The effect of internal forces on the stiffness of the end effector at the static equilibrium state is analyzed for a three-degree-of-freedom redundantly actuated planar parallel manipulator. Internal forces are represented by the deflection of the virtual springs attached to the end effector. It is shown that internal forces can be used to change the structural compliance of the parallel manipulator. The desired rotational compliance of the manipulator end effector is obtained by generating internal forces in a proper control scheme. The effect of internal forces during the peg-in-hole insertion operation was experimentally demonstrated.