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

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

This article was published in the journal, Ergonomics [© Taylor & Francis Ltd.] and the definitive version is available at: http://dx.doi.org/10.1080/00140139.2012.684889National 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

Use of seating to control exposures to whole-body vibration

In 100 vehicles, 67 conventional seats and 33 suspension seats were tested to determine the benefits that might be obtained by changing seats in the vehicles. Acceleration was measured on the floor and on the seat of 14 categories of vehicles (cars, vans, lift trucks, lorries, tractors, buses, dumpers, excavators, helicopters, armoured vehicles, mobile cranes, grass rollers, mowers and milk floats). Seat Transmissibilities and SEAT values were determined for all seats. This report and the work it describes were funded by the Health and safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy

The transmission of translational seat vibration to the head-I. Vertical seat vibration

Vibration in the three translational (fore-and-aft, lateral and vertical) and the three rotational (roll, pitch and yaw) axes of the head has been measured during exposure to whole-body random vibration. Using an instrumented bar gripped between the teeth, the influence of variations in bite grip and bite-bar mass on movements of the head were found to be small up to a mass of 375 g. The repeatability of measures of seat-to-head transmissibility within a single subject and the variability in transmissibility across a group of twelve subjects have been determined with two seating conditions: a rigid seat with a backrest and the same seat with no backrest. Seat-to-head transmissibilities associated with vertical seat vibration are presented at frequencies up to 25 Hz for all six axes of head vibration both with and without a backrest. Head motion occurred principally in the fore-and-aft, vertical and pitch axes of the head. The backrest increased the magnitude of head vibration in most cases. Intra-subject variability was generally small compared to inter-subject variability.</p

Evaluation of whole-body vibration in vehicles (in special Issue on the 2nd International Conference on Whole-body Vibration Injuries)

The vibration in 100 different vehicles has been measured, evaluated and assessed according to British Standard BS 6841 (1987) and International Standard ISO 2631 (1997). Vibration was measured in 14 categories of vehicle including cars, lift trucks, tractors, lorries, vans and buses. In each vehicle, the vibration was measured in five axes: vertical vibration beneath the seat, fore-and-aft, lateral and vertical vibration on the seat pan and fore-and-aft vibration at the backrest. The alternative methods of evaluating the vibration (use of different frequency weightings, different averaging methods, the inclusion of different axes, vibration dose values and equivalent r.m.s. acceleration) as defined in the standards have been compared. BS 6841 (1987) suggests that an equivalent acceleration magnitude is calculated using vibration measured at four locations around the seat (x -, y -, z -seat and x -backrest); ISO 2631 (1997) suggests that vibration is measured in the three translational axes only on the seat pan but only the axis with the most severe vibration is used to assess vibration severity. Assessments made using the procedure defined in ISO 2631 tend to underestimate any risks from exposure to whole-body vibration compared to an evaluation made using the guidelines specified in BS 6841; the measurements indicated that the 17 m/s1.75 "health guidance caution zone" in ISO 2631 was less likely to be exceeded than the 15 m/s1.75 "action level" in BS 6841. Consequently, ISO 2631 "allows" appreciably longer daily exposures to whole-body vibration than BS 6841

Transmission of yaw seat vibration to the head

The transmission of yaw-axis vibration to the heads of seated subjects has been investigated at frequencies below 5 Hz. The variability between and within subjects and the effects of backrest contact, visual environment and the position of the centre of rotation have been investigated. The subjects sat on a rigid flat seat and were exposed to random motion at a magnitude of 1·0 rad/s2r.m.s. (root-mean-square) for 2 min. Head motion was measured in six axes using a light-weight bite-bar held between the teeth. Twelve male subjects participated in a study of the effect of backrest contact and visual conditions and one male subject participated in a repeatability study. A "back-on" posture (subject's back in contact with the seat backrest) increased the frequency of maximum transmissibility from 2 to 3 Hz compared with a "back-off" posture. There was little change in transmissibility with the subjects sitting with their eyes open compared to their eyes closed. With increasing separations between a subject and the centre of rotation (at six distances from 0 to 500 mm with the subject facing outwards) there were large increases in lateral acceleration at the head

Effect of seating on exposures to whole-body vibration in work-vehicles

The vibration isolation efficiency of seating has been evaluated in 100 work vehicles in 14 categories (cars, vans, lift trucks, lorries, tractors, buses, dumpers, excavators, helicopters, armoured vehicles, mobile cranes, grass rollers, mowers and milk floats). Seat isolation efficiency, expressed by the SEAT value, was determined for all seats (67 conventional seats and 33 suspension seats) from the vertical acceleration measured on the floors and on the seats of the vehicles.For most categories of vehicle, the average SEAT value was less than 100%, indicating that the average seat provided some attenuation of vibration. However, there were large variations in SEAT values between vehicles within categories. Two alternative vibration frequency weightings (Wb from BS 6841, 1987; Wk from ISO 2631, 1997) yielded SEAT values that differed by less than 6%. Overall, the SEAT values determined by two alternative methods (the ratio of r.m.s. values and the ratio of vibration dose values) differed by less than 4·5% when using weighting Wb, although larger differences may be expected in some situations. The median SEAT value for the suspension seats was 84·6%; the median SEAT value for the conventional seats was 86·9% (based on weighting Wb and the ratio of r.m.s. values).Predicted SEAT values were obtained assuming that each seat could be interchanged between vehicles without altering its transmissibility. The calculations suggest that 94% of the vehicles investigated might benefit from changing the current seat to a seat from one of the other vehicles investigated. Although the predictions are based on assumptions that will not always apply, it is concluded that the severity of whole-body vibration exposures in many work environments can be lessened by improvements to seating dynamics

The transmission of translational seat vibration to the head-II. Horizontal seat vibration

The second part of this study of the six axes of head motion caused by translational seat vibration is concerned with the effect of fore-and-aft (x-axis) and lateral (y-axis) seat vibration. Seat-to-head transmissibilities have been determined at frequencies up to 16 Hz for each of the three translational and three rotational axes of the head during exposure to random vibration of the seat. Repeatability measures within a single subject and studies of the variability across a group of twelve subjects have been conducted with two seating conditions: a rigid seat with a backrest, and the same seat with no backrest. Fore-and-aft seat motion mainly resulted in head motion within the mid-sagittal plane (x-z plane). Without the backrest, transmissibilities for the fore-and-aft, vertical and pitch axes of the head were greatest at about 2 Hz. The backrest greatly increased head vibration at frequencies above 4 Hz and caused a second peak in the transmissibility curves at about 6 to 8 Hz. Lateral seat motion mainly caused lateral head motion with a maximum transmissibility at about 2 Hz. The backrest had little effect on the transmission of lateral vibration to the head. For both axes of excitation inter-subject variability was much greater than intra-subject variability.</p