by means of mechanical impedance techniques indicated that a two-degree-of-freedom system was sufficient to simulate the major dynamic characteristics of man in the frequencies below 10 Hz where seat vibration is most severe. Test procedures have been developed which utilize the man-simulator as the seat load. Results of these tests will be useful because they do not contain the subject variable and therefore can be compared directly with results obtained in other laboratories using the same methods.     

Dynamic leg volume changes when sitting in a locked and free floating tilt oé ce chair

It is well established that prolonged sitting may lead to swelling of the lower extremities. However, activation of the vein pump system by repeated walking breaks or dynamic tiltable foot-rests have been shown to reduce foot and leg oedema. Some oé ce chairs incorporate tilt mechanisms facilitating movements of the body from the feet up. The present study was undertaken to establish whether a beneficial effect on the transcapillary fluid balance of the legs by enabling such mechanisms could be documented. An oé ce chair where the tilt mechanism could be locked or open was used for the study. The seat position and seat activity level was recorded by a transducer system developed for the study. Calf volume and calf muscle pump activity was detected by mercury strain gauge plethysmography. In the locked position there was a steady increase in volume of mean=1.2% (range=0.8–1.8%) for all participants in the 30-min study period. On the other hand, for all participants there was a decrease in calf volume (mean=0.7%, range=0.1–1.2%, p=0.008) when the tilt mechanism was open (30-min period), irrespective of what study period came first. The study showed that upward seat deflection was not associated with concomitant venous obstruction, since such obstruction was detected in less than 2% of the time period with more than 50% of maximal upward deflection. A locked seat mechanism does not prevent activation of the vein pump mechanism, but the study indicates that oé ce chairs that permit variation in seat angle per se stimulate movements of the leg. This, in turn, activates vein pumps and counteracts local oedema formation in seated working postures.     

Dynamic leg volume changes when sitting in a locked and free floating tilt office chair

It is well established that prolonged sitting may lead to swelling of the lower extremities. However, activation of the vein pump system by repeated walking breaks or dynamic tiltable foot-rests have been shown to reduce foot and leg oedema. Some office chairs incorporate tilt mechanisms facilitating movements of the body from the feet up. The present study was undertaken to establish whether a beneficial effect on the transcapillary fluid balance of the legs by enabling such mechanisms could be documented. An office chair where the tilt mechanism could be locked or open was used for the study. The seat position and seat activity level was recorded by a transducer system developed for the study. Calf volume and calf muscle pump activity was detected by mercury strain gauge plethysmography. In the locked position there was a steady increase in volume of mean = 1.2% (range = 0.8-1.8%) for all participants in the 30-min study period. On the other hand, for all participants there was a decrease in calf volume (mean = 0.7%, range = 0.1-1.2%, p = 0.008) when the tilt mechanism was open (30-min period), irrespective of what study period came first. The study showed that upward seat deflection was not associated with concomitant venous obstruction, since such obstruction was detected in less than 2% of the time period with more than 50% of maximal upward deflection. A locked seat mechanism does not prevent activation of the vein pump mechanism, but the study indicates that office chairs that permit variation in seat angle per se stimulate movements of the leg. This, in turn, activates vein pumps and counteracts local oedema formation in seated working postures.     

Technical note: spine loading in automotive seating

For car manufacturers, seat comfort is becoming more important in distinguishing themselves from their competitors. Therefore, many studies on participative seat comfort are carried out. In this paper, an objective assessment approach is reported which evaluates the concept of "optimal load distribution", based on the identification of a close relationship between the pressure on the seat and the discomfort felt by the person sitting. An in vivo measurement of the pressure in the spinal disc, which is an indicator of the load in the spine, was performed. For this research, a pressure sensor was implanted with a canula in the middle of the disc intervertebralis of a participant. The local pressure on the disc was established for the participant in an automobile seat set in various seat positions. The results indicate that in the seat position with the pressure distribution corresponding to the most comfortable posture the pressure in the intervertebral disc is lowest. The pressure in this position is 0.5 bar, while in the upright seated position the pressure is 1.6 bar.     

Effect of time of day on the vertical spinal creep response

Vertical spinal creep (VSC) is height loss during sustained postures over a set period of time. While total stature has been demonstrated to decrease throughout the day, whether a diurnal effect applies to the VSC response has not been reported. The aim of this study was to investigate whether time of day had an effect on the magnitude of the VSC response in young subjects asymptomatic for musculoskeletal pathologies.The VSC response was recorded over 25 min while subjects remained seated on the stadiometer, at three times (morning, midday and afternoon) on the same day, in 48 asymptomatic volunteers aged 20-39 years.While no significant differences were calculated for the magnitude of the VSC responses on the three occasions of testing, differences in magnitude of VSC response may have been confounded by preloading activities.Where magnitude of the VSC response is the primary outcome measure, measurement protocols should control preloading activities and continue to conduct measurements at the same time of day, until further studies conclusively refute the existence of a diurnal effect.     

Vibration in operating heavy haul trucks in overburden mining

The objective of this study was to determine if the vibration in sagittal (x), coronal (y) and vertical (z) axes of the seat pan of the heavy haul trucks used in overburden mining, and the vibration experienced by the drivers at the third lumbar and seventh cervical vertebral levels in operating these trucks exceeded the ISO standards, thereby posing threat to safety. A new and an old truck of two different makes and different carrying capacities (200 and 300 series) were instrumented with a triaxial accelerometer on the seat pan. Fourteen drivers (8 male and 6 female) were instrumented with a triaxial accelerometer at C7 and L3 spinous processes. The vibration at the seat pan, C7, and L3 levels were recorded using an onboard computer with PCMCIA card and, compared against the ISO standards. The vertical vibration of the seat pan in the entire sample ranged from a low of .37 m/s² to a high of 11.73 m/s². The vibration at the lumbar level in the sample ranged between .34 and 2.72 m/s². The values for cervical level vertical vibration in the sample ranged between .2 and 2.22 m/s². The gender of the driver, truck make, and it's carrying capacity did not have a significant effect on the vibration. However, the body weight of the driver, the segment of the truck and the site of measurement revealed significant differences in vibration (p<0.001). Out of a total 36 combinations (4 trucks×9 recording locations) the seat pan vertical vibration exceeded the ISO standards 8 times for males and 9 times for females. The lumbar vertebral vertical vibration in males exceeded ISO standards 12 times and in females 11 times. The cervical vertebral vertical vibration exceeded ISO standards once in females only. However, in sagittal and coronal planes the whole body vibration exceeded the ISO standards many times. Heavy haul trucks (240 and 320 ton capacity) frequently generated vibrations in excess of ISO standards in overburden mining operation representing a health hazard.