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.     

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.     

Effects of horizontal whole-body vibration and standing posture on activity interference

Standing people are exposed to whole-body vibration in many environments. This paper investigates the effects of horizontal whole-body vibration and standing posture on task performance. Sixteen participants were exposed to random vibration (up to 4 Hz) whilst performing a timed pegboard task in two standing postures. Objective and subjective indicators of performance were used. Time taken to complete the task increased progressively with increases in vibration magnitude. The fore-and-aft posture generally showed greater performance decrements and postural interruptions (>1.0 ms^?2 root mean square) than the lateral. For both postures, performance was better during y-axis vibration than during x-axis vibration. Subjective ratings showed similar trends to time data. Impairments due to dual axis exposure were well predicted using root sum of squares calculations based on single axis components. These results indicate that best performance for those standing in moving environments will be achieved if individuals adopt a lateral posture with the most severe vibration in the y-axis.

Statement of Relevance: People have a need to work during transportation, either working for the transport provider or as a passenger. All modes of transport result in travellers being exposed to horizontal motion. This study demonstrates that task disturbance is affected by the orientation of the standing person to the vibration and, therefore, vehicle layouts can be optimised.     

The influence of seat backrest angle on perceived discomfort during exposure to vertical whole-body vibration

National and International Standards (e.g. BS 6841 and ISO 2631-1) provide methodologies for the measurement and assessment of whole-body vibration in terms of comfort and health. The EU Physical Agents (Vibration) Directive (PAVD) provides criteria by which vibration magnitudes can be assessed. However, these standards only consider upright seated (90°) and recumbent (0°) backrest angles, and do not provide guidance for semi-recumbent postures. This article reports an experimental programme that investigated the effects of backrest angle on comfort during vertical whole-body vibration. The series of experiments showed that a relationship exists between seat backrest angle, whole-body vibration frequency and perceived levels of discomfort. The recumbent position (0°) was the most uncomfortable and the semi-recumbent positions of 67.5° and 45° were the least uncomfortable. A new set of frequency weighting curves are proposed which use the same topology as the existing BS and ISO standards. These curves could be applied to those exposed to whole-body vibration in semi-recumbent postures to augment the existing standardised methods. PRACTITIONER SUMMARY: Current vibration standards provide guidance for assessing exposures for seated, standing and recumbent positions, but not for semi-recumbent postures. This article reports new experimental data systematically investigating the effect of backrest angle on discomfort experienced. It demonstrates that most discomfort is caused in a recumbent posture and that least was caused in a semi-recumbent posture.     

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.     

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.     

Development and design of a dynamic armrest for hydraulic-actuation joystick controlled mobile machines

Standard armrests used in conjunction with joysticks of heavy mobile machinery have been proven to inadequately meet operator needs, resulting in excessive static loading of shoulder musculature. During joystick operation, the trajectory of the user's forearm is governed by the motion of the controller, which creates horizontal and vertical movement of the forearm. The vertical motion of the forearm in the forward and backward motion create postures that stationary armrests cannot support thereby generating increased muscle activation and risk of repetitive strain injuries. The current paper describes the design process used in creating a dynamic armrest that replicates the operator's natural motion trajectories. By incorporating the natural motion paths into a dynamic armrest, the postural requirements and muscular activation of the operator's shoulder may be reduced.     

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.     

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.     

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.     

Vertical vibration frequency and sitting posture effects on muscular loads and body balance

This study examined the effects of whole body vibration and sitting posture on muscular load, body balance and discomfort. An electric vibrator, a surface electromyography measurement system and a balance evaluation system were used in the experiment. Nine test conditions were studied consisting of three vertical frequencies (no vibration, 20 and 40 Hz) and three sitting postures (erect, bent-forward and twisted). Study results showed that whole body vibration had significant effects on the muscular loads in the torso muscles, body balance and perceived discomfort. Adverse effects generally increased with high-frequency vibration. No significant muscular load difference and balance difference were observed among sitting postures at any frequency. Significant discomfort differences between the erect and twisted postures were found with no vibration or at low frequency.Relevance to industryAttention should be paid to the negative effects of vibration transmission on the human body at worksites. The results from this study should be useful for whole body vibration risk assessment and control measures.     

Performance of a complex manual control task during exposure to vertical whole-body vibration between 0·5 and 5·0 Hz

An experiment is described in which seated subjects performed first-order pursuit tracking with a simultaneous discrete task; performance with the discrete task was dependent on performance of the continuous task. Vertical, z-axis, whole-body sinusoidal vibration was presented at frequencies from 0·5 to 5·0Hz at an acceleration magnitude of 2·0 ms^?2 r.m.s. in three separate sessions. In the first session, inter-subject and intra-subject variability masked any disruption caused by the vibration. After further training, all vibration frequencies disrupted performance of the continuous task. Disruption was independent of vibration frequency below 3·15Hz and increased at 4·0 and 5·0Hz. A visual mechanism was assumed to account for the increased disruption at these higher frequencies. Mechanisms which may have been responsible for the disruption below 3·15 Hz are discussed. Effects of vibration on the discrete task were attributable to disruption in performance of the continuous task. The results illustrate the importance of adequately training subjects prior to investigating vibration effects.     

Influence of reading format on reading activity under uniaxial whole body vibration

In a railway vehicle, the vibrations are transmitted to the passengers through the various interfaces such as floor, seat, backrest etc. These vibrations affect the passenger comfort as well as their performance to do any work such as reading, writing, typing etc. In the present work, effects of vibration magnitude, direction of vibration, postures and reading formats have been studied on the reading activity. Thirty healthy male subjects have performed reading task, one at a time. All subjects were exposed to uni-axial whole body vibration in 1–20 Hz frequency range at 0.5, 1 and 1.5 m/s² rms vibration magnitude. The experimental task involved reading a paragraph under the different 54 experimental conditions (three magnitude, three direction, two posture and three reading format). The task performance has been evaluated in terms of time taken by the subjects to read a given paragraph and also the subjective evaluation of perceived difficulty on Borg's CR 10 scale. Perceived difficulty and performance degradation in reading have been found to increase with the increase in vibration magnitude in each direction of vibration. The perceived difficulty and performance degradation in reading have been observed to be higher in the fore-&-aft direction in with-backrest posture. In vertical and lateral vibration, perceived difficulty and performance degradation have been higher in without-backrest posture compare to with-backrest posture. The perceived difficulty and performance degradation have been lower for the triple-column format.     

Effect of alternating postures on cognitive performance for healthy people performing sedentary work

Prolonged sitting is a risk factor for several diseases and the prevalence of worksite-based interventions such as sit-to-stand workstations is increasing. Although their impact on sedentary behaviour has been regularly investigated, the effect of working in alternating body postures on cognitive performance is unclear. To address this uncertainty, 45 students participated in a two-arm, randomised controlled cross-over trial under laboratory conditions. Subjects executed validated cognitive tests (working speed, reaction time, concentration performance) either in sitting or alternating working postures on two separate days (ClinicalTrials.gov Identifier: NCT02863731). MANOVA results showed no significant difference in cognitive performance between trials executed in alternating, standing or sitting postures. Perceived workload did not differ between sitting and alternating days. Repeated measures ANOVA revealed significant learning effects regarding concentration performance and working speed for both days. These results suggest that working posture did not affect cognitive performance in the short term.

Practitioner Summary: Prior reports indicated health-related benefits based on alternated (sit/stand) body postures. Nevertheless, their effect on cognitive performance is unknown. This randomised controlled trial showed that working in alternating body postures did not influence reaction time, concentration performance, working speed or workload perception in the short term.     

Apparent mass distribution at the feet of standing subjects exposed to whole-body vibration

This study was carried out to investigate the influence of the body posture and of the foot support on the apparent mass distribution at the feet of standing subjects exposed to whole-body vibration. The apparent mass was measured at the driving point through a capacitive pressure sensor matrix, which allowed to separate the contributions of the different foot regions. The overall value was also determined using a conventional measurement system based on piezoelectric load cells. Ten male subjects performed 15 tests with three kinds of feet supports (flat rigid, anatomic rigid and flat soft) in five different postures. Static components of the pressure measurements were exploited to identify which fraction of the weight is supported by the rearfoot, the midfoot and the forefoot in the various test configurations. Factorial design of experiments on different response variables showed that the apparent mass is affected by the posture but not by the type of feet contact surface; conversely, the presence of insoles varies with the apparent mass distribution on the different feet parts.

Practitioner Summary: The response of standing subjects to whole-body vibration has always been considered as a global parameter measured at the driving point, neglecting the local phenomena occurring in different foot parts. We have experimentally identified the apparent mass distribution of subjects in different standing postures and with different foot supports.     

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.     

Lift performance and lumbar loading in standing and seated lifts

This study investigated the effect of posture on lifting performance. Twenty-three male soldiers lifted a loaded box onto a platform in standing and seated postures to determine their maximum lift capacity and maximum acceptable lift. Lift performance, trunk kinematics, lumbar loads, anthropometric and strength data were recorded. There was a significant main effect for lift effort but not for posture or the interaction. Effect sizes showed that lumbar compression forces did not differ between postures at lift initiation (Standing 5566.2?±?627.8 N; Seated 5584.0?±?16.0) but were higher in the standing posture (4045.7?±?408.3 N) when compared with the seated posture (3655.8?±?225.7 N) at lift completion. Anterior shear forces were higher in the standing posture at both lift initiation (Standing 519.4?±?104.4 N; Seated 224.2?±?9.4 N) and completion (Standing 183.3?±?62.5 N; Seated 71.0?±?24.2 N) and may have been a result of increased trunk flexion and a larger horizontal distance of the mass from the L5-S1 joint.

Practitioner Summary: Differences between lift performance and lumbar forces in standing and seated lifts are unclear. Using a with-in subjects repeated measures design, we found no difference in lifted mass or lumbar compression force at lift initiation between standing and seated lifts.     

The effect of posture and vibration magnitude on the vertical vibration transmissibility of tractor suspension system

The efficiency of suspension seat can be influenced by several factors such as the input vibration, the dynamic characteristics of the seat and the dynamic characteristics of the human body. The objective of this paper is to study the effect of sitting postures and vibration magnitude on the vibration transmissibility of a suspension system of an agricultural tractor seat. Eleven (11) healthy male subjects participated in the study. All subjects were asked to sit on the suspension system. Four (4) different sitting postures were investigated – i) “relax”, ii) “slouch”, iii) “tense”, and iv) “with backrest support”. All subjects were exposed to random vertical vibration in the range of 1–20 Hz, at three vibration magnitudes - 0.5, 1.0 and 2.0 m/s² r.m.s for 60 s. The results showed that there were three pronounced peaks in the seat transmissibility, with the primary resonance was found at 1.75–2.5 Hz for every sitting postures. The “backrest” condition had the highest transmissibility resonance (1.46), while the “slouch” posture had the highest Seat Effective Amplitude Transmissibility (SEAT) values (64.7%). Changes in vibration magnitude for “relax” posture from 0.5 to 2.0 m/s² r.m.s resulted in greater reduction in the primary resonance frequency of seat transmissibility. The SEAT values decreased with increased vibration magnitude. It can be suggested that variations in posture and vibration magnitude affected the vibration transmission through the suspension system, indicating the non-linear effect on the interaction between the human body and the suspension system.Relevance to industry: Investigating the posture adopted during agricultural activities, and the effects of various magnitudes of vibration on the suspension system's performance are beneficial to the industry. The findings regarding their influence on the human body may be used to optimize the suspension system's performance.     

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.     

The effects of whole-body restriction on task performance

Many occupations, particularly involving maintenance operations, require individuals to perform both physical tasks and mental tasks in restricted spaces. Researchers have examined physical task performance under various restrictions; however, little research has investigated the effects of restricted space on cognitive tasks. Cognitive task performance in restricted spaces presents cognitive demands (i.e. the task itself) as well as additional physical demands (e.g. awkward postures), which may adversely affect task performance or operator workload. This research focused on the effects of whole-body restrictions on cognitive task performance. An experiment was conducted that examined 9 levels of restriction created in a laboratory: an unrestricted control, 6 single whole-body restrictions at two severity levels (2 lateral, 2 sagittal and 2 vertical) and 2 multiple restrictions (sagittal/vertical, lateral/sagittal/vertical). An inspection task served as the cognitive task. Behavioural, physiological and psychophysical measures were collected and analysed to measure the operator and performance effects. Operator response differences were found among the various forms of restriction as well as the severity level of similar forms of restriction. Increasing restriction significantly affected the behavioural and physiological operator response as opposed to the cognitive response.