There is conventionally known, as a vehicle seat, a vehicle seat having a cushion material formed from polyurethane foam (hereinafter called urethane). As the cushion structure of such a vehicle seat, a structure is widely used in which a cushion material made of urethane is placed on a plate or a spring material, such as a contour mat (trade name) or the like provided at a seat frame or a seat back frame, and these are wrapped in a fabric material.
Therefore, it is known that the shape (the design shape) and the elastic characteristic of the cushion material have a great effect on the body pressure dispersability of the seated person and the vibration absorbability. Further, by structuring the cushion material by layering urethanes which have various characteristics, a cushion material can be obtained which has a spring characteristic (elastic characteristic) which is near the spring characteristic of the muscles of the buttocks or the like of the seated person. However, in such a structure, there are the problems that there is the sensation that the restoring force is insufficient and that the weight is heavy.
Thus, a seat has been conceived of in which, as the cushion material taking the place of urethane, a cushion structure is structured by stretching, over a seat frame, a pair of ground knit fabrics and a two-dimensional knit fabric or a three-dimensional solid knit fabric formed by connecting threads which are disposed between the ground fabrics (see, for example, Japanese Patent No. 5013089). The cushion material formed from this three-dimensional solid knit fabric or two-dimensional fabric is an elastic structure which is difficult to weaken, and is thinner than urethane, and exhibits an elastic characteristic in place of urethane.
However, in a conventional vehicle seat using such a three-dimensional solid knit fabric or two-dimensional fabric as the cushion material, the two-dimensional knit fabric is stretched over the seat frame at a tension such that the elongation is in the range of from 5% to 20%, and the three-dimensional solid knit fabric is stretched at a tension such that the elongation is less than 5%. Therefore, when a person sits down, large forces arise due to tension at the portions where the convex portions of the human body, such as the ischia, the coccyx, the shoulder blades and the like, contact the cushion material (clothes are ignored). Therefore, this becomes a cause of the supporting pressure becoming strong and a sensation that a foreign object exists arising. Further, if this acts over a long time period, there is the problem that pain arises in the muscles at the peripheries of the aforementioned ischia, coccyx, shoulder blades and the like. In particular, the muscles and the blood vessels in the peripheries beneath the ischial tuberosities, which support the majority of the body weight of the human body, are compressed, and there are cases in which pain or numbness due to interruption in blood circulation arises.
Further, in the above-described conventional seat, the two-dimensional knit fabric is stretched directly over the frame. Therefore, the problem is known that it is easy for there to arise a phenomenon, which is the so-called hammock sensation phenomenon, in which it is easy for the seated person to move in the front-rear and left-right directions within the frame, and due to the input of vibration at the time when the vehicle travels for example, the seated posture of the seated person changes or the seated person slides forward on the seat.