Abstract: A biomechanical seat construction (1) comprising a seat (2) and a back rest (3) with adjustable support (4) of the pelvis, the seat comprising a front seat part (5) and a rear seat part (6), and means (8) for displacement of the front seat part (5) and the rear seat part(6) vertically relatively to each other. In a first position the seat surface (5a) of the front seat part (5a) is generally positioned at a level which is essentially higher that the seat surface (6a) of the rear seat part (6) when both seat surfaces (5a,6a) are generally horizontal. Means is provided for displacement of the front seat part (5) and the back seat part (6) vertically relatively to each other to at least a second position. Said means comprise at least one arm (8) for supporting the rear seat part(6). In said second position the seat surface (6a) of the rear seat part (6) has an angle relatively to a horizontal plane.