**2. Comfort and discomfort definition**

Whether for pleasure or work, people want to be comfortable. Thus, when designing a seat, it is vital to start by analyzing the comfort that it will provide to the users. Therefore, understanding how comfort and discomfort are defined and evaluating them, is a major assignment. These two parameters do not have a consensus; hence multiple definitions and interpretations for comfort and discomfort can be found in the literature [19].

In 1958, Hertzberg defined comfort as the absence of discomfort. Hertzberg also postulated that comfort and discomfort could not coincide, so discomfort is not present when someone feels comfort [20]. Based on that statement, Shackel et al. and, later, Richards developed a concept where comfort and discomfort were defined as two states placed on opposite extremities of a linear scale [19, 21, 22].

However, in 1992 and 1996, Zhang and Helander and Zhang et al. developed a work that ostracized the previous linear concept and introduced the non-linear model of comfort and discomfort. Zhang et al. defined comfort and discomfort as independent, individual concepts associated with different underlying factors. Comfort is related to feelings of well-being and relaxation, and the esthetic impression of a product or environment influences it. In opposition, discomfort is associated with pain, soreness, numbness, and stiffness and is influenced by the product's physical constraints [23, 24]. In 2003, as a complement to his previous work, Herlander stated that comfort is an emotional state, whilst discomfort is a physical state of being [25].

De Looze et al. built a theoretical model that illustrates the non-linear relationship between comfort and discomfort and its relationship with the physical product. This model also distinguishes the three categories influencing comfort/discomfort assessment: the human, the product, and the environment [26–28].

In this model, it is possible to observe the differences between discomfort (left side of the model) and comfort (right side of the model). Also, it concludes that discomfort has a dominant effect on comfort. De Looze indicated that discomfort levels are influenced by the interaction of the human's physical capacity, like weight, physiological processes, muscle activation, body temperature, intradiscal pressure or nerve conduction. Relatively to the comfort side, it is highly influenced by the user's emotions, expectations, and esthetic design. Regarding the rail context, the emotions and expectations of the user (human level), as well as the train temperature (environment level), can influence the passenger's perception of comfort/discomfort (product level) [19, 26, 27].

In 2005, Moes introduced a different model concerning sitting discomfort. This model defines four initial parameters: the person, the seat, the purpose, and the usage. Then, five steps are taken before discomfort, interaction, effect on the internal body, perceived effects, appreciation of the effects and, finally, discomfort. The author also states that if a person is seated for a specific purpose, the interaction occurs.

For a train journey, the interaction can result in higher-pressure distribution in the seat interface. This pressure will lead to internal body effects, like nerve compression, which can change the user perceived effects and its appreciation and, consequently, lead to discomfort [26, 29].

The previous methods have different backgrounds and approaches but are unanimous in framing comfort as a combination of physical and psychological factors. Whilst the physical state can be influenced by external stimuli such as noise, temperature, or vibration, the psychological state is affected by the previous user experience and expectations [29–31]. Therefore, to be able to evaluate both, physical and psychological impact on the users' comfort, objective and subjective methods should be considered. The present research is developed following the De Looze model. Therefore, the interaction between the product, the user, and the environment is considered and evaluated.
