Static Factors in Sitting Comfort: Seat Foam Properties, Temperature, and Contact Pressure

Abstract

The seat characteristics have high relevance in overall comfort on any transportation means. In particular, the foam’s mechanical properties, interface pressure, and contact temperature play an important role in low- or no-vibration situations regarding static comfort. The present work presents the complete protocol for a static evaluation of different foams and seat covers to assess railway seats. Based on the evaluation of the foam’s mechanical properties and interface pressure profiles, it was concluded that higher-density foam (80 kg/m³) is the most favorable. Regarding the foam cover, a thermographic assessment demonstrated that the fabric cover that induces lower temperatures at passenger interface contact promotes higher comfort levels. It should be highlighted that experiments were conducted on real train seat cushions and environments using a thermographic camera and pressure map sensor.

1. Introduction

Public transportation users demand faster, safer, and more comfortable journeys. During a journey, a passenger spends most of the time seated, so it assumes a crucial impact on comfort [1]. This way, seats should fulfill their primary support function and, in addition, provide increased comfort. Perceived comfort is divided into static and dynamic categories, which are dependent on the presence or absence of vibration. Both are strongly connected, as one influences the other. Thus, they must be evaluated to properly quantify comfort levels. In a low-vibration situation, static comfort dominates the overall comfort evaluation, while in the opposite situation, the dynamics analysis is more relevant [2,3].

When seated, passengers are in close contact with seat cushions, which are considered one of the most critical factors affecting sitting comfort [4,5]. Therefore, evaluating the foam’s mechanical properties and its influence on passengers’ comfort is critical. Traditionally, seat cushions are made of polyurethane (PU) foams. Those have been universally used in multiple industries due to their versatile physical properties, such as excellent noise absorption and low thermal conductivity, comfort properties, low cost, and easy processing. PU foams are cellular polymers that are typically produced by reacting an isocyanate with polyols in the presence of blowing and crosslinking agents and additives [6,7,8].

There is no consensus on comfort definition. Thus, comfort quantitative evaluation is still ambiguous [9,10,11,12]. Several authors suggest evaluating seat physical characteristics and their relation with sitting comfort as the first approach to overcome the aforementioned limitation. From those physical properties, the hardness, the support factor (SAG factor), and the hysteresis loss were identified as strongly affecting seating comfort, becoming the main focus of most studies. Hardness corresponds to the foam’s firmness, the SAG factor indicates the supporting property, and the hysteresis loss correlates with resilience [7]. Understanding the influence of the seat’s physical characteristics on passengers’ sitting impressions is fundamental for designing the ideal foam, which can be accomplished by changing its mechanical properties to achieve higher comfort levels [3,13].

According to De Looze et al.’s [14] comfort model, which is applied as the reference model for the present study, comfort and discomfort result from the interaction between the product, the user, and the environment and can be evaluated by a combination of psychological and physical factors. The users’ feelings define psychological factors, being assessed based on questionnaires and rating scales (subjective evaluation). In contrast, the physical parameters depend on the human body weight, shape, and anthropometric dimensions, being defined by objective measurements [13,15,16,17]. The passenger’s pressure on the seat cushion strongly affects the seating interaction due to the biomechanical load imposed by the user’s musculoskeletal structure. That load leads to skin and tissue deformation, which may affect blood flow and the musculoskeletal system [18]. The skin has thermoreceptors capable of detecting temperature changes and four types of mechanoreceptors that provide information about touch, pressure, vibration, and cutaneous tension. The Meissner corpuscles are exceptionally efficient in transducing information regarding tactile and sensitive changes. The Pacinian corpuscles, responsible for detecting vibration, have a lower response threshold than the Meissner ones. Merkel’s disks and Ruffini’s corpuscles are slowly adapted mechanoreceptors. The former specialises in detecting pressure changes, and the latter can detect stretch and shear stresses. The thermoreceptors and the mechanoreceptors record the stressed surface changes and send the information to the brain, leading to comfort perception [19,20,21]. Therefore, although the foam’s mechanical properties influence comfort, the seat/skin interface pressure and temperature are also key factors.

The present article deals with the evaluation of static parameters and their influence on passengers’ discomfort. The foam properties of two samples were initially assessed in the lab. Then, those foams’ interface pressure and temperature were evaluated using a pressure map sensor and an infrared thermographic camera. Moreover, it was also possible to draw conclusions about the influence of the seat cover. In this way, it was hypothesised that the newly developed setup could represent a novel protocol for the comfort evaluation of in situ railway seats.

2. Influence Parameters

2.1. Influence of Foam Properties in Sitting Comfort Impressions

As previously described, foam cushions represent the primary contact between the user and the seat. Thus, some authors studied the relationship between foam’s physical characteristics and sitting comfort. This connection was assessed by means of subjective questionnaires and interface pressure measurements.

Lei et al. [22] related hysteresis loss with seating comfort once it correlates with resilience. Hysteresis Loss (HL) is defined as the percentage ratio between the area under the loading (0–75%) and unloading (75–0%) compression cycles Equation (1):

$$HL(\%)={\displaystyle \frac{A_{0–75\mathrm{\%}}}{A_{75–0\mathrm{\%}}}}$$

(1)

Ideally, foams should present a low hysteresis loss percentage. This value increases as resilience decreases [4,7]. Hysteresis loss is highly influenced by the cellular foam structure, specifically the cell/wall area ratio. During the compression cycle, the energy loss happens primarily due to the buckling affecting unrecovered cell walls and struts. Thus, to obtain low hysteresis, low cell/wall area ratios are required [23].

The SAG factor is another measure of foam’s suitability for sitting applications, obtained by the ratio of the compressive load at 65%, deformation $\left(F_{65\mathrm{\%}}\right)$, and the load necessary for 25% foam deformation $\left(F_{25\mathrm{\%}}\right)$, according to Equation (2):

$$SAG={\displaystyle \frac{F_{65\mathrm{\%}}}{F_{25\mathrm{\%}}}}$$

(2)

Once the SAG factor provides an indication of the foam-supporting property, high values are associated with higher comfort performance. The supportive factor is influenced by the foam’s cell structure and density. Regarding seating applications, an SAG value higher than 2.8 is recommended. Otherwise, users may feel the seat pad bottom [4,7,13,23,24].

Lee and Ferraiuolo [25] identified foam thickness and hardness as significantly affecting sitting comfort. Hardness is defined by the 25% Indentation Load Deflection (ILD). This relates to foam stiffness and defines the foam’s resistance to deformation when applying a load. High values correspond to high firmness, whereas low values are associated with lower firmness [6]. Using foam samples with equal density and hardness but varying thicknesses (50, 75, and 100 mm), Neal [26] concluded that foam thickness does not influence hysteresis loss but influences the 25% load as thicker foams reported higher forces.

Ebe and Griffin [13] conducted a study to relate the influence of foam properties, such as density, SAG factor, and hysteresis loss, in sitting comfort impressions. The sitting impressions of 12 subjects were obtained to judge the comfort of four foams with equal hardness (≈205 N) but varying density (45–65 kg/m³). The high-durability seat (55 kg/m³) was rated as the most comfortable, whereas the lowest-density foam (45 kg/m³) was the least. Moreover, the low-density foam presented the highest hysteresis loss (34%). In contrast, the high-durability foam demonstrated a lower hysteresis (29%). A high correlation was observed between the comfort scores and the SAG factor. A second experiment was performed to evaluate the influence of the foam’s hardness. The same 12 subjects rated the comfort perception of five foams with 25% ILD hardness between 120–285 N. Results suggested that hardness influences static comfort, but the preferred foam varied between subjects. Additionally, it should be highlighted that harder foams present the lowest hysteresis loss. Similar conclusions were reported by Moon et al. [4] when studying the production of PU foams with multiple layers. This foam is considered the best for seating applications, demonstrating a SAG factor of around 3, hardness of 150 N, and hysteresis loss of approximately 22.5%. Analysing the aforementioned studies, the ideal foam is characterised by a high SAG factor, low hysteresis loss, and intermedium to high density.

2.2. Influence of Interface Pressure in Sitting Impressions

While sitting, the human body is in direct contact with the seat cushion, exerting a pressure that is related to weight and contact area and defined by the user‘s musculoskeletal structure. Following this assumption, multiple authors defined comfort based on the seat–passenger interface pressure. The pressure profile is strongly connected with discomfort as proved by the correlation between objective and subjective evaluations [9,13,14,27].

Indeed, when a person is seated, more than 70% of his weight is supported by the seat cushion. Since the human buttock is not flat, the seat–passenger interface pressure varies over the seat surface. That feature leads to a significant pressure concentration in the ischial tuberosity area, resulting in pressure peaks that may provoke low cell oxygen content, prompting fatigue, pain, and discomfort. Although limiting the maximum pressure is fundamental to providing high comfort levels and reducing the harmful consequences for passengers, no consensus has been set yet [3,27,28,29]. Nevertheless, multiple authors stated that 32 mmHg, corresponding to the capillary pressure, should not be exceeded. Higher pressure may obstruct the capillaries, limiting blood circulation and leading to oxygen deprivation in the tissues, causing discomfort [2,16,28,30,31]. In opposition, other authors defended that a seat is still considered comfortable up to a maximum pressure of 43.50 mmHg under the ischial tuberosities and 21.75 mmHg elsewhere. However, those authors defined that the maximum mean pressure should never exceed 37.75 mmHg, since the skin capillaries close at this pressure [15,29]. Moreover, no consensus on defining the pressure distribution threshold has been reached. It is unanimous that a widely dispersed distribution without local peak concentration represents the ideal foam [2,15,16,29,30,31,32].

Pressure-distribution profiles provide information regarding multiple parameters. Those more related to discomfort are the contact area, maximum pressure peaks (especially those near the ischial tuberosities), and average pressure [33,34]. Moreover, the pressure profile can be affected by several factors, namely the user’s anatomical characteristics (weight, height, BMI-Body Mass Index, and buttock shape) and foam properties (hardness, SAG factor, hysteresis, thickness, and contour).

Several authors reported lower peak pressure concerning heavier subjects. Those tend to have larger contact areas, leading to lower peak pressures but higher mean pressures. In opposition, thinner subjects present higher peak pressure around the ischial tuberosities [15,35]. Li et al. [35] evaluated static factors affecting the comfort levels of high-speed trains. The authors group the volunteers’ results according to their stature. Those with larger stature reported higher average contact area, higher average pressure (similar to that of medium stature volunteers), and lower average peak pressure than the small stature subjects. Hu et al. [15] evaluated the effect of BMI on sitting-pressure distribution. Experiments ran with three subjects with varying BMIs: underweight, normal, and obese. The obese subject generated the largest contact area, whereas the normal subject demonstrated lower maximum and mean pressures and reduced contact areas. Moreover, results showed that the contact area would linearly increase according to increased subjects’ weight and height.

The association between local and whole-body comfort was evaluated by Xu et al. [28]. Pressure measurements and subjective rankings were used to assess discomfort. Body parts were divided into four segments: back, waist, hips, and thighs. The hips were the most significant body part influencing whole-body discomfort, followed by the back, waist, and thighs. A similar investigation was conducted by Peng et al. [32]. However, local comfort was divided into eight parts: shoulders, mid-back, side back, waist, buttocks, and upper, side, and lower thigh. The shoulder, waist, and buttocks were revealed to be the body parts most affecting overall comfort.

Pressure distribution may also be influenced by foam’s physical properties, particularly hardness, SAG factor, hysteresis loss, and thickness. In fact, multiple authors strongly link foam’s hardness with interface pressure. Softer seats tend to induce higher comfort levels. Those generate a significantly larger contact area than that of a rigid seat, reducing pressure peaks. Additionally, higher pressure peaks around the ischial tuberosities are expected for rigid foams since these are less flexible than the soft ones. Thus, both seat and buttock deformations enlarge the contact area regarding a soft seat, whereas the subject’s buttocks deformation mainly defines the rigid foam contact area [13,15,32]. As aforementioned, the SAG factor should be higher than 2.8 (to prevent users from feeling bottoming) and the hysteresis loss should be as reduced as possible [4,7,13,23,24]. The foam thickness is another important physical parameter to evaluate. Multiple authors stated that higher foam thicknesses induce higher comfort levels. Ebe and Griffin [13] assessed foam thickness’s influence on sitting impressions. Results highlighted that thin foams had more bottoming and, consequently, higher discomfort levels. Using a finite element model, Kumar et al. [36] investigated the influence of foam thickness (40, 60, 80, and 100 mm) on interface pressure. The authors concluded that increasing the foam thickness significantly reduces contact stress and, consequently, reduces interface pressure.

Foam contour may also influence interface pressure. Different contours affect the seat–user interaction, resulting in varying interface pressures [37]. Tang et al. [38] evaluated the contour effect on sitting impressions. Pressure measurements and local discomfort questionnaires evaluated discomfort. Three contours were produced: flat, bilateral-protruding cushion, and front-protruding cushion. The flat foam was defined as promoting higher comfort levels; subjectively, it was rated with the highest overall comfort, and, objectively, it presented the lowest pressure peaks.

Lastly, the foam cover is an essential element of the sitting system as it directly promotes contact between the user and the seat. Depending on the material, this element also alters the sitting interface pressure. Wagner et al. [37] studied the influence of seat cover properties on perceived comfort. Two identical seats with different seat cover materials (leather and fabric) were evaluated by 30 subjects. Evaluation comprehended both objective (pressure measurements) and subjective (discomfort rating) methods. Moreover, subjects were blindfolded and received no information about the seats. The subjective evaluation noted that neither the leather nor the fabric cover was uncomfortable, as 16 subjects preferred the leather seat and 14 the fabric seat. Regarding the objective evaluation, the leather seat presented a maximum pressure of 14 mmHg higher than that of the fabric cover.

2.3. Interface Temperature Influence on Perceived Comfort

Thermal comfort is an important parameter to consider when assessing passengers’ comfort. Multiple authors stated that there is evidence that the heat exchange at the seat–user interface influences perceived comfort [39,40,41,42]. The heat exchanges between the human body and its surroundings in a railway vehicle can be compared to that in a car seat. Those heat exchanges are influenced by four mechanisms of heat transference: convective, radiative, conductive, and evaporative, and can be calculated as follows Equation (3):

$$Q=Q_{cv}+Q_{cd}+Q_r+E_{sk}$$

(3)

where $Q$ represents the total heat exchanges, $Q_{cv}$ is the convective heat transfer, $Q_{cd}$ relates to the conductive heat transfer, and $E_{sk}$ means the evaporative heat transfer. In a closed environment, as in the case of rail vehicles, the human body exchanges more heat with surroundings by radiation and conduction. Additionally, conduction heat exchanges assume a critical significance as a considerable portion of the body surface area is in contact with the seat [43]. This way, the seat cover is also a determinant factor in thermal comfort.

When the contact pressure is associated with thermal accumulation, discomfort and harmful consequences may be raised by the users [44]. Current investigations focus more on analysing pressure-distribution comfort than the seat–passenger interface temperature [45]. This way, a research gap was identified.

Infrared thermography is a technique capable of being applied to investigate thermal exchanges. Thermographic cameras operate following the principles of Stefan–Boltzmann, Wien, Planck, and Kirchhoff laws [46]. The Planck law states that all bodies above an absolute zero temperature (–273.15 °C, i.e., 0 K) are constituted by small particles in continuous motion in a random direction that emit electromagnetic radiation [47,48]. Therefore, thermographic cameras detect the infrared radiation emitted by a body and convert it into a thermal image. In conclusion, this non-destructive and non-invasive technique allows the mapping of materials to locate temperature variations [49]. Thus, this methodology can help investigate the existence of microclimates at the seat cushion interaction [7].

Sales et al. [47] employed infrared thermography to evaluate the thermal response of seat cushions and backrests of eight different chairs (with varying designs and materials). Results demonstrated a similar behavior of leather and fabric covers (main interest cover materials of the present section), as they reported final temperatures around 30 °C, promoting a temperature increase of approximately 6–7 °C.

Liu et al. [44] reported that foam thickness interferes with thermal conductivity. The temperature at the body–seat contact surface increases as the foam thickness increases. By applying an infrared thermographic camera on four different wheelchair cushions, Bui et al. [50] concluded that lower temperatures at the seat–user interface promote higher comfort levels.

Stockton and Rithalia [51] investigated the influence of both seating temperature and interface pressure on sitting discomfort levels. Contrary to what was reported by other authors, results suggested that high temperatures were not necessarily associated with discomfort, and low interface pressures may not be linked to comfort. These contradictory results highlighted the need to perform more experimental campaigns regarding static sitting comfort and the parameters capable of affecting it.

3. Experimental Campaign

The mechanical properties, interface pressure, and temperature of four foams were assessed to investigate which foam and seat cover is more suitable for railway seating applications. Foams were characterised based on their density, thickness, SAG factor, hardness and hysteresis loss, and induced interface pressure and temperature. The suitability of the infrared thermography method for performing temperature mapping on the current application should be highlighted. Both leather and fabric have high emissivity, 0.96 and 0.92, respectively [52]. This parameter induced a reduced influence of the environment on the method. In opposition, if the materials’ emissivity were low, the environment would significantly impact the method, limiting its suitability [53].

Long-distance railways generally have two-seat categories defined based on seat performance. Those with higher comfort levels are rated as comfort class seats, whereas those with a suitable but less comfortable performance are rated as standard class seats. Seats differ in dimensions and cushion thickness. Comfort seats have thicker foam cushions than the standard. Currently, the Portuguese Alfa Pendular (AP) comfort seats present a total thickness of 190 mm and a 60 mm thickness under the users’ ischial tuberosities. The standard seat’s thicknesses are 130 mm and 40 mm for total and local thickness, respectively. The present study uses the current AP train, comfort, and standard seats, as well as two foam samples with mechanical properties equal to those previously employed before the 2017 AP renovation [54,55]. While the current foams have a 40 kg/m³ density, the previous foams have 80 kg/m³. Moreover, the current AP seats have a leather cover, while the older AP seat covers are made of fabric.

3.1. AP Comfort and Standard Seats

The Portuguese tilting AP train is operated as a single unit comprising six cars, four motors, and two hauled vehicles. These train seats are divided into two categories: Cars 1 and 2 are comfort-class seats, whereas the remaining seats are classified as standard seats (see Figure 1). As previously stated, although visually similar, comfort and standard seats differ in dimensions. The former is larger than the latter, presenting a higher seating area, thicker surface and seatback foams, a larger seat frame, and higher space between seats [54,55,56].

Introduced in 1999, the AP trains were renovated in 2017, which significantly changed the seat design but retained the structural frame. The major change established by this renovation comprised the replacement of seat foams and covers. In opposition to the original seats, produced with 80 kg/m³ foams and fabric covers, the new seats are made of 40 kg/m³ density foams with leather covers. The difference of the foam and cover properties may change the passenger’s perceived comfort [54,55,56,57].

3.2. Experiment 1: Mechanical Properties Assessment

Following both Refs. [58,59] ISO 2439 and ISO 3386 recommendations, an experimental setup was developed to measure and calculate foam hardness, SAG factor, and hysteresis loss. The setup consists of a hydraulic actuator (1) connected to a 200 kN load cell (2) and a posterior 5 kN load cell (to increase the measurement resolution) (3). A circular plate (4) was attached to the 5 kN load cell, which performs complete contact with the seat sample (5). The actuator’s internal LVDT measured the plate displacement. A control-and-acquisition system was used to control the actuator speed and record the force. Figure 2 illustrates the experimental setup.

Both Refs. [58,59] ISO 2439 and ISO 3386 experiments perform the vertical movement of the plate (200 mm diameter) at a constant speed of 100 mm/min. Figure 3a,b shows the experimental tests conducted on both current AP seats, namely on comfort and standard foam cushions without and with seat covers, respectively. In its turn, Figure 3c,d represents the experimental tests conducted on the older AP seats without and with seat covers, respectively.

Hysteresis loss and SAG factor experiments run only in foams without cover. These were applied to the 25% ILD hardness experiments to avoid damage to the covers.

Table 1. Hysteresis loss percentage and force at 75% deformation for the four analysed cushions.

Seat Cushion	Hysteresis Loss %	${\mathit{F}}_{75\mathit{\%}}$ (N)
Current comfort seat	13.7	1024
Current standard seat	13.8	598
Older comfort seat	15.1	1255
Older standard seat	14.9	542

presents the hysteresis loss percentage and the force was achieved at 75% deformation.

Similar results were obtained regarding the cushions of the same foam type, which may indicate that the foam thickness does not influence the hysteresis loss percentage, complying with the observations conducted by Neal [26]. Comparing the results of the current and older seats, the latter presented slightly higher, but not significant, hysteresis loss. Therefore, considering only this property, both seats are expected to perform well and similarly. Moreover, the results have demonstrated lower values than those reported by Ebe and Griffin [13] and Moon et al. [4]. Figure 4a,b illustrates the hysteresis loss of current and older AP seat cushions, respectively.

Regarding the SAG factor, as mentioned above, it should be, ideally, superior to 2.8 for seating applications [4,7,13,23,24].

Table 2. SAG factor results.

Seat Cushion	SAG Factor
Current comfort seat	2.7
Current standard seat	2.0
Older comfort seat	3.2
Older standard seat	2.6

presents the calculated values for both foam types.

Observing the results, the older foams demonstrated a higher SAG factor than the new ones. Its results are closer and superior to 2.8 regarding the standard and comfort cushions, respectively. Current and older seat foams are different in terms of density but equal in dimensions. Therefore, based on the current results, it may be concluded that density is affecting the foam-supporting capacity. Moreover, the current standard seat has the lowest supporting factor of all foams. When comparing both seat types’ (comfort and standard) SAG factors, an increase is noticed for the thicker seat, which may indicate the influence of foam thickness on this physical property.

The 25% ILD hardness evaluated hardness. To understand the effect of the seat cover, cushions were compressed with and without cover, and the increasing hardness percentage was calculated.

Table 3. Foam hardness results with and without a seat cover.

Seat Cushion	25% ILD Foam Hardness (N)	25% ILD Foam+Cover Hardness (N)	Increasing %
Current comfort seat	288	360	26
Current standard seat	250	320	28
Older comfort seat	271	354	44
Older standard seat	167	240	30

exhibits the 25% ILD cushion hardness with and without seat cover and the increase %.

By analysing the results of Table 3, it can be concluded that current foams have a higher hardness than older ones. Moreover, hardness increases as the foam thickness rises, which is demonstrated by comfort foams that show a higher 25% ILD hardness than the standard ones, in agreement with Neal [26]. Additionally, the seat cover highly influences the system’s (foam and cover) hardness. While the leather cover promoted a maximum increase of 28%, the fabric cover corresponded to a 44% hardness increase. It should be highlighted that if a cover is stretchable and loosely connected to the foam, the mechanical properties of the seat are more dependent on the cover, as the foam can freely deform. In opposition, a stiff cover tightly attached to the foam may limit the foam deformation and, consequently, influence the seat performance [18].

Additionally, an extra experiment assessed the foam and cover capacity for absorbing energy. That consisted of releasing a ball (52 mm diameter and mass of 127 g) and measuring its initial and bump heights (Figure 5). That was constituted by the seat (with and without cover) (1), a motion sensor CBR 2 (Texas Instruments) (2) [60] capable of measuring and recording distance, velocity, and acceleration, connected with a computer (3) and the ball (4). The calculation of energy absorption %, follows Equation (4):

$${\% E}_{abs}={\displaystyle \frac{v_2}{v_1}}\times 100={\left({\displaystyle \frac{h_2}{h_1}}\right)}^{{\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}\times 100$$

(4)

where $v_1$ and $h_1$ are corresponding to the ball’s initial speed and height and $v_2$ and $h_2$ represent the ball’s rebound speed and height. It was assumed that the ball acts as a rigid body. This way, the ball did not lose energy, but the seat cushion suffered deformation due to the collision [61].

Measurements were recorded at a 50 Hz sample frequency. Experiments were run five times for each cushion with and without the seat cover.

Table 4. Mean energy absorption percentage with and without seat cover.

Seat Cushion	Energy Absorption % (Foam)	Energy Absorption % (Foam + Cover)
Current comfort seat	68	82
Current standard seat	66	76
Older comfort seat	71	73
Older standard seat	58	69

presents the experiments’ mean energy absorption percentage.

The cover increases the energy absorption percentage, especially the leather cover. At the current AP seats, energy absorption increases by 14% and 10%, respectively, for the comfort and standard seats. That absorption percentage increase is less noticed for the older seats, as the fabric cover only increased 2% and 11% of the energy absorption for the comfort and standard cushions, respectively. This way, based on the hardness increase and the energy absorption percentage, the leather cover appears to be more suitable for seating applications. Nevertheless, more experiments need to be performed to properly evaluate the cover influence.

3.3. Experiment 2: The Influence of Seat-Passenger Interface Pressure and Temperature

A set of dedicated experimental tests was developed and performed to evaluate interface pressure and temperature influence on sitting comfort. The University of Minho Ethics Committee previously approved the experiment’s realisation. Before the beginning of the experiments, the volunteers provided their informed consent. Three volunteers (two male and one female) participated in the experiment.

Table 5. Volunteers’ anthropometric characteristics.

Subject	Gender	Age (Years)	Mass (kg)	Height (m)
S1	Female	25	58	1.70
S2	Male	26	80	1.87
S3	Male	39	115	1.85

illustrates the subjects’ anthropometric characteristics.

The experimental setup was designed to simultaneously assess pressure and temperature at the seat–passenger interface. The pressure was measured using a CONFORMat (Tekscan Inc, Providence Highway, Norwood, MA, USA), developed by Tekscan Inc. [62], presenting a sensitive area of 471.4 mm × 471.4 mm. The pressure map was positioned at the seat surface, where the subjects sat. A FLIR A325sc thermal imaging camera (FLIR head office, Wilsonville, OR 97070, USA) was positioned perpendicularly to the seat to evaluate the seat temperature [63]. Both the pressure map and thermographic camera were connected to individual computers containing specialised software. Figure 6 illustrates the complete experimental setup.

The experiment consisted of sitting for 10 min while pressure was measured and recorded during the entire sitting experience at a 1 Hz sample frequency. Maximum peak pressure $\left(P_{max}\right)$, average pressure $\left(P_{avg}\right)$, and contact area were analysed. When the 10 min periods were concluded, thermal images of the seat were obtained. Moreover, following the aforementioned studies of multiple authors that evidenced the importance of matching objective and subjective static evaluations, volunteers rated their total and local sitting comfort impressions using a seven-point scale (

Table 6. Subjective 7-point comfort/discomfort rating scale.

Ranking	Comfort/Discomfort Level
–3	Very uncomfortable
–2	Uncomfortable
–1	Slightly uncomfortable
0	Neutral
1	Slightly comfortable
2	Comfortable
3	Very comfortable

). Local comfort was divided into four body parts as demonstrated in Figure 7.

The experiment was performed by the three subjects in the current AP seats to investigate the effects of users’ anthropometric characteristics. After assessing the effects, Subject S1 experimented with the older seats and fabric cover to compare seats (current and original), matching results with foam’s mechanical properties and assessing the best seat configuration (foam and cover). Figure 8 describes a flowchart of the experimental setup.

3.3.1. Influence of Anthropometric Characteristics

The three subjects performed the experiments at the current AP train seats (comfort and standard) to evaluate the influence of users’ anthropometric characteristics on perceived comfort. Pressure results regarding the comfort seat can be found in

Table 7. Maximum and mean pressure and contact area regarding the current comfort AP seat.

Sub.	${\mathit{P}}_{\mathit{m}\mathit{a}\mathit{x}}$ (mmHg)				${\mathit{P}}_{\mathit{a}\mathit{v}\mathit{g}}$ (mmHg)				Contact Area (cm2)
	R1	R2	L1	L2	R1	R2	L1	L2
S1	37	26	29	20	29	20	23	15	1076
S2	57	18	50	17	46	16	44	14	1060
S3	41	29	65	27	35	22	58	22	1339

.

By observing the results, both maximum and mean pressures demonstrated values above the maximum, 32 mmHg. Some authors defined this threshold as the trigger for developing discomfort and increasing the risk of health consequences, particularly for longer sitting exposures [2,16,28,30,31]. Those pressures and contact areas revealed an increasing tendency related to the subject’s weight. Moreover, pressure peaks were obtained at the ischial tuberosities regions (R1 and L1); S2 (57 mmHg) and S3 (65 mmHg) are even above the pressure threshold (43.50 mmHg) stated for that specific region [15,29]. Additionally, Hu et al. [15] and Peng et al. [29] suggested that the mean pressure should never exceed 37.75 mmHg elsewhere in the sitting region, which is true for the present case. S1 and S3, the lightest and heaviest pressure profiles, are illustrated in Figure 9.

As observed, the two subjects demonstrated different pressure profiles, being the S3 larger (X direction) and the S1 with a higher length (Y direction). Comparing both contact areas, as expected, the heaviest subject produced a higher contact area than the lightest individual.

Table 8. Maximum and mean pressure and contact area regarding the current standard AP seat.

Sub.	${\mathit{P}}_{\mathit{m}\mathit{a}\mathit{x}}$ (mmHg)				${\mathit{P}}_{\mathit{a}\mathit{v}\mathit{g}}$ (mmHg)				Contact Area (cm2)
	R1	R2	L1	L2	R1	R2	L1	L2
S1	38	21	35	24	28	16	28	19	1100
S2	51	15	46	34	41	13	38	25	1035
S3	50	49	46	35	37	38	40	29	1350

presents the pressure distributions regarding the standard seat.

As for the comfort seat, all subjects presented maximum pressures above 32 mmHg for the standard seat as well. Nevertheless, those pressure peak values are slightly inferior to those of the comfort seat. The same decreasing tendency was observed for the mean pressure. This fact can be justified by the highest contact areas promoted by the standard seat.

Regarding the interface temperature, results present the maximum, minimum, and average temperatures, and the difference between maximum and minimum temperatures for the sitting area and for a profile line. Figure 10a shows a 3D thermal image in the course of the experiment. Otherwise, Figure 10b presents the region of interest and a profile line of the seat. The profile line clearly distinguishes between the seat’s initial temperature and its increase due to the subject’s presence.

Results concerning the comfort seat are presented in

Table 9. Temperature records regarding the comfort seat.

Scheme.	Sitting Area				Sitting Profile Line
	$\mathbf{Min}. (°\mathbf{C})$	$\mathbf{Max}. (°\mathbf{C})$	$\mathbf{Max}-\mathbf{Min} (°\mathbf{C})$	$\mathbf{Avg}. (°\mathbf{C})$	$\mathbf{Min}. (°\mathbf{C})$	$\mathbf{Max}. (°\mathbf{C})$	$\mathbf{Max}-\mathbf{Min} (°\mathbf{C})$	$\mathbf{Avg}. (°\mathbf{C})$
S1	28	34.1	6.1	31.8	25.2	32.9	7.7	31.8
S2	27.1	33.1	6.0	31.3	25.0	31.2	6.1	28.5
S3	24.4	30.2	5.8	27.7	23	28.2	5.3	26.4

.

Regarding the sitting area, similar temperatures were obtained for the difference between maximum and minimum values. This result may indicate that the user’s anthropometric characteristics do not influence the temperature increase. The sitting line revealed that subject S1 induced higher temperature differences when compared to the seat’s initial temperature. Moreover, the same subject also induced the highest maximum and average temperatures in the sitting area. This is the only female subject, which may justify the results. Therefore, although the passengers’ anthropometric features (weight and height) do not impact the temperature, it may be observed that gender is a possible cause of influence. Nevertheless, it should be highlighted that due to the reduced number of subjects, the results only reveal a hint. However, to produce further conclusions, more experiments need to be performed. Regarding the standard seat, the results are exhibited in

Table 10. Temperature records regarding the standard seat.

Subject	Sitting Area				Sitting Profile Line
	$\mathbf{Min}. (°\mathbf{C})$	$\mathbf{Max}. (°\mathbf{C})$	$\mathbf{Max}-\mathbf{Min} (°\mathbf{C})$	$\mathbf{Avg}. (°\mathbf{C})$	$\mathbf{Min}. (°\mathbf{C})$	$\mathbf{Max}. (°\mathbf{C})$	$\mathbf{Max}-\mathbf{Min} (°\mathbf{C})$	$\mathbf{Avg}. (°\mathbf{C})$
S1	26.2	32.9	6.7	30.5	23.9	31.3	7.4	27.6
S2	25.1	31.7	6.7	29.6	21.8	31	9.2	28.1
S3	24.3	32.1	7.8	29.8	23.3	31.1	7.8	28.5

.

The results of standard seats revealed a similar tendency to that of comfort. Both demonstrated equivalent maximum and average temperatures in the sitting area and profile line.

Subjectively, three subjects unanimously ranked the ischial tuberosity areas (R1 and L1) as “Slightly uncomfortable”, which, in fact, exhibited the highest pressure peaks. Therefore, complying with the findings of Xu et al. [28] and Peng et al. [29], the ischial tuberosity area seems to be the one most influencing the perception of sitting comfort. Additionally, based on the mentioned assumption, it is also possible to conclude that pressure profiles were strongly linked to discomfort rankings [9,13,14,27]. Regarding temperature, the similarities between experiment results did not allow for conclusions to be made about its impact on sitting impressions. That influence will be further analysed in the following sub-section. It should be highlighted that the small number of volunteers is the main limitation of this experiment.

3.3.2. Influence of Seat and Cover Features

To verify the influence of the seat and the cover on sitting comfort, the S1 subject performed experiments with the current and older AP seats. As aforementioned, these seats have foams with similar dimensions but different densities. Moreover, their covers are produced from different materials. This way, it will be possible to investigate the influence of foam mechanical properties on interface pressure and the impact of interface temperature promoted by the seat cover.

Results regarding the interface pressure of current and older AP comfort seats are depicted in

Table 11. Maximum and mean pressure and contact area regarding the current and older AP comfort seat for subject S1.

Seat Type	${\mathit{P}}_{\mathit{m}\mathit{a}\mathit{x}}$ (mmHg)				${\mathit{P}}_{\mathit{a}\mathit{v}\mathit{g}}$ (mmHg)				Contact Area (cm2)
	R1	R2	L1	L2	R1	R2	L1	L2
Current	37	26	29	20	29	20	23	15	1076
Older	33	16	27	17	28	14	21	14	983

.

Generally, the older seat presented a lower average and peak pressures (–4 mmHg), while the main peak pressure occurs at the same region as the current seats (R1). Therefore, based on the previously found correlation between discomfort and pressure distribution, it can be concluded that the older foams induce higher comfort levels. This result is also sustained by the lower hardness presented by the older foams [13,15,32]. Pressure profiles (Figure 11) highlighted the differences between both foams. The current foam (Figure 11a) induced multiple pressure peaks at the ischial tuberosities regions and a generally higher average pressure. Concerning the older foam (Figure 11b), a reduced number of localised pressure peaks around the ischial tuberosities are observed. Those are coupled with a globally distributed pressure, reducing the average pressure.

As described in

Table 12. Maximum and mean pressure and contact area regarding the current and older AP standard seat for subject S1.

Seat Type	${\mathit{P}}_{\mathit{m}\mathit{a}\mathit{x}}$ (mmHg)				${\mathit{P}}_{\mathit{a}\mathit{v}\mathit{g}}$ (mmHg)				Contact Area (cm2)
	R1	R2	L1	L2	R1	R2	L1	L2
Current	38	21	35	24	28	16	28	19	1100
Older	33	16	19	14	25	13	16	12	981

, similar pressure tendencies were obtained on the standard seats.

Standard and comfort seats only differ in terms of dimensions, namely thickness. Comparing the similarity of results of those seat types, it can be concluded that thickness does not influence pressure distribution. These findings contradict Ebe and Griffin [13] and Kumar et al. [36], who stated that increasing foam thickness reduces interface pressures.

Seat cover influence was assessed by the thermographic images. The evaluation methodology was the same as previously conducted, where a sitting area and a defined profile line were applied.

Table 13. Temperature records regarding the different seating covers.

Seat Type/Cover	Sitting Area				Sitting Profile Line
	$\mathbf{Min}. (°\mathbf{C})$	$\mathbf{Max}. (°\mathbf{C})$	$\mathbf{Max}-\mathbf{Min} (°\mathbf{C})$	$\mathbf{Avg}. (°\mathbf{C})$	$\mathbf{Min}. (°\mathbf{C})$	$\mathbf{Max}. (°\mathbf{C})$	$\mathbf{Max}-\mathbf{Min} (°\mathbf{C})$	$\mathbf{Avg}. (°\mathbf{C})$
Comfort/Leather	28	34.1	6.1	31.8	25.2	32.9	7.7	31.8
Comfort/Fabric	19	27.1	8.1	24.1	17.1	25.1	7.9	20.9
Standard/Leather	26.2	32.9	6.7	30.5	23.9	31.3	7.4	27.6
Standard/Fabric	18.7	27.5	8.8	24.1	16.7	26.3	9.6	23.6

presents the thermal results of both seat covers.

Table 13 shows that fabric cover seats have lower minimum, maximum, and average temperatures independently of the seat type. Similar results were found within the same cover seats, meaning the non-influence of seat thickness. For the leather cover, both comfort and standard seats presented minimum temperatures of approximately 28 $°\mathrm{C}$, whereas the maximum temperature was around 33 $°\mathrm{C}$ (see Figure 12).

When considering the fabric seats, the temperature results slightly decreased. Likewise, minimum temperatures of around 19 $°\mathrm{C}$ and maximum temperatures of 27 $°\mathrm{C}$ were found, as illustrated in Figure 13.

Moreover, it should be noted that regarding the average temperature, fabric covers presented a temperature of 24 °C, whereas that of leather covers was around 31 °C. These results contradict the findings of Sales et al. [47], who reported that leather and fabric covers have similar thermic behaviours, and those of Liu et al. [44], once foam thickness did not influence results when using the same cover material. It should be highlighted that the work developed by Sales et al. [47] uses a single subject. Therefore, it is used as a reference but not as a gold standard.

Based on the assumption stated by Bui et al. [50] that lower temperatures at the seat–user interface promote higher comfort levels, it can be concluded that fabric covers are more suitable for railway seat applications than leather covers. Results agree with those reported by Bartels [64], Wölfling et al. [65], and Wegner et al. [37], where fabric covers revealed superior seat comfort over leather covers. Nevertheless, it should be highlighted that leather covers demonstrated a higher capacity to absorb energy and reduce hardness increases. The latter parameter may be linked to the cover foam connection [18]. Therefore, there is still space for fabric cover improvements and attachment within rail seats.

The present study’s findings can be used to improve the static comfort levels of new railway seats. As observed, higher-density foams and fabric covers can increase those. Even so, fabric covers must be improved, as their mechanical properties have been demonstrated to be weaker than those revealed by leather covers. Indeed, the ideal cover would combine the mechanical properties of leather and the thermal properties of fabric.

4. Conclusions

In the absence of vibration, static comfort factors mainly influence passengers’ perceived comfort. Static comfort is usually assessed using foam mechanical properties and interface pressure experiments. Additionally, the influence of temperature at the seat–passenger interface was also evaluated by employing an infrared thermographic assessment. This methodology revealed its suitability due to the high emissivity of fabric and leather covers.

Map pressure allows for the analysis of the seat’s influence on the user as a response to the applied body load distribution. In contrast, thermographic analysis permits measuring the opposite effect, i.e., what the passenger induces on the seat (increased temperature). In this way, these two methods can be considered complementary to the sitting comfort analysis.

An analysis of the influence of users’ anthropometric characteristics on interface pressure was performed. The results partially agree with those reported by Li et al. [35], as higher mean pressures and larger contact areas were obtained for heavier people. However, contrary to this author, higher peak pressures were also demonstrated for heavier individuals. Moreover, the pressure profile of the lighter subject and the heavier individual also show significant differences. While the former shows a longer pressure profile, the latter evidences a wider profile. The pressure in the ischial tuberosities area significantly influences the subjective results of discomfort. All pressure peaks were located in this area, which was rated as “Slightly uncomfortable” by all subjects. Thus, it is possible to conclude that the pressure profiles are directly related to discomfort levels.

To assess the best foam and cover for railway seating applications, two foams with different densities and two covers made of different materials (leather and fabric) were analysed. The foams have the same properties as the current (lower density) and older (higher density) AP train seats. Older foams exhibited better pressure profiles (lower average and maximum pressures) and distributed the pressure more uniformly over the contact area. Combining these results with the assumption that lower pressure levels are associated with higher comfort levels, it is possible to conclude that the higher-density foams have a better static performance than the lower-density ones. This statement is also sustained by their mechanical characteristics, especially the high SAG factor and the low 25% ILD hardness. Both foams show similar results regarding hysteresis loss and energy-absorption capacity, so these are not considered differentiating factors. Moreover, by comparing comfort and standard seats, it was noticed that foam thickness does not influence pressure measurements, thus contradicting what has been reported by other authors.

Finally, regarding the selection of a more appropriate cover, it was observed that the fabric cover promotes maximum, minimum, and average temperatures lower than the leather cover. Thus, this one is the most suitable for application in railway seats since lower interface temperatures promote higher comfort levels.

Moreover, it should be highlighted that the experiments elapsed with real seat foams and not isolated foams as reported in most bibliographic research. Nevertheless, the reduced number of volunteers participating in the experiment represents its main limitation. Besides the need to expand the subject’s sample size, it could be rather interesting to replicate the present study using children. This would allow us not only to determine the children’s static comfort levels but also to determine a possible interface pressure pattern. Additionally, in the developed experiments, more properties could be assessed for the cover: breathability, moisture and water resistance, biodegradability, environmental impact, and even tear and pulling resistance. Lastly, the present study could be replicated with in-movement railways. This way, it would be possible to rank the influence of static properties over dynamic ones.