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.     

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.     

PHYSIOLOGICAL BASIS OF TRACTOR DESIGN

Energy expenditure of tractor driving varies from 1-4 kcal/min depending on the particular agricultural task performed. The paper is concerned with principles underlying the design and positioning of tractor controls and seats in such a way as would enable the operator to carry out this strenuous work under the most favourable conditions.

Energy consumption and electrical skin capacity of the operator were measured during tractor driving with a number of different types of seat. The most satisfactory seat had a parallel seat suspension and hydraulic damping of tractor vibration. The extra cost of this seat above that most commonly supplied was insignificant in relation to the total cost of the tractor.

A study was made of the positioning relative to the operator of brake and clutch pedals, foot rest, steering column and steering wheel. The techniques employed in this study were : measurement of O₂ consumption and heart rate of the operator and of forces required in depressing the pedals and turning the steering wheel. In each instance the optimal positions of control and operator are illustrated by a detailed diagram.

In many existing tractors there is a separate brake pedal for each wheel (for rapid turning) and for both wheels together (for braking). A new type of brake with a single pedal (accident proof) is described. This brakes both wheels unless the steering wheel is also turned, when the brake on the outer wheel is released.

A standard commercial tractor was modified according to the principles described. Energy consumption of operators was found to be 13-29 per cent below that with unmodified equipment and the heart rate 40-45 per cent lower. It is believed that fatigue is reduced more than the energy figures suggest.     

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.     

Qualitative models of seat discomfort including static and dynamic factors

Judgements of overall seating comfort in dynamic conditions sometimes correlate better with the static characteristics of a seat than with measures of the dynamic environment. This study developed qualitative models of overall seat discomfort to include both static and dynamic seat characteristics. A dynamic factor that reflected how vibration discomfort increased as vibration magnitude increased was combined with a static seat factor which reflected seating comfort without vibration. The ability of the model to predict the relative and overall importance of dynamic and static seat characteristics on comfort was tested in two experiments. A paired comparison experiment, using four polyurethane foam cushions (50, 70, 100, 120 mm thick), provided different static and dynamic comfort when 12 subjects were exposed to one-third octave band random vertical vibration with centre frequencies of 2.5 and 5.5 Hz, at magnitudes of 0.00, 0.25 and 0.50 m.s^-2 rms measured beneath the foam samples. Subject judgements of the relative discomfort of the different conditions depended on both static and dynamic characteristics in a manner consistent with the model. The effect of static and dynamic seat factors on overall seat discomfort was investigated by magnitude estimation using three foam cushions (of different hardness) and a rigid wooden seat at six vibration magnitudes with 20 subjects. Static seat factors (i.e. cushion stiffness) affected the manner in which vibration influenced the overall discomfort: cushions with lower stiffness were more comfortable and more sensitive to changes in vibration magnitude than those with higher stiffness. The experiments confirm that judgements of overall seat discomfort can be affected by both the static and dynamic characteristics of a seat, with the effect depending on vibration magnitude: when vibration magnitude was low, discomfort was dominated by static seat factors; as the vibration magnitude increased, discomfort became dominated by dynamic factors.     

Qualitative models of seat discomfort including static and dynamic factors

Judgements of overall seating comfort in dynamic conditions sometimes correlate better with the static characteristics of a seat than with measures of the dynamic environment. This study developed qualitative models of overall seat discomfort to include both static and dynamic seat characteristics. A dynamic factor that reflected how vibration discomfort increased as vibration magnitude increased was combined with a static seat factor which reflected seating comfort without vibration. The ability of the model to predict the relative and overall importance of dynamic and static seat characteristics on comfort was tested in two experiments. A paired comparison experiment, using four polyurethane foam cushions (50, 70, 100, 120 mm thick), provided different static and dynamic comfort when 12 subjects were exposed to one-third octave band random vertical vibration with centre frequencies of 2.5 and 5.5 Hz, at magnitudes of 0.00, 0.25 and 0.50 m x s(-2) rms measured beneath the foam samples. Subject judgements of the relative discomfort of the different conditions depended on both static and dynamic characteristics in a manner consistent with the model. The effect of static and dynamic seat factors on overall seat discomfort was investigated by magnitude estimation using three foam cushions (of different hardness) and a rigid wooden seat at six vibration magnitudes with 20 subjects. Static seat factors (i.e. cushion stiffness) affected the manner in which vibration influenced the overall discomfort: cushions with lower stiffness were more comfortable and more sensitive to changes in vibration magnitude than those with higher stiffness. The experiments confirm that judgements of overall seat discomfort can be affected by both the static and dynamic characteristics of a seat, with the effect depending on vibration magnitude: when vibration magnitude was low, discomfort was dominated by static seat factors; as the vibration magnitude increased, discomfort became dominated by dynamic factors.     

Evaluation of the seating of Qantas flight deck crew

In 1985 Qantas Airways (Australia) requested an ergonomics assessment of three pilots' seats so that one could be selected for fitting in all new aircraft as well replacement in existing aircraft. The Ipeco seat was chosen. In 1991, after all aircraft were fitted with the Ipeco seats, the company then requested a further evaluation of the seat to see if it was acceptable to the pilots and if there were any outstanding problems. A seat feature checklist plus a body chart discomfort rating scale was given to the total crew of 1030 pilots. The results from the 202 respondents indicated that although the pilots found the Ipeco seat an improvement on the Weber seat there were some modifications required. The main problems included insufficient adjustment range of the lumbar support area and the thigh supports, and infrequent replacement of the seat cushion. The body charts supported the checklist results in that the main areas of discomfort indicated were the buttocks and low back. Recommendations for improvements in design of the Ipeco seat, training in use and maintenance are presented. The method used in this study has application for field assessment of seating in a wide range of occupations, particularly bus drivers, truck drivers and train drivers, who spend long hours seated without being able to take breaks.     

Effects of aircraft seat pitch on interface pressure and passenger discomfort

Seat pitch, defined as the distance from a point on the back of one seat to the same point on the seat in front, is one of the most important factors influencing aircraft seating comfort. This study assessed the influence of different airline seat pitches on subjective ratings of discomfort and body-seat interface contact pressures. This was a laboratory within-subjects study using an aircraft interior mock up to vary seat pitch. Twelve participants completed 1 h of sitting in each of five different seat pitches (28inches, 30inches, 32inches, 34inches, and 36inches). Interface pressure mats measured seat and backrest pressure distribution, subjective rating scales were used to measure overall and local body region discomfort. The results showed that overall body and local body region discomfort ratings tend to be lower when the seat pitch increased from 28 inches to 36 inches (p < 0.05). For pressure variables, the upper back average contact area, upper/lower back average contact pressure, upper/lower back average peak contact pressure, right buttock average contact area, left/right thigh buttock average peak contact pressure, and left buttock average peak contact pressure were significantly affected by seat pitch(p < 0.05). Separate analyses support that seat pitch was more strongly correlated with backrest interface pressure than with seat pan pressure. In conclusion, seat pitch was found to be an important factor associated with body-seat contact pressure and discomfort ratings.     

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 postural support of seats: a study of driver preferences during simulated tractor operation

The overall comfort of four current production tractor seats and some individual features of seat design have been assessed by a method of paired comparisons. The experiment was carried out under both static and dynamic conditions and subjects had to adopt natural postures similar to those adopted when ploughing. The results suggest that operators can differentiate seat comfort differences reliably. Subjects' posture has proved important particularly when assessing the height of the back rest. On the other hand most appraisals were not significantly affected by the conditions (dynamic or static) of the simulation. An exception to this was the cushion length. This experiment has finally produced enough information on the relative importance of various aspects to enable us to design an experimental seat to be used in a series of experiments to assess specific features of seat aspects. The ultimate objectives of this investigation are to determine what aspects of seat and workplace design affect the comfort of the operator and how these can be altered to improve the tractor driver's posture.     

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.     

Predicting the discomfort caused by tractor vibration

A field study was conducted to investigate how the discomfort caused by the vibration of an agricultural tractor can be predicted from objective measurements of the vibration in the cabin. Eleven professional drivers judged the vibration discomfort produced by four different tractors on sixteen different test runs. At the same time, for all the tests, the multi-axis vibration in the cabin was measured on the floor, the seat pan and the seat backrest. For each of the 704 tests carried out, the discomfort caused by the vibration was predicted from the measured vibration in the cabin using a total of twenty different analysis procedures. The relative merits of the different prediction procedures were investigated by comparing, on an individual basis for each driver/tractor combination, the statistical significance of the correlations between the subjective judgements and the predicted values. There was considerable variability in the drivers' subjective responses, but it was concluded that, overall, the best procedure for predicting the vibration discomfort in an agricultural tractor is that recommended by ISO 2631 (International Organization for Standardization 1978), using the frequency weighted rms values of the vibration (0·5–20?Hz) measured on the seat pan in the three orthogonal directions, and taking the square-root-of-the-sum-of-the-squares of the values in order to combine the directions as recommended in Amendment 1 to ISO 2631 (International Organization for Standardization 1982).     

Development of a work seat for industrial sewing operations: results of a laboratory study

A hydraulically adjustable chair with seven independent variables - seat height, seat angle, seat rocking, seat swivel, back-rest distance, back-rest height and back-rest angle-was constructed to evaluate comfort and biomechanical factors relevant to the design of a chair for industrial sewing operations. The purpose of this investigation was to determine which of these factors were significant. Four assessment methods: overall body discomfort, localized body discomfort, electromyography (EMG) and height reduction (vertebral disc shrinkage) were used to evaluate these seven independent variables. Two female subjects attended eight 6 hour experimental sessions. The results indicate that to accommodate 90% of the female population, the seat height should be adjustable in the range 51 to 61 cm, the back-rest distance should be adjustable in the range 10 to 15 cm (measured from the centre of the back-rest to the centre of the seat-pan), the back-rest height should be fixed at 25 cm (measured from the centre of the back-rest to the centre of the seat-pan), and the seat should be allowed to swivel freely.     

The effects of the duration on the subjective discomfort of a rigid seat and a cushioned automobile seat

The subjective discomfort caused by the seat would affect the judgements of discomfort for the seated subjects. However, there have been few studies concerned with the discomfort on the rigid seat in static states, especially for a relatively long duration. This paper investigated the subjective discomfort caused by a rigid seat and a cushioned automobile seat for an hour. Twelve students (eight males and four females) rated the overall discomfort on a category-ratio scale and the local body discomfort on a 6-point rating scale every 10 min caused by two seats in two separate days. The static discomfort increased with increasing time, and the rigid seat caused greater discomfort than the cushioned seat. The local discomfort on the back dominated on the automobile seat, whereas the local discomfort on the buttock area dominated on the rigid seat. We established the empirical equations to predict the relations between the discomfort and duration of the two types of seats, for benchmark in the future studies on vibration and noise discomfort.     

Optimum seat pan and back-rest parameters for a comfortable tractor seat

An experimental set up was fabricated to measure the pressure distribution on the seat pan and back-rest of a tractor seat. Experiments were conducted with four different seat pans having radius of curvatures of 60, 75, 90 and cm, four backrests with radius of curvatures of 30, 60, 90 and cm, and three back-rest inclinations of 0°, 5° and 10° on representative Indian tractor operators. The subjective assessment of perceived comfort at the seat-operator interface was also recorded. Experiments were conducted in a randomized block design and the data obtained were analysed using suitable computer packages. Results indicate that all the parameters, namely seat pan, back-rest profile curvatures and the back-rest angle of inclination, affect the pressure distribution. It is concluded that a seat pan with radius of curvature 75cm, back-rest with radius of curvature 30 cm and back-rest inclination of 10° are the most suitable parameters for Indian tractor operators.     

Optimum seat pan and back-rest parameters for a comfortable tractor seat

An experimental set up was fabricated to measure the pressure distribution on the seat pan and back-rest of a tractor seat. Experiments were conducted with four different seat pans having radius of curvatures of 60, 75, 90 and infinity cm, four back-rests with radius of curvatures of 30, 60, 90 and infinitity cm, and three back-rest inclinations of 0 degrees , 5 degrees and 10 degrees on representative Indian tractor operators. The subjective assessment of perceived comfort at the seat-operator interface was also recorded. Experiments were conducted in a randomized block design and the data obtained were analysed using suitable computer packages. Results indicate that all the parameters, namely seat pan, back-rest profile curvatures and the back-rest angle of inclination, affect the pressure distribution. It is concluded that a seat pan with radius of curvature 75 cm, back-rest with radius of curvature 30 cm and back-rest inclination of 10 degrees are the most suitable parameters for Indian tractor operators.     

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

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

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

Modeling of human model for static pressure distribution prediction

Vehicle comfort, the key factor that influences the purchase of automobile products, is becoming increasingly important. However, the processes of traditional empirical and experimental approaches to design a new, more comfortable seat is complicated, time consuming and costly. The finite element method could facilitate, accelerate and economize this process. In the present work, a complete human FE model is established based on the Hybrid III dummy, the appropriate element size of 10 mm was ascertained. The body segment mass was verified by comparing segment mass percentages obtained from this model with previous data. The further validation study of the human model was achieved via the human pressure distribution experiments over human-rigid seat interaction under three postures, the validation reveals that the simulation results agree well with the experimental data. On this basis, the human model was applied to predict the interactions between human body and an automobile seat, then the contact pressure distribution, additional information about the contact shear stresses distribution and stress distribution within the soft tissue were obtained through simulation. The human model presented in this paper can reflect the interaction between human body and automobile seat precisely.Relevance to industryThe results deduced that the model is capable of realistically predicting pressure distribution, the present model allows the evaluation of seating comforts in a virtual phase of seat development, and the study can be taken as reference for vehicle seat design and biomechanical evaluation.     

Tractor stability indicator

Tractor overturns are a major cause of death in farm operations. The overturns are the result of interactions between the tractor operator, the tractor and the environment. Numerous variables involved in tractor overturn have been identified. Previous stability analyses have resulted in attempts to design devices to warn or prevent tractor rollover but to date nothing has proved successful. A newer approach to the rollover problem is to develop instrumentation that will give the operator instantaneous cues concerning the tractor's stability as it is operated. The device is perceived as a learning instrument which will increase both cognitive and motor skills.     

Technical note: spine loading in automotive seating

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