A support structure for supporting an instrument manipulator comprises a proximal link and a distal link configured to extend from the proximal link. The support structure further comprises a base joint coupling the proximal link of the support structure to a base, and the proximal link is configured to rotate about a first axis associated with the base joint. The support structure further comprises a counterbalance mechanism comprising a counterweight block configured to move linearly within the support structure. The counterweight block has a counterweight mass to counterbalance a combined mass of the support structure and the instrument manipulator as the distal link extends from the proximal link.