A seat backing comprising a closed hollow plastic jacket defining a chamber fillable with a fluid and comprising a valve serving for feeding the fluid, having a bearing region intended for putting on a seat, a sitting region intended for receiving a sitting person, and a transition region formed between the bearing and sitting regions and interconnecting them, - wherein the sitting region is inclined with respect to the bearing region whereby an upper sitting section and a lower sitting section of the sitting region are formed; - whereby the transition region comprises a sitting partial region and a bearing partial region which are respectively formed around the sitting and the bearing regions so that these partial regions integrally interconnect the bearing face and the sitting face; - wherein in the unloaded state the transition region, adjacent to the upper sitting region, substantially comprises a curved cross section, whereas it forms, adjacent to the lower sitting region, two partial regions which meet in an acute angle and form an edge, - wherein the faces of the partial regions (ATB, STB) in the upper and in the lower sitting sections at least partially define a sitting face which is increased when the seat backing is used, resulting in a horizontal and vertical movement support of the pelvis of the user, to enable dynamic or active sitting. - wherein an angle between the sitting region and the bearing region is about 4° to 14°, and characterized in that - the bearing partial region is defined substantially symmetrically to the sitting partial region of the transition region, and - the upper sitting section is provided with a transition region rounded in top view, wherein the lower sitting section is provided with a transition region substantially rectangular in top view and having rounded edges, and in that the sitting or bearing face is rounded on one side and are rectangular with rounded edges on the other side. - wherein the sitting region is inclined with respect to the bearing region whereby an upper sitting section and a lower sitting section of the sitting region are formed; - whereby the transition region comprises a sitting partial region and a bearing partial region which are respectively formed around the sitting and the bearing regions so that these partial regions integrally interconnect the bearing face and the sitting face; - wherein in the unloaded state the transition region, adjacent to the upper sitting region, substantially comprises a curved cross section, whereas it forms, adjacent to the lower sitting region, two partial regions which meet in an acute angle and form an edge, - wherein the faces of the partial regions (ATB, STB) in the upper and in the lower sitting sections at least partially define a sitting face which is increased when the seat backing is used, resulting in a horizontal and vertical movement support of the pelvis of the user, to enable dynamic or active sitting. - wherein an angle between the sitting region and the bearing region is about 4° to 14°, and characterized in that - the bearing partial region is defined substantially symmetrically to the sitting partial region of the transition region, and - the upper sitting section is provided with a transition region rounded in top view, wherein the lower sitting section is provided with a transition region substantially rectangular in top view and having rounded edges, and in that the sitting or bearing face is rounded on one side and are rectangular with rounded edges on the other side.