A movable support arm positions a surface platform for a keyboard, display monitor, pointing device, wrist support or the like relative to a base structure such as the underside of a desk top. Articulated members between a base member and the platform are coupled by joints having rotational couplings on two mutually perpendicular axes, forming universal joints. The joints have axially facing rotational engagement surfaces that can be splined or otherwise made engageable, and are displaceable toward and away from one another, the latter preferably by a spring coupling that is axially limited in extension. The rotational engagement surfaces resist rotation when engaged. A line such as a cable traverses the joints and normally applies tension via a spring, causing all the joints to lock and to hold the supporting platform at a given position and orientation. A manually operable control is provided to extend the length of the line and disengage all the joints simultaneously, whereupon the support platform can be moved freely to a new position anywhere in a three dimensional range, as well as tilted, etc. The control is released at the new position to again hold the platform in place. The line runs centrally through the joints. A counterbalance spring and line are displaced from the centerline, the cable winding around a horizontal one of the joints to vary tension on the spring for offsetting part of the weight of the support.