In this note we deal with the problem of force/tracking control of constrained manipulators without velocity measurements interacting with an infinitely stiff environment. We use a reduced order model for which the coordinates space is restricted to a subset\Omega where the constraints Jacobian is guaranteed to be non singular. In contrast to other similar approaches, we do not assume that the generalized trajectories belong to the subset\Omega all the time thus, our contribution is to prove that if the trajectories start within the subset\Omega\Gamma they remain in it. Furthermore, we prove uniform asymptotic stability considering only position and force measurements. Keywords: Output feedback, tracking control, holonomic constraints. 1 Introduction Starting with [22, 13, 10] the interaction with an infinitely stiff environment is modeled by holonomic (algebraic) constraints imposed to the manipulator's motion. This kind of motion is often referred to in the literature ..