Review of anthropometric considerations for tractor seat design

Tractor driving imposes a lot of physical and mental stress upon the operator. If the operator's seat is not comfortable, his work performance may be poor and there is also a possibility of accidents. The optimal design of tractor seat may be achieved by integrating anthropometric data with other technical features of the design. This paper reviews the existing information on the tractor seat design that considers anthropometry and biomechanical factors and gives an approach for seat design based on anthropometric data. The anthropometric dimensions, i.e. popliteal height sitting (5th percentile), hip breadth sitting (95th percentile), buttock popliteal length (5th percentile), interscye breadth (5th and 95th percentile) and sitting acromion height (5th percentile) of agricultural workers need to be taken into consideration for design of seat height, seat pan width, seat pan length, seat backrest width and seat backrest height, respectively, of a tractor. The seat dimensions recommended for tractor operator's comfort based on anthropometric data of 5434 Indian male agricultural workers were as follows: seat height of 380 mm, seat pan width of 420–450 mm, seat backrest width of 380–400 mm (bottom) and 270–290 mm (top), seat pan length of 370±10 mm, seat pan tilt of 5–7° backward and seat backrest height of 350 mm.

Relevance to industry

The approach presented in this paper for tractor seat design based on anthropometric considerations will help the tractor seat designers to develop and introduce seats suiting to the requirements of the user population. This will not only enhance the comfort of the tractor operators but may also help to reduce the occupational health problems of tractor operators.     

Transmission of vertical vibration through a seat: Effect of thickness of foam cushions at the seat pan and the backrest

The perception of vehicle ride comfort is influenced by the dynamic performance of full-depth foam used in many vehicle seats. The effects of the thickness of foam on the dynamic stiffness (i.e., stiffness and damping as a function of frequency) of foam cushions with three thicknesses (60, 80, and 100 mm), and the vibration transmitted through these cushions at the seat pan and the backrest were measured with 12 subjects (6 males and 6 females). With increasing thickness, the stiffness and the damping of the foam decreased. With increasing thickness of foam at the seat pan, the resonance frequencies around 4 Hz in the vertical in-line and fore-and-aft cross-axis transmissibilities of the seat pan cushion and the backrest cushion decreased. For the conditions investigated, it is concluded that the thickness of foam at a vertical backrest has little effect on the vertical in-line or fore-and-aft cross-axis transmissibilities of the foam at either the seat pan or the backrest. The frequencies of the primary resonances around 4 Hz in the vertical in-line transmissibility and the fore-and-aft cross-axis transmissibility of foam at the seat pan were highly correlated. Compared to sitting on a rigid seat pan with a foam backrest, sitting with foam at both the seat pan and the backrest reduced the resonance frequency in the vertical in-line transmissibility of the backrest foam and increased the associated transmissibility at resonance, while the fore-and-aft cross-axis transmissibility of the backrest was little affected. Compared to sitting without a backrest, sitting with a rigid vertical backrest increased the resonance frequency of the fore-and-aft cross-axis transmissibility of the seat pan cushion and increased the transmissibility at resonance.Relevance to industryThe transmissibility of a seat is determined by the dynamic properties of the occupant of the seat and the dynamic properties of the seat. This study shows how the thicknesses of foam at a seat pan and foam at a backrest affect the in-line and cross-axis transmissibilities of the foams at the seat pan and the backrest. The findings have application to the design of vehicle seats to minimise the transmission of vibration to the body.     

Biomechanical model to predict loads on lumbar vertebra of a tractor operator

A biomechanical model is important for prediction of loads likely to arise in specific body parts under various conditions. The biomechanical model was developed to predict compressive and shear loads at L4/L5 (lumbar vertebra) of a tractor operator seating on seats with selected seat pan and backrest cushion materials. A computer program was written to solve the model for various inputs viz. stature and weight of the tractor operators, choice of operating conditions, and reaction forces from seat pan and backrest cushions. It was observed that maximum compressive and shear forces ranged 943–1367 N and 422–991 N, respectively at L4/L5 of tractor operators steering the tractor with leg and hand control actions and occasionally viewing the implement at back. The compressive forces were maximum (1202–1367 N) with coir based composite seat backrest cushion materials (thickness of 80 mm, density of 47.19 kg/m³) and were minimum (943–1108 N) with high density polyurethane foam (thickness of 44 mm, density of 19.09 kg/m³) for the seats.Relevance to industryThe biomechanical model of a tractor operator is important for theoretical understanding the problem of sitting and is also valuable in prediction of compressive and shear loads at L4/L5 of operator under various operating conditions. It will help in design of tractor seat for operator's comfort.     

Adaptation and sensitivity to postural change in sitting

We used 3 psychophysics methods to determine perceptible changes in seat height, seat pan angle, and backrest angle using an experimental chair. In the method of adjustment, the chosen chair settings were affected by the initial setting. For example, a high initial setting of the seat height led to a high selected setting and a low setting led to a low value. The difference between settings was referred to as not noticeable difference (NND). The method of limits was used to determine acceptable chair settings using verbal limits such as "too high" and "too low." Using the method of constant stimuli, just noticeable differences (JNDs) were determined for chair height (1.5 cm), seat pan angle (1.2 degrees) and backrest angle (1.7 degrees). The corresponding values for NNDs and verbal limits were about twice as large: chair height (2.5 cm), seat pan angle (4 degrees) and backrest angle (3 degrees). NNDs and verbal limits are unobtrusive measures that are considered more valid than JNDs, which exaggerate the need for adjustability. The results have practical implications for the design of office chairs.     

Effect of lumbar support on seating comfort predicted by a whole human body-seat model

Automobile seat greatly affects the ride comfort of drivers in a prolonged driving. Not only the layout parameters of automobile seats, such as seat height, cushion inclination angle, backrest inclination angle, etc, but also the backrest surface related with lumbar support all affect the seating comfort. The human body-seat system includes the three-dimensional data of body based on anatomy and anthropometry, three-dimensional data of seat and adjustable assembly interaction between body and seat based on human body kinematics. Body height and driving posture are adjusted in POSER software, then the solid model of human skin, skeleton and muscle are created in ANSA software, and the integrated model of body-seat system is created in ABAQUS software. The adjustment of the lumbar support parameters is achieved by setting boundary condition of lumbar support region of seats. The finite element model of human body-seat system is validated by comparison to available literature results. At last the finite element model is applied to analyze the effect of lumbar support parameters of seats on the interaction between body and seat under the action of gravity. The pressure value and distribution, contact area, total force of backrest and intervertebral disc stress are obtained. The result shows that the optimal thickness of seat's lumbar support size for the seating comfort is 10 mm after comprehensive comparison and evaluation.Relevance to industry: This study investigated the effects of lumbar support on seating comfort, and can be used to protect the lumbar health. The modeling and simulation method can be applied for the optimization design of vehicle seats.     

Human–seat interface analysis of upper and lower body weight distribution

The human–seat interfaces were analyzed to determine the differential distribution of the body weight to the components of seat. Fifteen volunteers were tested on a simulated seat system with two piezoelectric force platforms, one placed as chair seat pan and the other placed on the floor surface as footrest. The seated configurations included back inclines (75° and 80°), upright (90°) and reclines (95°, 105° and 115°), absence or presence of armrest (adjusted at 62–68 cm of height), forward and backward sloping of the seat pan, and supported and unsupported back. The armrest and backrest assemblies were isolated from the force platforms. The difference in the body weight (kgf) to the sum of forces recorded at seat pan and feet yielded the extent of weight transferred to other features (e.g., backrest and armrest). The weight distributed at seat was 10–12% less at back inclines (p<0.01) as compared to upright unsupported sitting. With the backrest reclined beyond 95°, the weight at seat gradually decreased by 9% at 115° recline. The load distributed at feet varied narrowly; however, it was significantly greater (p<0.01) at upright supported back, compared to unsupported back. The height of the armrest was optimized at 68 cm, since the weight distribution at seat pan consistently reduced by 12% at that height, as compared to the absence of armrest (F_(4,524)=8.80, p<0.05). The suggested height of the armrest corresponded to 40% of the body stature of the selected volunteers. The load distributed at feet was 18% greater with the presence of armrest, indicating that a part of the weight of the upper leg fell on the seat pan, when the armrest was absent. The weight fell on the seat in slouch posture was 5% less than in upright sitting, while the weight at feet was marginally higher in slouch than in upright posture. The study maintained that the horizontal as well as 5° forward slope of the seat might be the preferred choice, since the load distributed at seat was highest at backward sloping seat for all conditions of supported and unsupported back. The study reaffirms that the backrest and armrest have conjoint influence in reducing the load distributed at seat, which in turn might help in mitigating stress on the spinal and other paraspinal structures.

Relevance to Industry

The human–seat interface analysis and understanding of body weight distribution to the components of seat may be beneficial for ergo-design application in optimizing parameters for chair configurations that provide comfort and safety to the user.     

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.     

Free shoulder space requirements in the design of high backrests

The objective of this study was to determine the influence of scapular support on the effects of lumbar support and to prove that a high and straight backrest is inappropriate. In literature the importance of a lumbar support is noted, although data about optimal dimensions is an under-researched topic and in earlier studies on force distribution and muscle activity the backrest had a fixed form. The lumbar support is needed to maintain the lumbar lordosis but no studies deal with the question of the precise dimensions of the backrest at shoulder level. With a specially designed apparatus, forces on shoulder and seat were measured separately, and the force on the pelvis calculated, while varying seat and backrest inclination within the range from 0° to 17°. Seat-to-backrest angle (at the level of lumbar support) was kept constant at 90°. The distance between the tangent to the lumbar support and the parallel tangent to the scapular support was varied from 0, 2, 4, 6 and 8 cm. This distance is called the free shoulder space. Electromyography was measured at the erector spinae at the levels of the L1, T8 and T5 vertebrae. For all seat angles, a free shoulder space of d=0 cm resulted in the highest back muscle activity. In agreement with the biomechanical model, EMG activity reduced with an increase of seat tilt and increase of free shoulder space. With increasing free shoulder space, a larger part of the total backrest force was carried by the lumbar support. This study shows that a high and straight backrest overrules lumbar support. Offering free shoulder space of at least 6 cm reduces back muscle activity and allows for lumbar support.     

Free shoulder space requirements in the design of high backrests

The objective of this study was to determine the influence of scapular support on the effects of lumbar support and to prove that a high and straight backrest is inappropriate. In literature the importance of a lumbar support is noted, although data about optimal dimensions is an under-researched topic and in earlier studies on force distribution and muscle activity the backrest had a fixed form. The lumbar support is needed to maintain the lumbar lordosis but no studies deal with the question of the precise dimensions of the backrest at shoulder level. With a specially designed apparatus, forces on shoulder and seat were measured separately, and the force on the pelvis calculated, while varying seat and backrest inclination within the range from 0 degrees to 17 degrees. Seat-to-backrest angle (at the level of lumbar support) was kept constant at 90 degrees. The distance between the tangent to the lumbar support and the parallel tangent to the scapular support was varied from 0, 2, 4, 6 and 8 cm. This distance is called the free shoulder space. Electromyography was measured at the erector spinae at the levels of the L1, T8 and T5 vertebrae. For all seat angles, a free shoulder space of d=0 cm resulted in the highest back muscle activity. In agreement with the biomechanical model, EMG activity reduced with an increase of seat tilt and increase of free shoulder space. With increasing free shoulder space, a larger part of the total backrest force was carried by the lumbar support. This study shows that a high and straight backrest overrules lumbar support. Offering free shoulder space of at least 6 cm reduces back muscle activity and allows for lumbar support.     

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.     

Static three-dimensional modelling of prolonged seated posture.

Prolonged seating can cause musculo-skeletal problems in the long term if poor postures are adopted. A three dimensional static model of the body to calculate the intervertebral disc compression at the fifth lumbar disc was built. SAMMIE, a computer aided ergonomics package was used for modelling a computer operator workstation and determination of joint centre locations and joint angles in a seated posture. Experimentation was also performed to determine the body-mass distribution on the seat used. No significant difference between male and female body-mass distribution was found. The static model found postures with the seat pan and backrest reclined to predict the lowest disc compression. Postures in which the operator was bent forwards gave the highest disc loadings.     

Evaluation of a new work seat for industrial sewing operations: results of three field studies

A newly developed work seat for industrial sewing operations was compared with a traditional sewing work seat to evaluate the effectiveness of design features. The new seat was designed with special seat-pan and backrest features to accommodate the musculoskeletal geometry of a low sit-stand posture. The seat-pan consisted of a pelvic support which supported the ischial tuberosities and areas behind them, and a thigh support which maintained the thighs at a 15 degrees downward angle, resulting in a 105 degrees trunk-thigh angle. The backrest consisted of a lumbar support which preserved lumbar lordosis and a thoracic support which supported the upper back during backward leaning. The traditional work seat was similar to an office chair (i e, a large horizontal seat-pan and a wide backrest) with the exception of having a higher than normal seat-height. This investigation consisted of three studies to compare the seats: (1) A user comfort and acceptance experiment which compared the initial psychophysical responses of 50 industrial sewers when introduced to the new seat; (2) a backrest usage experiment which compared the duration of backrest use among 10 industrial sewers; and (3) a follow-up experiment to evaluate chair preference after extended use of the new seat. The results of the user comfort and acceptance experiment found that the new work seat had greater comfort and user preference; the results of the backrest usage experiment found that the new seat had greater backrest use than the traditional seat; the results of the follow-up experiment found that the preference for the new seat was maintained over time and not due to a Hawthorne Effect.     

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.     

Intervention on seat adjustment among drivers of forest tractors

The aim of this study was to clarify the ergonomics of forest tractor drivers' sitting conditions, to study how well drivers had adjusted the seats of forest tractors and to study the short-term effect of the backrest adjustment and the use of accessory lumbar support on neck-shoulder and low-back symptoms of drivers. The subjects in this field study were 100 male forest tractor drivers aged 21–50 years (37 ± 7 years) from central and eastern Finland. The drivers were visited twice, and a two-week intervention on seat adjustment was carried out between the visits. The methods used were interview and assessment of the seat. The height and inclination of the seat, inclination of the backrest and the stiffness of the spring were measured; after the measurements, the technician adjusted the seat. Half of the drivers were given an accessory lumbar support (Camp 21025) and advice concerning its use. Pain, stiffness and fatigue of the low-back and neck-shoulder at the end of the shift were reported to have diminished among nearly all drivers. There were no differences between the intervention groups regardless of whether the inclination of the backrest was adjusted or not or whether the drivers had used the lumbar support or not during the two-week intervention period. Relevance to industryThe results of this study can be taken into account when improvements are made in the ergonomics of the seats of forest tractors.     

Design of combine harvester seat based on anthropometric data of Iranian operators

Harvesting operation with agricultural combines imposes excessive physical loads on the operators and the poorly designed seats may be an important contributory factor in this regard. This aims of this field study were to evaluate the possible mismatch between seat dimensions of existing harvesting combines and anthropometric characteristics of 200 Iranian operators and to propose seat dimensions based on anthropometric principles. The anthropometric dimensions of each individual operator were compared to the relative seat dimensions using the equations proposed in the literature. The results showed a considerable mismatch for different seat dimensions including upper backrest width (100%), seat height (97%), armrest height (83.7%), seat width (52.8%), lower backrest width (40.6%) and seat depth (39.5%). This meant that the existing combine seats were too high, too narrow and too shallow and had armrests and backrests which did not match with body dimensions of the majority of the operators. It was shown that compared to the existing designs, the new proposed dimensions (including seat height = 400 mm, seat width = 450 mm, seat depth = 410 mm, armrest height = 290 mm, backrest height = 420 mm, upper backrest width = 360 mm and lower backrest width = 400 mm) better matched to the operator's anthropometry, with the match percentages ranging from 77% to 100%.Relevance to industryThe design and manufacturing of agricultural machinery should be made based on the anthropometric characteristics of actual users to avoid unnecessary demands on them. This study provide additional data on the operator's anthropometry that can be used as a starting point for designing more appropriate agricultural machinery or used by other researchers in the field.     

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.     

Sitting comfort in an aircraft seat with different seat inclination angles

Passengers' comfort experience during flights is important in choosing their flights. The focus of this study is passengers’ perceived comfort in different climbing angles during ascent. Twenty-six participants were invited to experience three inclination angles including 3°, 14° and 18° in a Boeing 737 cabin. The angle of 3° was used to simulate cruising stage and the other two were used to simulate different climbing angles. Participants experienced each setting for 20 min where the perceived comfort, their heart rate variability(HRV), and their body contact pressure values on the backrest and seat pan were recorded with questionnaires, HRV bands and pressure mats respectively. The results indicate a preference of 14° inclination angle resembling the cruising angle (3°) and having the slowest moving speed of the center of pressure (COP) on both the backrest and seat pan.     

How does a backrest work?

After discussing the distribution of forces in the stable upright posture and illustrating the importance of the lumbar lordosis for minimising muscular effort in this position, the loadings on the spine and muscles of the back are outlined during upright sitting. It is shown that the backrest locates the lumbar spine so that the CG of the superincumbent body parts can be positioned above the vertebrae, permitting the gravity load to be transmitted to the seat without the counteracting torques which muscles would have to provide if this position was not adopted. The forces arising from other sitting positions are then discussed, and some conclusions drawn for seat design.     

Malaysian sitting anthropometry for seat fit parameters

This article presents sitting anthropometry data for Malaysians focusing on seat fit parameters. It offers additional data regarding seat design requirements as there have been only a few efforts made to establish anthropometry data in Malaysia. An analysis using a measurement composed of 16 common and important dimensions in seat design was done with 216 subjects. Data collection was done by using direct measuring methods with standard equipment. Malaysian automobile seat fit parameters, namely backrest width, backrest height, cushion width, and cushion length were established from collected anthropometric dimensions: interscye breadth (5th percentile female and 95th percentile male), sitting shoulder height (5th percentile female), hip breadth (95th percentile female), and buttock–popliteal length (5th percentile female), respectively. From the data collected, this study finds that the fit parameter dimensions are bigger than other Asians' 95th percentile values but smaller than the Filipino and the Thai data being compared in this study. The female 5th percentile values for cushion length and cushion width are the smallest. In addition, two local automobile seats were measured and compared with the data. It was observed that the cushion lengths of both seats were too long for the 5th percentile female of the current data. This study provides seat fit parameters–the most fundamental part of automotive seat design, especially for the Malaysian population. © 2010 Wiley Periodicals, Inc.     

Developing a simplified finite element model of a car seat with occupant for predicting vibration transmissibility in the vertical direction

The transmissibility of seat depends on the dynamics of both the seat and the human body, and shows how the amplification and attenuation of vibration varies with the frequency of vibration. A systematic methodology was developed for finite element (FE) modelling of the dynamic interaction between a seat and the human body and predicting the transmissibility of a seat. A seat model was developed to improve computational efficiency before models of the seat pan and backrest were calibrated separately using load–deflection and dynamic stiffness measurements, joined to form the complete seat model, and integrated with the model of a manikin for further calibration. The calibrated seat model was combined with a human body model to predict the transmissibility of the seat. By combining a calibrated seat model with a calibrated human body model, and defining appropriate contacts between the two models, the vibration transmissibility with a seat–occupant system can be predicted.