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):
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
, and the load necessary for 25% foam deformation
, according to Equation (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/m
3). The high-durability seat (55 kg/m
3) was rated as the most comfortable, whereas the lowest-density foam (45 kg/m
3) 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):
where
represents the total heat exchanges,
is the convective heat transfer,
relates to the conductive heat transfer, and
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.