mechanism 21 chair member 12 will remain in the desired position with no pressure or force exerted by the mechanism itself dynamic posture shifting occurs automatically when the user moves his or her upper body forward or rearward to the desired task posture but unlike conventional mechanisms the tilt mechanism 21 does not offer or generate any return force the resulting sensation to the user is a slow smoothly flowing high viscosity fluid motion when the desired tilt position is reached the user simply stops upper body movement and the chair position is retained by the mechanism 21 with reference to fig3 chair member 12 can be tilted through an angular range u03c6 which in the preferred embodiment is no greater than about 30 degrees with reference to fig6 and 7 the mobile base 17 consists of a shallow conical housing 40 that is preferably circular in configuration and which includes three equi angularly spaced cylindrical rollers 33 mounted at its periphery a three point support for mobile base 17 is preferred because it provides excellent stability for the chair 11 over the range of tilt positions of the chair member 12 and the sit stand position of the user to provide optimum stability mobile base 17 should have a minimum diameter of.