# Static Factors in Sitting Comfort: Seat Foam Properties, Temperature, and Contact Pressure
## 2 Influence Parameters
### Influence of Interface Pressure in Sitting Impressions
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. [PERSON] and [PERSON] [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, [PERSON] 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]. [PERSON] 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. [PERSON] 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.