The influence of seat backrest angle on human performance during whole-body vibration

This study investigated the effects of reclined backrest angles on cognitive and psycho-motor tasks during exposure to vertical whole-body vibration. Twenty participants were each exposed to three test stimuli of vertical vibration: 2-8 Hz; 8-14 Hz and 14-20 Hz, plus a stationary control condition whilst seated on a vibration platform at five backrest angles: 0° (recumbent, supine) to 90° (upright). The vibration magnitude was 2.0 ms(-2) root-mean-square. The participants were seated at one of the backrest angles and exposed to each of the three vibration stimuli while performing a tracking and choice reaction time tasks; then they completed the NASA-TLX workload scales. Apart from 22.5° seat backrest angle for the tracking task, backrest angle did not adversely affect the performance during vibration. However, participants required increased effort to maintain performance during vibration relative to the stationary condition. These results suggest that undertaking tasks in an environment with vibration could increase workload and risk earlier onset of fatigue. PRACTITIONER SUMMARY: Current vibration standards provide guidance for assessing exposures for seated, standing and recumbent positions, but not for semi-recumbent postures. This paper reports new experimental data systematically investigating the effect of backrest angle on human performance. It demonstrates how workload is elevated with whole-body vibration, without getting affected by backrest angle.     

The vibration of inclined backrests: perception and discomfort of vibration applied parallel to the back in the z-axis of the body

This study determined how backrest inclination and the frequency of vibration influence the perception and discomfort of vibration applied parallel to the back (vertical vibration when sitting upright, horizontal vibration when recumbent). Subjects experienced backrest vibration at frequencies in the range 2.5 to 25 Hz at vibration magnitudes up to 24 dB above threshold. Absolute thresholds, equivalent comfort contours, and the principal locations for feeling vibration were determined with four backrest inclinations: 0° (upright), 30°, 60° and 90° (recumbent). With all backrest inclinations, acceleration thresholds and equivalent comfort contours were similar and increased with increasing frequency at 6 dB per octave (i.e. velocity constant). It is concluded that backrest inclination has little effect on the frequency dependence of thresholds and equivalent comfort contours for vibration applied along the back, and that the W (d) frequency weighting in current standards is appropriate for evaluating z-axis vibration of the back at all backrest inclinations. STATEMENT OF RELEVANCE: To minimise the vibration discomfort of seated people, it is necessary to understand how discomfort varies with backrest inclination. It is concluded that the vibration on backrests can be measured using a pad between the backrest and the back, so that it reclines with the backrest, and the measured vibration evaluated without correcting for the backrest inclination.     

The vibration of inclined backrests: perception and discomfort of vibration applied parallel to the back in the z-axis of the body

This study determined how backrest inclination and the frequency of vibration influence the perception and discomfort of vibration applied parallel to the back (vertical vibration when sitting upright, horizontal vibration when recumbent). Subjects experienced backrest vibration at frequencies in the range 2.5 to 25 Hz at vibration magnitudes up to 24 dB above threshold. Absolute thresholds, equivalent comfort contours, and the principal locations for feeling vibration were determined with four backrest inclinations: 0° (upright), 30°, 60° and 90° (recumbent). With all backrest inclinations, acceleration thresholds and equivalent comfort contours were similar and increased with increasing frequency at 6 dB per octave (i.e. velocity constant). It is concluded that backrest inclination has little effect on the frequency dependence of thresholds and equivalent comfort contours for vibration applied along the back, and that the W _d frequency weighting in current standards is appropriate for evaluating z-axis vibration of the back at all backrest inclinations.

Statement of Relevance: To minimise the vibration discomfort of seated people, it is necessary to understand how discomfort varies with backrest inclination. It is concluded that the vibration on backrests can be measured using a pad between the backrest and the back, so that it reclines with the backrest, and the measured vibration evaluated without correcting for the backrest inclination.     

Equivalent comfort contours for vertical seat vibration: effect of vibration magnitude and backrest inclination

This study determined how backrest inclination and the frequency and magnitude of vertical seat vibration influence vibration discomfort. Subjects experienced vertical seat vibration at frequencies in the range 2.5-25 Hz at vibration magnitudes in the range 0.016-2.0 ms(-2) r.m.s. Equivalent comfort contours were determined with five backrest conditions: no backrest, and with a stationary backrest inclined at 0° (upright), 30°, 60° and 90°. Within all conditions, the frequency of greatest sensitivity to acceleration decreased with increasing vibration magnitude. Compared to an upright backrest, around the main resonance of the body, the vibration magnitudes required to cause similar discomfort were 100% greater with 60° and 90° backrest inclinations and 50% greater with a 30° backrest inclination. It is concluded that no single frequency weighting provides an accurate prediction of the discomfort caused by vertical seat vibration at all magnitudes and with all backrest conditions. PRACTITIONER SUMMARY: Vertical seat vibration is a main cause of vibration discomfort for drivers and passengers of road vehicles. A frequency weighting has been standardised for the evaluation of vertical seat vibration when sitting upright but it was not known whether this weighting is suitable for the reclined sitting postures often adopted during travel.     

Objective and subjective responses of seated subjects while reading Hindi newspaper under multi axis whole-body vibration

Train passengers often read newspapers while traveling. Vibration is one of the key factors that may occasionally inhibit this activity. An experimental study was, therefore, conducted to investigate the extent of interference perceived in reading task by seated subjects in two postures under random vibration. 30 healthy male subjects were exposed to vibration magnitudes of 0.4, 0.8 and 1.2 m/s² in mono, dual and multi axis in the low frequency range 1–20 Hz. The task required subjects to read a given paragraph of Hindi national newspaper, in two seated postures (lap posture with backrest support and table posture with leaning over the table). The reading performance was evaluated by both degradation in performance in terms of time required to complete the task and subjective rating using Borg CR10 scale. Both the methods of reading performance evaluation exhibit progressive increase with an increase in vibration magnitude for both the subject postures in all the direction of vibration and are found to be higher in lateral and vertical direction among mono axes. The effects of multi axis vibration on perceived difficulty have been found to be similar to dual axes vibration and greater than mono axes vibration; however degradation in reading performance in multi axis vibration was also found to be similar to that for lateral direction. A comparison of the effect of postures by both evaluation methods revealed that the reading performance was adversely affected for table posture in all direction of vibration, however for lap posture, only the X-axis vibration effect was more severe.

Relevance to industry

Available ride comfort standards for vehicles do not include the effects of vibrations on passenger activities. Assessment of activity discomfort would be useful for vehicle design optimization to facilitate activity comfort.     

Subjective response of standing persons exposed to fore-aft,lateral and vertical whole-body vibration

The aims of this study were to propose multiply scale factors for evaluation of discomfort of standing persons and to investigate whether there exist differences between multiplying factors used for evaluation of discomfort of standing persons and those of seated persons exposed to WBV. Twelve male subjects were exposed to twenty-seven stimuli that comprise three acceleration magnitudes (0.2, 0.4, and 0.8 m/s² r.m.s.) along fore-aft (x), lateral (y) or vertical (z) direction. The subjects with seated or standing posture on the platform of the vibration test rig rated the subjective discomfort for each stimulus that has frequency contents ranging from 1.0 Hz to 20 Hz with a constant power spectrum density. The order of presentation of the test stimuli was fully randomized and each stimulus was repeated three times. The subjective scale for discomfort was calculated by using the category judgment method. The best combinations of multiplying factors were determined by calculating correlation coefficients of regression curves in-between subjective ratings and vibration magnitudes. In all the directions, body posture significantly influenced on subjective discomfort scales. Particularly in the fore-aft and lateral direction, the upper limit of all the categories for the standing posture resulted in higher vibration acceleration magnitudes than those for the seated posture. In contrast, in the vertical direction, only the upper limit of category “1: Not uncomfortable” for standing posture was observed to be higher than that for seated posture. The best agreement for ISO-weighted vibration acceleration occurred at x factor of 1.8 and y factor of 1.8 in the standing posture and x factor of 2.8 and y factor of 1.8 in the seated posture. The results suggest that seated people respond more sensitively and severely in perception of discomfort to fore-aft and lateral vibration than standing people do while standing people respond more sensitively and severely to vertical vibration than seated people do. Thus the effects of body postures on multiplying factors should be considered in evaluation of discomfort caused by whole-body vibration.Relevance to industryThis study reports differences in subjective response of standing persons to fore-aft, lateral and vertical whole-body vibration. The results obtained in this study propose the fundamental data on the sensitivity to whole-body vibration exposed with standing posture.     

Apparent mass of small children: experimental measurements

A test facility and protocol were developed for measuring the seated, vertical, whole-body vibration response of small children of less than 18 kg in mass over the frequency range from 1 to 45 Hz. The facility and protocol adhered to the human vibration testing guidelines of BS7085 and to current codes of ethics for research involving children. Additional procedures were also developed which are not currently defined in the guidelines, including the integral involvement of the parents and steps taken to maximize child happiness. Eight children were tested at amplitudes of 0.8 and 1.2 m/s(2) using band-limited, Gaussian, white noise acceleration signals defined over the frequency interval from 1 to 50 Hz. Driving point apparent mass modulus and phase curves were determined for all eight children at both test amplitudes. All results presented a single, principal, anti-resonance, and were similar to data reported for primates and for adult humans seated in an automotive posture which provided backrest support. The mean frequency of the apparent mass peak was 6.25 Hz for the small children, as compared to values between 6.5 - 8.5 Hz for small primates and values between 6.5 - 8.6 Hz for adults seated with backrest support. The peak value of the mean, normalized, apparent mass was 1.54 for the children, which compares to values from 1.19 to 1.45 reported in the literature for small primates and 1.28 for adults seated with backrest support. ISO standard 5982, which specifies a mean, normalized, apparent mass modulus peak of 1.50 at a frequency of 4.0 Hz for adults seated without backrest support, provides significant differences.     

Apparent mass of small children: experimental measurements

A test facility and protocol were developed for measuring the seated, vertical, whole-body vibration response of small children of less than 18 kg in mass over the frequency range from 1 to 45 Hz. The facility and protocol adhered to the human vibration testing guidelines of BS7085 and to current codes of ethics for research involving children. Additional procedures were also developed which are not currently defined in the guidelines, including the integral involvement of the parents and steps taken to maximize child happiness. Eight children were tested at amplitudes of 0.8 and 1.2 m/s² using band-limited, Gaussian, white noise acceleration signals defined over the frequency interval from 1 to 50 Hz. Driving point apparent mass modulus and phase curves were determined for all eight children at both test amplitudes. All results presented a single, principal, anti-resonance, and were similar to data reported for primates and for adult humans seated in an automotive posture which provided backrest support. The mean frequency of the apparent mass peak was 6.25 Hz for the small children, as compared to values between 6.5 – 8.5 Hz for small primates and values between 6.5 – 8.6 Hz for adults seated with backrest support. The peak value of the mean, normalized, apparent mass was 1.54 for the children, which compares to values from 1.19 to 1.45 reported in the literature for small primates and 1.28 for adults seated with backrest support. ISO standard 5982, which specifies a mean, normalized, apparent mass modulus peak of 1.50 at a frequency of 4.0 Hz for adults seated without backrest support, provides significant differences.     

Predictive discomfort of non-neutral head-neck postures in fore-aft whole-body vibration

It seems obvious that human head-neck posture in whole-body vibration (WBV) contributes to discomfort and injury risk. While current mechanical measures such as transmissibility have shown good correlation with the subjective-reported discomfort, they showed difficulties in predicting discomfort for non-neutral postures. A new biomechanically based methodology is introduced in this work to predict discomfort due to non-neutral head-neck postures. Altogether, 10 seated subjects with four head-neck postures--neutral, head-up, head-down and head-to-side--were subjected to WBV in the fore-aft direction using discrete sinusoidal frequencies of 2, 3, 4, 5, 6, 7 and 8 Hz and their subjective responses were recorded using the Borg CR-10 scale. All vibrations were run at constant acceleration of 0.8 m/s2 and 1.15 m/s2. The results have shown that the subjective-reported discomfort increases with head-down and decreases with head-up and head-to-side postures. The proposed predictive discomfort has closely followed the reported discomfort measures for all postures and rides under investigation. STATEMENT OF RELEVANCE: Many occupational studies have shown strong relevance between non-neutral postures, discomfort and injury risk in WBV. With advances in computer human modelling, the proposed predictive discomfort may provide efficient ways for developing reliable biodynamic models. It may also be used to assess discomfort and modify designs inside moving vehicles.     

Determination of reference values of apparent mass responses of seated occupants of different body masses under vertical vibration with and without a back support

The biodynamic response of human body seated without a back support and exposed to vertical whole-body vibration have been standardized in ISO 5982 and DIN 45676 in terms of driving-point mechanical impedance and apparent mass. A comparison of ranges defined in two standards, however, reveal considerable differences in both the magnitude and phase. Greater differences are more evident for the three body mass groups, which suggests the lack of adequate reference values of biodynamic responses of seated human subjects of different body masses. In this experimental study, the biodynamic responses of seated humans within three different body mass ranges are characterized under different magnitudes of vibration and three different sitting postures in an attempt to define reference values of apparent mass for applications in mechanical-equivalent model development and anthropodynamic manikin design. Laboratory measurements were performed with adult male subjects of total body mass in the vicinity of 55, 75 and 98 kg (nine subjects for each mass group) seated with and without an inclined back support and exposed to three different magnitudes of white-noise vertical vibration (0.5, 1.0 and 2.0 m/s² unweighted rms acceleration) in the frequency range between 0.5 and 20 Hz. The measured data were analyzed to derive the mean magnitude and phase responses for the three body masses, posture and excitation conditions. The mean magnitude responses of subjects within three mass groups were compared with idealized ranges defined in ISO 5982 and mean values described in DIN 45676 for no back support condition. The results revealed significant differences between the mean measured and standardized magnitudes, suggesting that the current standardized values do not describe the biodynamic responses of seated occupant of different masses even for the back not supported condition. The mean measured responses revealed most important effect of body mass, irrespective of the sitting posture. The reference values of apparent mass responses of seated body subject to vertical whole-body vibration are thus defined for three mass groups and different back support conditions that may be considered applicable for ranges of excitations considered. The responses of the body seated without a back support, also revealed notable influences of excitation magnitude, particularly on the primary peak frequencies.     

Vibration and comfort HI. Translational vibration of the feet and back

The effects on discomfort of the frequency and direction of the translational vibration of a footrest and flat firm backrest have been studied in two experiments. At frequencies in the range 2.5-63 Hz, the first experiment determined the levels of fore-and-aft, lateral and vertical vibration of the feet of seated subjects which caused them discomfort equivalent to that from 0.8 m/s² r.m.s. 10 Hz vertical vibration of a firm flat seat. The levels of fore-and-aft, lateral and vertical vibration at the back of a seat which were equivalent to 0.8 m/s² r.m.s. 10 Hz vertical seat vibration were determined in the second experiment. The vibration of the feet or back occurred without simultaneous vibration at the seat.

Individual and group equivalent comfort contours are presented. It is concluded that the data provide a useful initial indication of the relative contribution of foot and back vibration to discomfort. Equivalent comfort contours for foot vibration were similar for all three directions of vibration. The contours for vibration of the back show a high sensitivity to fore-and-aft vibration. The results obtained from two additional studies show that vibration from a backrest and other variations in seating conditions can influence subject comfort.     

Inter-individual postural variability in seated drivers exposed to whole-body vibration

Long-term occupational exposure to whole-body vibration (WBV) is a cause of low back pain for seated drivers. Poor and long-term seated postures are considered as a cofactor in the risk. It depends on the vehicle's ergonomics and tasks. Differences in posture may also be observed between operators doing identical tasks. An experiment has been performed in order to simultaneously measure posture and WBV for 12 drivers in 3 vehicles (loader, dumper and excavator) during controlled tasks. The inter-individual postural variability has been evaluated. The positions and movements of the body were measured with the CUELA system (computer-assisted recording and long-term analysis of musculoskeletal loads). Significant differences were observed between the three vehicles in the WBV, positions and movements of the body. Significant postural differences were observed between drivers (EN 1005-4 2005). Individual strategies for performing a task were also identified.     

Effect of the seating condition on the transmission of vibration through the seat pan and backrest

Changes in the seating condition may change the body posture which could affect the transmission of vibration through a vehicle seat. This study investigates the effect of different seating conditions on the transmission of vibration through a car seat. Ten male subjects sat on the passenger seat of a sedan car driven at 60 km/h adopting one of six conditions at a time. The VDV was measured on the seat and backrest. Backrest contact affected the VDV measured on the seat pan in the z- and y-axis only. Increasing the backrest angle increased the VDV at the backrest in the x-direction and reduced the VDV at the backrest in the z-direction. With the increase in the backrest angle, the total VDV at the backrest became higher than the total VDV on the seat pan. The study showed no effect of foot position and contact with a headrest on the VDVs.Relevance to industryThis research presents the effect of the seating condition on the transmission of vibration through the seat pan and backrest of a car seat. Research of this kind may help seat manufacturers recommend seating conditions that reduce discomfort caused by whole-body vibration.     

A parametric investigation on seat/occupant contact forces and their relationship with initially perceived discomfort using a configurable seat

The present work investigates the contact forces between sitters and seat as well as their correlations with perceived discomfort. Twelve different economy class aeroplane seat configurations were simulated using a multi-adjustable experimental seat by varying seat pan and backrest angles, as well as seat pan compressed surface. Eighteen males and 18 females, selected by their body mass index and stature, tested these configurations for two sitting postures. Perceived discomfort was significantly affected by seat parameters and posture and correlated both with normal force distribution on the seat-pan surface and with normal forces at the lumbar and head supports. Lower discomfort ratings were obtained for more evenly distributed normal forces on the seat pan. Shear force at the seat pan surface was at its lowest when sitters were allowed to self-select their seat-pan angle, supporting that a shear force should be reduced but not zeroed to improve seating comfort.

Practitioner Summary: The effects of seat-pan and backrest angle, anthropometric dimensions and sitting posture on contact forces and perceived discomfort were investigated using a multi-adjustable experimental seat. In addition to preferred seat profile parameters, the present work provides quantitative guidelines on contact force requirement for improving seating comfort.     

The effect of posture and seat suspension design on discomfort and back muscle fatigue during simulated truck driving

Several studies have shown a relationship between low-back problems and exposure to seated whole-body vibration. The amount of vibration transmitted to the operator is influenced by the posture of the subject in the vehicle. The aim of this study was to determine whether a truck seat with a gas spring in its suspension is superior to the standard spring seat in slowing the onset of muscle fatigue and reducing the level of discomfort experienced during road vibrations while maintaining typical driving postures. The experiment used a 2 x 3 (2 seats x 3 postures) repeated measures design. It was conducted on six males free from low-back pain. Subject comfort was rated before and directly after exposure to typical vibrations. Muscle fatigue using centre frequency was determined during vibration exposure, and the magnitude and phase of acceleration transfer were calculated from the base plate to the seat pan and from the seat pan to the bite bar. None of comfort, fatigue rate or fatigue average were affected by seat type or seat suspension design in the short term, 10 min vibration exposure. Fatigue and comfort measures could continue to be used to detect postural defects, but the more sensitive measures of seat/driver interactions remain mechanical ones using motion-measuring techniques such as accelerometry and correcting for the heavily damped nature of the system. Until more sophisticated manikins are available the characteristics of vibration-attenuating seats should be confirmed using live humans.     

Whole-body vertical biodynamic response characteristics of the seated vehicle driver: Measurement and model development

The vertical driving-point mechanical impedance characteristics applicable to seated vehicle drivers are measured in the 0.625–10 Hz frequency range with excitation amplitudes ranging from 1.0 to 2.0 m s⁻² using a whole-body vehicular vibration simulator. The measurements are performed for seated subjects with feet supported and hands held in a driving position. Variations in the seated posture, backrest angle, and nature and amplitude of the vibration excitation are introduced within a prescribed range of likely conditions to illustrate their influence on the driving-point mechanical impedance of seated vehicle drivers. Within the 0.75–10 Hz frequency range and for excitation amplitudes maintained below 4 m s⁻², a four-degree-of-freedom linear driver model is proposed for which the parameters are estimated to satisfy both the measured driving-point mechanical impedance and the seat-to-head transmissibility characteristics defined from a synthesis of published data for subjects seated erect without backrest support. The parameter identification technique involves the solution of a multivariable optimization function comprising the sum of squared magnitude and phase errors associated with both the mechanical impedance and seat-to-head transmissibility target values, subject to limit constraints identified from the anthropometric and biomechanical data. The model response, however, is found to provide a closer agreement with the mechanical impedance target values than that with the seat-to-head transmissibility. From the model, the main body resonant frequencies computed on the basis of both biodynamic response functions are found to be within close bounds to that expected for the human body.

Relevance to industry

The development of an appropriate analytical seated vehicle driver model should provide means of estimating the forces and motions being transmitted within the body under specific vehicular vibration environments. Furthermore, its use in conjunction with a corresponding model for the vehicle seat should allow the prediction of the driver's vibration exposure levels and the seat's ability to attenuate the vibration in particular vehicles.     

Effects of differences in office chair controls, seat and backrest angle design in relation to tasks

In this study the influence of chair characteristics on comfort, discomfort, adjustment time and seat interface pressure is investigated during VDU and non-VDU tasks: The two investigated office chairs, both designed according to European and Dutch standards are different regarding: 1) seat cushioning and shape, 2) backrest angle and 3) controls. Thirty subjects in total, both male and female, participated in two experiments: twenty in the first and ten in the second.Significant differences are found for ease of adjustment and adjustment time of controls, independent of the tasks. Related to tasks, a significant difference was found for the backrest range of motion. For non-VDU tasks a larger range of backrest motion was preferred by 70% of the subjects. The chair design differences were most clear for comfort and adjustment time of controls, followed by comfort of backrest angle. No differences are found between seat pan comfort and discomfort, first impressions and peak interface pressure.     

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.     

Characterization of posture and comfort in laptop users in non-desk settings

Laptop computers may be used in a variety of postures not coupled to the office workstation. Using passive motion analysis, this study examined mean joint angles during a short typing/editing task in college students (n=20), in up to seven positions. Comfort was assessed after task execution through a body map. For three required postures, joint angles in a prone posture were different than those while seated at a couch with feet either on floor or on ottoman. Specifically, the prone posture was characterized by comparatively non-neutral shoulders, elbows and wrists, and pronounced neck extension. Significantly greater intensity and more regions of discomfort were marked for the prone posture than for the seated postures. It is recommended that the prone posture only be assumed briefly during laptop use. Exposure to laptops outside of the office setting should be assessed in future epidemiologic studies of musculoskeletal complaints and computer use.     

Subjective response to whole-body vibration The effects of posture

In an investigation of the effects of posture on subjective responses to whole-body vibration, 20 undergraduate subjects produced equal sensation contours adopting three postures each on different occasions. The postures adopted were standing, sitting upright and sitting slouched.

The results indicated significant differences in the contour shapes from the three postures, and the level set in the sitting postures were significantly lower than in the standing posture. No difference was obtained between the two sitting postures.

Implications of these findings are discussed regarding the role of transmissibility in subjective response to vibration, and the necessity to produce different standards for different postures.