## 1 Introduction
[PERSON] and [PERSON] [19] also found that discomfort significantly depended on the incline angle of the backrest when the subjects experienced whole-body vertical vibration, and they determined various equivalent comfort contours for various backrest angles at 0\({}^{\circ}\), 30\({}^{\circ}\), 60\({}^{\circ}\), and 90\({}^{\circ}\). [PERSON] and [PERSON] [20] reported that different backrest angles (0\({}^{\circ}\), 22.5\({}^{\circ}\), 45\({}^{\circ}\), 67.5\({}^{\circ}\), and 90\({}^{\circ}\)) had significant impacts on discomfort. Through field measurements, [PERSON] [21] found that the vibration dose value (VDV) increased with an increasing backrest angle. However, several studies failed to show a significant effect of the backrest incline angle on discomfort under whole-body vibration. [PERSON] and [PERSON] [22] indicated that the discomfort was independent of the backrest angle when the direction of vibration at the backrest was parallel to the back in the z-axis of the body. [PERSON] and [PERSON] [23] proposed that backrest inclination (15\({}^{\circ}\), 30\({}^{\circ}\), 45\({}^{\circ}\), and 60\({}^{\circ}\)) did not influence discomfort caused by vibration on fast boats. The inclination of a backrest can also lead to different local discomfort in the vibration environment. [PERSON] and [PERSON] [16; 20] found that the body part experiencing the most discomfort shifted from the buttock area to the back region when the angle between the backrest and the seat pan increased from 0\({}^{\circ}\) to 60\({}^{\circ}\) under whole-body vibration at \(x\)- or \(z\)-axis. They further indicated that local discomfort was felt on the body part supporting the greatest proportion of the weight at the vibrating surface (e.g., the back, buttocks, or the thighs), and that the most uncomfortable body part was changed by varying the backrest angle [24]. [PERSON] and [PERSON] [23] found that the most uncomfortable body part was the buttocks when the angle between the backrest and the seat pan was 90\({}^{\circ}\) and the discomfort shifted to the upper and middle back when the angle was 45-60\({}^{\circ}\). The physiological parameters (i.e., the gender, stature, weight, etc.) are the potential factors related to discomfort in a seat [4; 13]. Male and female subjects report different stress load conditions and experience different levels of discomfort [25; 26]. [PERSON] and [PERSON] [27] indicated that there were significant differences between males and females regarding sensitivity to pressure from the backrest and the seat pan. Subjects with different statures preferred different sizes of seats; relatively, taller subjects reported less discomfort in larger seats than in smaller seats, and vice-versa [1; 8]. [PERSON] and [PERSON] [28] reported that -state showed significant effects on local discomfort in the neck, shoulders, hips, and thighs. [PERSON] and [PERSON] [29] studied driving discomfort, in the field and laboratory, by grouping subjects into tall, middle, and short groups. They found marginal effects of stature on discomfort; subjects in the middle stature group rated the lowest discomfort, possibly because contemporary car seats accommodate subjects of middle height the best. [PERSON] and [PERSON] [9] investigated seat discomfort by grouping subjects by stature, weight, and gender. They indicated that the larger contact area between the human-seat interface led to lower average pressure and peak pressure for the larger BMI subjects, and therefore the large male group had the lowest discomfort ratings. [PERSON] and [PERSON] [30] detected that a subject with a large BMI could have a high ratio of pressure exerted on the backrest bolster relative to the insert, which implied poor fit and increased discomfort. The above studies mainly investigated sitting discomfort in a fixed (could be multi-group) posture, under a static or dynamic environment.