Influence of lumbar support on tractor seat comfort based on body pressure distribution

To solve the problem of lumbar spine injuries of tractor drivers, lumbar support devices were added to the tractor. The purpose of study is to design lumbar supports with different protrusion thicknesses to adjust the load on the operator’s lumbar under whole-body vibration. Integrating pressure distribution measurement with subjective assessment, pressure distribution on the contact surface between ten male tractor drivers and the seat under conditions of different lumbar supports and driving speeds and the influence of the presence of lumbar supports and their thickness on the subjects’ comfort were analyzed. The results demonstrated that there existed a certain correlation between the pressure distribution indexes and the drivers’ subjective evaluation, and the pressure distribution indexes could reflect ride comfort objectively. The 3 cm thick lumbar support did not improve ride comfort significantly, while the 9 cm thick support exerted too much stress on the waist, causing the lumbar spine to lean forward excessively and aggravating lumbar fatigue. Most subjects in the study would prefer to increase the thickness of the lumbar supports to approximately 6 cm. Based on the pressure distribution test, this study conducted an analysis of the influence of lumbar supports in tractors, which could provide a reference design for tractor seats.


Introduction
Unlike passenger vehicles, the drivers of agricultural tractors are likely exposed to a high level of whole-body vibration due to mainly working on off-road terrain. Modern tractors integrate different suspended seat designs, including self-leveling seats for larger tractors and compact mechanical seats in small tractors, which are used for minimizing the vibration transmitted to the driver [1]. Vibrations transmitted from tractor components to the human body can lead to reduced performance and discomfort, and may cause perpetual back pain [2][3][4].
The vibrations in the tractor exerted to arms or legs of tractor operators are transmitted through the seats. The backrest acts as a channel for such force in many cases, otherwise the muscular tissue of the torso must be tensed to provide a semi-rigid path for strength transmission [5]. The additional load on the vertebral column will be increased due to the resulting muscle tension, especially in the lumbar spine that is the main path for transferring load from the upper part of the body to the lower [6,7]. Ng et al. (1995) pointed out that the pressure is a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 mainly concentrated in the lumbar region in the seat backrest, showing a worse distribution in comparison with thighs and hips [8]. Therefore, the applied research about lumbar support of tract seat has broad practical prospects [9,10]. Corlett et al. (1984) emphasized the significance of the lumbar lordosis to minimize muscular effort in sitting positions [11]. The lumbar vertebrae is located by the backrest in order to position the center of gravity of the upper body above the spine, allowing the transfer of gravitational loads to the seat without the counteracting torque provided by muscles [12]. The Human Performance Institute of Western Michigan University developed a test chair to determine the position and extent of lower back support that a user needs when sitting in a work chair [13]. The results showed that about 70% of the participants selected asymmetric lower back support (the sum of the contact pressures on the left side of the low back was unequal to the right side of the low back), while nearly a quarter of participants chose double the support on one side of their lower back versus the other. Relevant studies have shown that, among the seat parameters, lumbar support protrusion and backrest angle have the most significant influence on back muscular movements [14]. In the light of the experiment about car seats accomplished by , the EMG amplitude at L1 level and disc pressure was decreased by approximately 50% through increasing the lumbar support protrusion from 0 to 50 mm [15]. Hosea et al. (1986) explored the impacts of lumbar supports' thicknesses on back muscular activity in a road test. The results demonstrated that the lumbar support protrusions of 30 mm and 50 mm generated tantamount back muscle movements in the lumbar and thorax, while the 70-mm protrusion increased the EMG amplitude [16]. Lower-level muscle activities will lead to less muscular discomfort and fatigue [17].
Although there are considerable factors those influence the lower back during the operation of tractors, continuous flexion of the lower back may have connections to the occurence of soreness [18]. Lumbar support can lighten the load on the spine [19,20], and studies conducted by  indicated that lumbar support could reduce injuries of lumbar intervertebral disc [19,21]. The most momentous feature of lumbar support lies in that it should be positioned surrounding the lumbar spine of the seated operator. Installation of a lumbar support on the seat backrest can play a strong supporting role for the driver's waist, effectively preventing lumbar diseases caused by insufficient supporting force on lumbar [22][23][24]. Research has shown that a reasonable lumbar support helps to keep the lumbar vertebra in a natural bending state and reduce the load balance support of the trunk [25]. But if the cushion is overly thin, it will fail to furnish the lumbar spine with ample strut. Under the action of gravity, the transformation of the natural curved arc of lumbar vertebra may give rise to discomfort on the lower back [26]. On the other hand, the great thickness of the cushion will enlarge the anteversion angle of lumbar vertebra, which also poses an adverse impact on the natural bending arc of waist spine, making the human body unwell [27]. In a word, the optimal design of lumbar support conforming to the structure of the human lumbar vertebrae can promote relieving drivers' discomfort successfully, which is beneficial to their occupational health [28].
Unlike former researches on riding comfort of road vehicles, this study introduced the method of seat pressure in precision agriculture to increase the scenario of applied research. The operating conditions of agricultural machinery such as tractors are very poor, and the vibration damage is more serious than that of automobiles [29]. But there are few researches on comfort, with no lumbar support device especially for tractors on the market. The goal of this study was designing appropriate lumbar supports with different protrusion thicknesses to reduce the pressure load on the operator's waist and improve the seat comfort during agricultural operations in vibration environment by attenuating the natural frequency vibration that posed severe damage to the lumbar and spinal system. On the basis of physiology and comfort research, the method of integrating pressure distribution with subjective evaluation was adopted to determine the pressure distribution of the driver's seat cushion and backrest under different lumbar support conditions and at different driving speeds of the tractor, providing basis and reference for the development of smart seats that are beneficial to the operator's occupational health [30].

Participants
Ten male tractor operators, age (33.35±3.08) years, height (1.74±0.06) m, and weight (76 ±4.58) kg, with more than six years of experience in tractor driving were recruited in this experiment. They have good health in lumbar spine. The study was performed in a suburban farmland. Ten subjects operated the same tractor and drove along straight and curved roads for 1 km without any obstacles. Before the experiment, the operators were familiar with the experimental procedure and details and had a 20-minute training. The study was approved by the college of engineering of China Agricultural University, Beijing, China. The individuals in this manuscript have given written informed consent (as outlined in PLOS consent form) to publish the case details.

Tested lumbar supports
Considering serious driving fatigue of tractor drivers when operating high-power tractors, John Deere 1204 high-power tractor with a power of 88kw was selected for this test. The tractor was equipped with the pneumatic suspension and a suspension seat with the size of 360×350×300 mm.
The dimension of the lumbar support has a significant impact on the driver's driving posture and visual field, which should be designed on the basis of human body sizes. The width of lumbar spine ranges between the width of chest and that of shoulder. In view of potential changes in the position of the lumbar spine at special moments such as the turning of the tractor, the range of lumbar spine width was expanded to 1.2 times of the original. According to the 50th percentile size of adult male [31], the lumbar support length was designed adjustable, ranging from 29.0 to 41.7 cm. The waist length of the human body should be about 65% of the shoulder height in the sitting position [32]. Consequently, the waist support length was defined as 37 cm. In the light of the structure of the human body template in the sitting position and the adjustment scope of each joint angle in 'GB/T 14779-1993 Requirements for Functional Design of Sitting Human Body Templates' [33], the accommodative range of the human waist joint angle is from 168º to 195º, and the length of the waist joint protrusion is from 0 to 10 mm. Therefore, the thickness of the lumbar support protrusion was set to be 3 cm, 6 cm and 9 cm respectively in this paper. In order to meet the requirements of human body structure size, cab space and seat dimension at the same time, the size of the lumbar support was determined as 30×37 cm. Moreover, the support was made of polypropylene material as shown in Fig 1, whose thickness could be adjusted freely by using the knob to realize the needs of the thickness of 3 cm, 6 cm and 9 cm.

BPMS system
A body pressure measurement system (BPMS, model BCE5350-1, sensing area: 41.66×38.61 cm, 1558 of sensing elements, Tekscan, USA) was used as the pressure testing device, as shown in Fig 2. It consists of scanning electronics-evolution handles, software, and thin-film sensors. The handle was connected to the sensors, collected data from the sensors, and then processed and sent this data to the computer via USB connection.

Experiment process
In this study, two pressure cushions were installed on the backrest and cushion of the seat correspondingly for pressure measurements. The pressure cushion had to be calibrated before the test started. Tiled the pressure cushion on the seat, and connected it to the sensor handle. The subjects placed total body weight on the seat, then the software BPMS recorded the stress of the seat cushion for calibration. Before data acquisition, with regard to parameter setting of the software, the sampling frequency was 4 frames per second, with each frame included 1558 pressure values, and the recording duration was defaulted to 10 min. With the purpose of making the pressure data more evenly distributed, the upper limit of the pressure was demanded to be slightly larger than the maximum of the measured value. So the upper limit was set to 30 KPa.
The experiment added lumbar supports with different thicknesses of 0 (i.e., no lumbar support), 3 cm, 6 cm and 9 cm to the tractor seat. And the tractor moved at three specified driving speeds of 3 km/h, 6 km/h and 9 km/h. The pressure distribution data of the subject's back and  hip under different conditions of lumbar supports and driving speeds was collected during the operating process, and it was needed to assess the corresponding subjective comfort through the form of the comfort scale. This experiment adopted a within-subject design. Each participant was tested three times randomly.
After the pressure cushion was laid flat on the seat cushion and backrest without any wrinkles and fixed to the seat, the subject adjusted to a comfortable sitting position. In the experiment with no lumbar support, the tractor was driven for 10 minutes at a constant speed of 3 km/h, 6 km/h and 9 km/h respectively in the farmland. The grounds were nude without any grass or stones. They were parallel routes, and each test ride was performed on uncompacted ground, which was dry. The pressure distribution data of the backrest and seat cushion were recorded in real time. Subsequently, adjusted the lumbar support to the thickness of 3 cm, 6 cm and 9 cm respectively, and repeated the above experimental steps. Each subject was required to complete 12 sets of experiments. The test scene is shown in Fig 3. After each group of operations, the subjects had to fill out the questionnaire with a 10-level scale. The driver's subjective feelings were divided into the following degrees: extremely uncomfortable, very uncomfortable, moderately uncomfortable, slightly uncomfortable, normal, acceptable, not uncomfortable, slightly comfortable, moderately comfortable and very comfortable, the corresponding scores varied from 1 to 10. The subjective comfort evaluation questionnaire is shown in Table 1.

Analysis of pressure indicator
The basic evaluating indicators used in this paper include maximum pressure (P m ), average pressure (P v ) and contact area (A r ), as well as comprehensive analysis indicators, e.g., the average percentage of pressure activation units (NC), dynamic seat pressure change rate (DSPD) and the center coordinate variation curve of longitudinal pressure. The pressure indicators in this paper can describe the driver's ride comfort in static and dynamic situations, allowing comparisons of different tractor seats, as shown below. Based on these, the pressure distribution of the seat backrest and cushion was analyzed.
1. The maximum pressure represents the pressure peak value in all contact points between human operators and the seat surface, which reflects the stiffness of the seat cushion. The

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Influence of lumbar support on seat comfort larger the value, the more uncomfortable the seat is. It can be calculated with the Eq (1): Where P i is the pressure value at the i-th contact point, N is the total of contact points. 2. The average pressure means the average value of the pressure in all contact points between human operators and the seat surface, which can reflect the stiffness, hardness and material of the seat. It is defined as Eq (2): 3. The contact area refers to the sum of the area of the contact surface between the human body and the seat under the action of the pressure point sensor when the subject sits on the seat, and shows the fit degree of the seat to the back and hip.
Where N S(range) means the number of sensors whose pressure values are between 0 and 13 KPa, 14~40 KPa and 41~100 KPa; N S is the number of active sensors measuring pressure higher than 0 KPa. 5. Uniform pressure distribution can characterize satisfactory comfort. The dynamic seat pressure change rate (DSPD) represents the change rate of the seat pressure (SPD) in a continuous time domain during the test procedure, which can embody the ability of the seat cushion to maintain a uniform pressure distribution within a period of time. The computing formula is shown as Eqs (4) and (5): Where T means the pressure duration.

Analysis of pressure distribution under different lumbar support conditions
Research suggests that under the correct sitting posture, the result of the pressure distribution on the seat cushion demonstrates that the pressure on the ischial tuberosity is the largest, centered on which, the pressure values decrease outwards smoothly. In contrast, the result of the pressure distribution on the backrest shows that there exists the maximum pressure on the scapula and lumbar vertebrae. The pressure values reduce evenly outward from the center of the joints [34]. Comparing the pressure distribution cloud charts of the backrest and seat cushion under different lumbar support conditions in the pressure measurement test, as shown in Fig 4 which takes the speed of 6 km/h for example, it can be found that the pressure distribution was relatively uniform. With the aid of lumbar support, the pressure on the hips eased to a certain extent, and the pressure on the backrest exhibited a homogeneous distribution. The maximum pressure (P m ), average pressure (P v ) and contact area (A r ) were obtained from BPMS software directly, and the indexes of DSPD and NC could be calculated according to the measured data. The calculation results are shown in Table 2. It can be found from Table 2 that the P m , P v and A r values of the seat cushion were all higher than those of the backrest under the same conditions. With the increase of the thickness of lumbar support, both P m and P v values on the backrest and seat cushion tended to increase generally, while A r values decreased relatively. Within the pressure range of 14~40 KPa, the NC in the seat tests with lumbar support was significantly higher than that in the seat tests without lumbar support. More precisely, the NC of the backrest caused by the lumbar support with a thickness of 6 cm was relatively low, and the seat cushion with the 9 cm of lumbar support brought the optimum NC. As for the pressure range of 41~100 KPa, it can be seen that there existed no pressure cells at the backrest, and the NC at the seat cushion increased as the  increase of the thickness of lumbar support, indicating that excessive lumbar support would put more pressure on the hip. On the other hand, the smaller the DSPD value, the better the ability of the seat cushion to maintain a uniform pressure distribution over a period of time.
For the seat installed with appropriate lumbar support device that served to keep an even distribution of the subject's lumbar pressure in the process of operation, the DSPD values of the backrest would be reduced prominently. Nevertheless, as the thickness of the support increased, the DSPD values of the backrest would drop concomitantly, while those of the cushion showed a trend of increasing unduly. This was bound to make the waist pressure unevenly distributed and reduce the comfort of the waist. With the thickening of the lumbar support, the backrest fitted the waist region of the human back better gradually. The increase of contact area made the waist bear more stress, and relieved the pressure on the seat cushion and shoulders accordingly. Considering that the perception sensitivity of the shoulder and back of the human body to the discomfort of contact force was higher than that of the waist, improvement of the overall comfort of the backrest benefited from the moderate thickening of the lumbar support. However, in the light of Table 2, the maximum contact pressure at the lumbar support had exceeded 25 KPa. If the thickness of the support was further increased, it would pose a negative influence on the operator's driving experience. In addition, the sitting position of subjects also has a deep connection to the thickness of the lumbar support. An unreasonable lumbar support has a tendency to cause the lumbar vertebra of the human body to lean forward excessively, which is not conducive to forming a good driving posture.

Analysis of pressure distribution at different speeds
According to the different driving cycles of the tractor, the pressure distribution cloud charts of the backrest and the seat cushion under different driving speeds were acquired. Taking the lumbar support of 3 cm as instance, Fig 5 illustrates the stress distribution. As can be seen from it, the backrest exerted the highest pressure on the lumbar vertebrae, which provided better support for the lumbar vertebrae in lumbar curvature. Besides, the pressure distribution of the cushion was consistent with the expectation. The maximum pressure appeared on the ischial tuberosity, and the smooth transitive pressure was discovered from the ischial tubercle to the anterior thigh. Table 3 shows the comparison of the pressure parameters of the backrest and cushion under the driving speed of 3 km/h, 6 km/h and 9 km/h. It can be seen that the P m , P v and A r values increased monotonically with the increment of speed in general. No pressure unit was detected at the backrest under any working conditions within the pressure range of 41~100 KPa, and the NC value at the seat cushion had a positive relationship with the driving speed. Meanwhile, in the pressure range of 14~40 KPa, the activation percentage of pressure sensors at the backrest and seat cushion increased by 29.9%~83.2% and 1.3%~16.4% respectively with the speed. By comparison, within the pressure scope of 0~13 KPa, as the speed rose, the NC value of the backrest and seat cushion declined by 3.4%~9.5% and 2.2%~9.4% separately. The dynamic pressure distribution rule revealed that the DSPD value became larger with the velocity increasing, which suggested that the ability of the seat cushion to maintain pressure stability was weakened.
Furthermore, Fig 6 shows the average change trend of the longitudinal pressure center coordinates of the backrest and cushion with 3 cm thick lumbar support over time under each operation condition. The most notable finding was that the coordinate of the longitudinal

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Influence of lumbar support on seat comfort pressure center at the cushion tended to posted an increasing trend when the velocity went up from 3 km/h to 6km/h and 9 km/h, whereas the center coordinate at the backrest decreased with an opposite trend. This phenomenon implied that the vibration change resulting from the rise of velocity had a remarkable impact on the force of the subject's rear buttock and low waist. Simultaneously, during the execution procedure of operations, the coordinate of the longitudinal pressure center at the backrest gradually moved down, and that of the seat cushion moved forward, which concurred with the subjective sensation in the actual persistent sitting position.

Relationship between subjective evaluation and pressure distribution
The activation ratio of the pressure sensor can be regarded as an exact description of the comfort level. The increment of P v would bring about the decrease of comfort. The dynamic pressure change rate functioned as the reflection of the cushion's capacity of preserving stress stability. The larger the change rate value, the lower the human comfort. According to the subjective comfort assessment results with different lumbar supports at different speeds in Fig 7, it can be found that with the thickening of the lumbar support device, the comfort of the waist, elbow, thigh and the whole body had a tendency of increasing first and then decreasing, and the shoulder comfort kept descending constantly, which demonstrated that an appropriate thickness of the lumbar support could improve the overall comfort of the human body. The correlation between the subjective evaluation and indicator parameters of the pressure distribution was analyzed by using SPSS software. The results are shown in Table 4. The results discovered that the subjective comfort had a strong correlation with P v , NC (0~13) , NC (14~40) , NC (41~100) and DSPD (correlation coefficient |r|>0.4, significance P<0.05). More specifically, there existed a highly positive correlation between NC (14~40) and the subjective rating of the comfort of ischium, hip and thigh, while the other indexes and NC (14~40) were negatively related. Comparatively, the estimation of subjective comfort was in weak relativity to P m and A r (|r|<0.4).

Discussion
Compared with previous researches about ride comfort of road vehicle, this study applied the seat pressure method in precision agriculture to expand the scenario of applied research, and designed appropriate lumbar supports for tractor drivers to improve the seat comfort during agricultural operations in vibration environment. The experimental results demonstrated that furnishing the seat with lumbar support equipment reduced the stress on the buttocks and waists of tractor drivers during execution of tasks. When the lumbar support measured 3 cm thick, the improvement of comfort was not evident; when the thickness of the support increased to 6 cm, the DSPD value on the backrest and cushion was relatively small, making a rational pressure distribution on the human lumbar and the top score of subjective comfort; as the prominence thickness was added to 9 cm, the high value of P v and P m of the backrest meant that excessive pressure load was exerted to the back of the human body in this situation, which was easy to cause spinal injury. This reflected that lumbar supports too thin or too thick would pose a negative effect on the subjective comfort.
On the other hand, in the driving cycle of the tractor, the pressure index parameters of the backrest and seat cushion would increase with the increase of driving speed, apart from NC (0~13) and NC (41~100) of the backrest. Then the vibration damage to the driver's rear buttock and lower waist would also be aggravated. In a continuous sitting position, the longitudinal pressure center coordinates of the backrest tended to move downward, while that of the seat cushion would move forward. Tractor traveling speeds had a significant effect on subjective comfort of the seat (P = 0.02). Vibration caused by high speeds may induce discomfort, fatigue, pain, reduced performance of the operator and the inability to fully control the tractor. The increased velocity caused the tractor to vibrate more intensely, and the whole body vibration during driving processes ought to be the reason for lower back pain in tractor drivers. Hence, in the experiment, the driver's discomfort increased sharply with the increase of speed. Changes in the lumbar support prominence of seats could significantly reduce incidences of tractor operators' symptom of discomfort. The vibration intensity posing an impact on the human body should be attached great importance to.
It is apparent that there exists a substantive interaction between support devices and fit degrees [35]. For instance, changes in the lumbar support profile are bound to transform the pressure distribution of backrest and may have prominent effects on sitting postures based on anthropometric measurements. In this paper, the comfortable quality would be improved or reduced with the increase of the lumbar support thickness. It suggested that the discrepancies of the tractor seat backrest angles were responsible for the difference in results. During the experiment, it was discovered that increasing the inclination angle the backrest could improve the comfortability strongly. In this case, the support bulged against the upper lumbar region of the subject's back, instead of matching well with the curvature of the spine, to generate a highpressure area. The lower part of the subject's spine was left unsupported due to the convex surface, which would negatively affect discomfort in the area. Ideally the thickness of the lumbar support should be regulable. There existed a noticeable effect of time on driving comfort (P = 0.03). The discomfort of lumbar support increased significantly with time. The adjustability of tractor lumbar supports is essential to better satisfy operators' preferences and decrease lumbar flexion in sitting position, in order to guarantee that the support protrusion fits the profile of the operator's lumbar. Larger subsequent studies are needed to investigate this effect more thoroughly.
Although the definition of lumbar protrusion on tractor seats remains unclear, the common ground in this experiment was that the lumbar support protrusions on the tractor seat were all vertical to the backrest plane, with the operator's dorsum and hip touching this plane during the test procedure. It was found that most drivers had a disposition to select the posture with kyphotic or flat lumbar curvatures when seated on seats with protruding lumbar supports in prolonged driving, which reduced the possibility that most subjects determined proper lumbar support. Schultz et al. (1985) found that postures flattening the waist curve can reduce the active muscle tone required to keep the posture and may relieve fatigue and discomfort in muscle [36]. The results of this test argued that, more than half of the subjects tended to change the lumbar support configuration during the operating process if permitted. This implied that the subjects may adjust the lumbar support setting in sitting position for sustained driving, which was consistent with the conclusion raised by Reed et al. (1996) [37]. It can be seen that norms in the lumbar support parameters are short of standardization. The anticipated seat profile will emerge if the driver is seated in an adjustable and comfortable position, which makes the problem more complicated. Owing to diverse anthropometric measurements and preferences for sitting positions, specific lumbar supports are probable to correspond to various effectual protrusions or perpendicular positions relative to the lumbar vertebra depending on the driver and sitting posture. Support parameters in tractors are rarely quantified in the literature despite that any aspect of the seat surface profile can be deemed as an influence factor. Also, the majority have more significance when accounting for body pressure distribution instead of posture. However, the longitudinal backrest profile is an important exception, especially the lumbar support of the lower back area, considering that the reacting force engendered by the seat is directed to the driver's lumbar spine. It has been proved that the specification of lumbar supports and the size of the occupant will bring about a series of optional seatback angles [38]. Consequently, in this experiment, backrest angles may transform the degree of lumbar support selected by drivers. The resulting confounding effects and variability would affect the accuracy of the experimental results to some extent. In addition, studies have shown that women prefer more lumbar supports than men [39]. Therefore, it is essential to set up preliminary data about lumbar support preferences of different genders for long-term tractor driving and explore gender differences in subjective comfort and lumbar spine posture.
Conclusively, the results suggested that most subjects were more willing to increase the variability capability of the tractor's lumbar supports to approximately 6 cm. This research could potentially be applied to provide tractor drivers with tangible and visible biofeedback, identify stress in high-risk areas, assess postural and seating abnormalities and areas of operator discomfort, assist in scientific selection of the lumbar support and positions, and enhance driver satisfaction with lumbar supports, including surfaces and thicknesses.

Conclusion
With the thickening of the lumbar support, the comfort of the waist, elbow, thigh and the whole body tended to increase first and then decrease, and the shoulder comfort kept descending constantly, which demonstrated that an appropriate thickness of the support could improve the overall comfort of the human body. The vibration change resulting from the rise of velocity affected the force of the subject's rear buttock and low waist significantly. The results showed that there was a certain relevance between the pressure distribution indicators and the subjective assessments of drivers, which could objectively represent the comfort of human body. The 3 cm lumbar support did not substantially improve ride comfort, and the lumbar support of 9 cm put too much pressure on the waist, resulting in excessive forward inclination of the lumbar spine and aggravating lumbar fatigue. Most subjects showed a preference for increasing the variability capacity of the lumbar support to about 6 cm.
This research can serve as a basis for designing, implementing, and evaluating agricultural machinery vehicle seating technologies to improve the quality of agricultural work environments and enhance drivers' comfort. Furthermore, the information and laws of seat pressure can assist designers to develop smart seats with more application value, in order to effectively ameliorate pressure on the waist of the driver and improve driving comfort.