# Static Factors in Sitting Comfort: Seat Foam Properties, Temperature, and Contact Pressure
## 2 Influence Parameters
### 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]. [PERSON] 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. [PERSON] 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 [PERSON] 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 [PERSON] et al. [32]. However, local comfort was divided into eight parts: shoulders, mid-back, side back, waist, buttocks, and upper, side, and lower thigh.