A system for performing an operation on a subject , said system comprising: a mechanical mounting member adapted to support a surgical tool guide for use in operating on the subject ; a support member rigidly positioned relative to the bed on which the subject is lying, said mechanical mounting member being attached to said support member by a moveable element (14 or 26); a bone attachment element adapted to attach said mechanical mounting member to a bone of the subject in the region where said operation is to be performed, such that said mechanical mounting member and its supported tool guide moves in correspondence with motion of said bone of said subject; characterised in that the system comprises a force sensor associated with said bone attachment element , said force sensor adapted to output a force signal in accordance with a force exerted on said bone attachment element resulting from a force applied by a surgical tool in said tool guide; and control circuitry receiving said force signal and adapted: (i) when said force exceeds a predetermined threshold level, to output a drive signal to a controlled motion drive configured to apply a reactive force to said mounting member to counteract said force applied by said surgical tool in said tool guide, and (ii) when said force does not exceed a predetermined threshold level, to become inactive, such that said mechanical mounting member can move freely with motion of said bone . a mechanical mounting member adapted to support a surgical tool guide for use in operating on the subject ; a support member rigidly positioned relative to the bed on which the subject is lying, said mechanical mounting member being attached to said support member by a moveable element (14 or 26); a bone attachment element adapted to attach said mechanical mounting member to a bone of the subject in the region where said operation is to be performed, such that said mechanical mounting member and its supported tool guide moves in correspondence with motion of said bone of said subject; characterised in that the system comprises a force sensor associated with said bone attachment element , said force sensor adapted to output a force signal in accordance with a force exerted on said bone attachment element resulting from a force applied by a surgical tool in said tool guide; and control circuitry receiving said force signal and adapted: (i) when said force exceeds a predetermined threshold level, to output a drive signal to a controlled motion drive configured to apply a reactive force to said mounting member to counteract said force applied by said surgical tool in said tool guide, and (ii) when said force does not exceed a predetermined threshold level, to become inactive, such that said mechanical mounting member can move freely with motion of said bone . (i) when said force exceeds a predetermined threshold level, to output a drive signal to a controlled motion drive configured to apply a reactive force to said mounting member to counteract said force applied by said surgical tool in said tool guide, and (ii) when said force does not exceed a predetermined threshold level, to become inactive, such that said mechanical mounting member can move freely with motion of said bone . <EOS>