A repositionable, lockable support arm assembly for surgical and other tools includes a base arm having a lower end and an upper end, a distal arm having a proximal end and a distal end, and a central joint, typically a rotational joint, directly or indirectly linking the upper end of the base arm to the proximal end of the distal arm. A lower joint, typically a spherical joint, is positioned at the lower end of the base arm, and an upper joint, also typically a spherical joint, is located at the distal end of the distal arm. A locking mechanism is coupled to the base arm at a location above the lower joint and is configured to simultaneously deliver locking forces to the lower joint, to the rotational joint, and to the upper joint. The kicking mechanism usually includes a powered, bilateral force generator for actuating the locking mechanism.